A reactor construction

Abstract

The present invention relates to a reactor construction (1) comprising an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising: a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6); a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising: one or more drain tanks (10) in communication with the molten salt drain system; a buffer water tank (15) comprising an inner wall (16) and an outer wall (17) and buffer water (21) in the gap between the inner wall (16) and the outer wall (17) of the buffer water tank (15), said buffer water tank (15) surrounding the one or more drain tanks (10), wherein a first piping structure (22) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (23) in thermal contact with a reservoir water tank (30), said reservoir water tank (30) being at a level above the buffer water tank (15), said reservoir water being in thermal contact with an environment and / or a second piping structure (24) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (25) in thermal contact with an environment, said heat exchanger (25) being at a level above the buffer water tank (15) and / or a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15).

Description

[0001] A REACTOR CONSTRUCTION

[0002] Field of the invention

[0003] The present invention relates to a reactor construction for a small modular reactor (SMR) such as a molten salt reactor (MSR) where the reactor construction comprises a cooling system for the decay heat from the nuclear fission reaction. The reactor construction comprises a water tank for transferring heat to enable a fully passive decay heat removal system. The present invention furthermore relates to a method of transferring heat from a molten salt.

[0004] Background of the Invention

[0005] Molten salt reactors (MSRs) are based on a critical concentration of a fissile material dissolved in a molten salt. The molten salt comprising the fissile material is commonly referred to as the fuel salt or molten fuel salt. Research was conducted on MSRs initially at the Oak Ridge National Laboratory (ORNL) in the 1950's and 1960's but is yet to be successfully commercialised. MSRs have several advantages over other reactor types, including those being in commercial use nowadays. MSRs are capable of breeding fissile U-233 from thorium, of producing much lower levels of transuranic actinide waste than uranium / plutonium fueled reactors; capable of operating at high temperatures, capable of avoiding accumulation of volatile radioactive fission products in solid fuel rods and capable of combusting larger amounts of fissile material than is possible in conventional reactors. Other particular attractive features of an MSR are the operation at ambient or low pressure and the retention of the fission products as strongly bonded salts which are conventionally either fluoride salts or chloride salts.

[0006] Much of the attention of the safety systems for nuclear reactors is focused on the removal of heat arising from the decay of the fission products especially if a breakdown occurs of the normally functioning cooling pumps of the nuclear plants sometimes described as a loss of cooling accident (LOCA). These occurrences can be the result of a loss of offsite power (LOOP) or for other reasons. Conventional light water reactors (LWRs) are of the Generation II type and a few are of the Generation III type. The Generation II and III types use active systems for cooling with residual heat removal heat exchangers. Advanced designs (Gen III and Gen III+) rely extensively on passive safety systems at least for a certain grace period to allow time for an active system to be up and running to remove the large amounts of decay heat for most prevailing nuclear power plants.

[0007] Molten salt reactors (MSRs) are characterized as Gen IV designs along with other advanced small modular reactors (SMRs) such as high temperature gas cooled reactor (HTGR). These reactor types are expected to rely on passive and inherent safety features for an indefinite period of time. The reactor vessel auxiliary cooling system (RVACS) and the direct reactor auxiliary cooling system (DRACS) are examples of such relatively new safety systems. RVACS and DRACS are particularly suited as cooling systems for SMR's whereas these systems are less attractive for conventional nuclear reactors where the demand to heat removal may be an order of magnitude larger.

[0008] ORNL-314 "MOLTEN-SALT REACTOR PROGRAM QUARTERLY PROGRESS REPORT" from 15 December 1960 discloses heat removal in relation to the operation of the original Molten Salt Reactor Experiment (MSRE). The document discloses that the heat is removed from molten fuel salt after the molten fuel salt has been drained into several drain tanks. It is described that in order to make the heat removal as nearly uniform as possible throughout the tank, forty immersed bayonet coolers are used, with boiling water as coolant. Water-cooling was selected above gas, molten salt, or NaK because of its simplicity and relative independence from utilities failures. However, the immersion of water contained in bayonets directly into the molten fuel salt imposes a risk in case of a breach of the bayonet material, typically a metal alloy accompanied by a contact between the molten fuel salt and water leading to sudden burst of high-temperature steam.

[0009] KR20090021722 discloses a cooling system for removing decay heat from a high temperature gas cooled reactor (HTGR). The system is aimed at minimizing the parasitic heat loss during normal operation. The decay heat from the reactor core is transferred to a piping system for water where the water pipe is air cooled. The water piping is in thermal contact with a water reservoir via a water bath heat exchanger and the water is actively pumped through the water piping. A passive air-cooling system is used as the main cooling system and only the air- cooling device is used during normal operation and the water-cooling part of the system is used in the event of an accident. Though a large capacity of passive cooling can be obtained in the event of an accident with a nuclear reactor, especially a high-temperature gas reactor, the system also relies on active cooling to minimize parasitic heat loss during normal operation.

[0010] US2019035510 discloses a reactor vessel auxiliary cooling system (RVACS) for removing decay heat from an MSR and also for spent molten fuel salt. The cooling system comprises a conduit structure that defines a sealed closed circuit through which a cooling gas or a fluid circulates through natural convection. In some embodiments, the cooling system is always functioning so that the cooling system continuously extracts heat from the nuclear core. The heat is further transferred directly to an environment such as the external environment via a thermal transfer for example mediated by a large roof structure above the reactor building.

[0011] KR20220106618 discloses a reactor cooling system for a nuclear reactor onboard a maritime vessel. A water reservoir is in fluid communication with the reactor vessel compartment. The reservoir may periodically be filled with seawater and provides a reservoir for cooling the reactor. The system is described as a passive cooling system of the floating nuclear power plant using the seawater and has the advantage of improving the safety of the floating nuclear power plant by removing the uncertainty due to the equipment failure of a pump as this is not required. The nuclear reactor is continuously cooled during operation. Summary of the invention

[0012] It was an object of the present invention to provide a cooling system for an MSR that enables a fully passive method of transferring decay heat from the molten fuel salt of the MSR.

[0013] A further object of the present invention was to provide a cooling system for an MSR to be situated on a maritime structure with a limited amount of space.

[0014] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0015] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0016] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0017] - one or more drain tanks in communication with the molten salt drain system;

[0018] - a buffer water tank comprising an inner wall and an outer wall and buffer water in the gap between the inner wall and the outer wall of the buffer water tank, said buffer water tank surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank. In accordance with another aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0019] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0020] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0021] - one or more drain tanks in communication with the molten salt drain system;

[0022] - a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within the conjoined inner and outer tube of the tube system, said tube system surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the tube system, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the tube system and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the tube system.

[0023] In accordance with another aspect of the invention, there is provided a method of transferring heat from a molten salt comprising the steps of

[0024] - providing a molten fuel salt in one or more drain tanks for molten fuel salt;

[0025] - surrounding the one or more drain tanks for molten fuel salt with a buffer water tank comprising buffer water in a gap between an inner wall and an outer wall of the buffer water tank, said molten fuel salt transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall to heat the buffer water into steam;

[0026] - providing a first piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a heat exchanger in the reservoir water in a reservoir water tank above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the heat exchanger in the reservoir water in the reservoir water tank to the buffer water tank and / or

[0027] - providing a second piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a gas heat exchanger in thermal contact with an environment comprising gas, said gas heat exchanger being above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the gas heat exchanger to the buffer water tank and / or

[0028] - providing a seawater heat exchanger in thermal contact with the buffer water, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank, wherein buffer water circulates through the seawater heat exchanger.

[0029] D E T A I L E D D E S C R I P T I O N

[0030] The reactor construction comprises an upper compartment above a drain tank compartment, said upper compartment comprising:

[0031] - a molten salt reactor (MSR), comprising a reactor vessel with a molten fuel salt;

[0032] - a molten fuel salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0033] - one or more drain tanks in communication with the molten fuel salt drain system; - a buffer water tank comprising an inner wall and an outer wall and buffer water in the gap between the inner wall and the outer wall of the buffer water tank, said buffer water tank surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank.

[0034] We find that such a reactor construction has numerous advantages.

[0035] Most of the conventional cooling systems for a nuclear reactor implies that the cooling is carried out for the reactor itself. For small modular reactors (SMRs) such as molten salt reactors (MSRs) the cooling system extensively rely on passive and inherent safety systems such as cooling directly for the reactor in a DRACS system or for the reactor vessel in a RVACS system. In such systems where a passive cooling takes place for example without pumps, the cooling is ongoing during operation of the reactor. There is therefore a constant cooling of the reactor in the above-mentioned systems leading to a parasitic heat loss instead of the heat produced by the reactor being used for e.g., steam generation.

[0036] The proposed reactor construction does not cool the reactor or the reactor compartment during normal operation and the parasitic heat loss is avoided even when it is a passive cooling system. Instead, the decay heat is removed from the molten fuel salt after the molten fuel salt has been drained into the drain tanks for example during an emergency event or during a planned maintenance operation.

[0037] The proposed reactor construction also addresses a specific accident event, the so-called "molten salt spill" scenario. This is the scenario where large amounts of molten fuel salts are not contained in the reactor or the drain tanks and can be caused accidentally during a planned maintenance operation or by extreme events such as an aircraft impact, projectile impacts, or an unusual hurricane. Most of the spilled molten salt will eventually flow downwards in any construction in such an event and come to rest on floors or similar. In the proposed reactor construction, the spilled-over molten fuel salt can be collected with a fuel salt catcher installed at the lowest position in the drain tank compartment in an event of a molten fuel salt leakage. This enables a cooling of the spilled-over molten fuel salt because the cooling system operates in the drain tank compartment which is the lower part of the reactor construction.

[0038] When cooling a reactor especially when directly cooling the reactor core with a cooling medium such as air or water, there is inevitably an irradiation of the cooling medium taking place leading to a procedure of dealing with large volumes of contaminated cooling medium. The proposed system reduces this disadvantage by simply not cooling the reactor but the molten fuel salt in the drain tanks where no nuclear fission process takes place. The decay processes in the molten fuel salt will however lead to irradiation and activation also of the surroundings but the radiation level will decline over time.

[0039] The provision of the buffer water around the drain tanks provides an effective heat sink for the decay heat and works instantly once the molten fuel salt starts filling up the drain tanks during a draining of the molten fuel salt from the reactor. The build-up of heat in the buffer water is further treated with at least one further heat removal system so that the decay heat is transferred to an ultimate heat sink of choice or two types of ultimate heat sinks working parallel. Both these transfers of heat to the ultimate heat sink are also passive heat transfers and therefore presents a strong safety case. The choice of buffer water (water) as the cooling medium reduces the volume of the cooling system compared to a primary choice of air as the cooling medium.

[0040] Molten salt reactors (MSRs) and molten salts

[0041] Molten salt reactors (MSRs) are based on obtaining criticality with a fissile material most often dissolved in a molten salt. When the MSR uses a fissile material dissolved in a molten salt, the salt is referred to as the fuel salt (or molten fuel salt).

[0042] The nuclear fission produces energetic neutrons typically at an energy range from 100 keV to 2 MeV. The probability of a fission event occurring depends on the neutron energy. In a so-called fast reactor, the unmoderated (fast) neutrons produced from the fission events interact directly with other nuclei. Thermal and epi-thermal nuclear fission reactors rely on moderators to first reduce the energy of the energetic neutrons of 100 keV to 2 MeV to thermal neutrons which are typically stated to be 0.025 eV which is the kinetic energy at ambient temperature. Such thermal neutrons have a higher probability of inducing a fission event for e.g., U-235 as a prominent example of a fissile material. To summarise, nuclear fission reactors can thus be operated by two d ifferent principles, namely fast reactors and thermal / epi-thermal reactors. In a fast reactor, the energetic neutrons interact directly with fissile material to produce energy, fission products and energetic neutrons. In thermal and epi-thermal reactors, the energetic neutrons produced by fission exchange energy with a moderator material such as graphite and eventually interact with fissile material to produce energy, fission products and more energetic neutrons.

[0043] When using a molten fuel salt, the fissile material is dissolved in the molten salt comprising carrier salts and the carrier salts are preferably fluoride- or chloridebased salts.

[0044] In one embodiment, the MSR comprises a molten salt being a molten fuel salt circulating in channels or interstices in a graphite core.

[0045] Another type of reactor also referred to as an MSR is a reactor where the fuel is a solid and a molten salt is used as a coolant salt for accommodating the temperature increase in the solid being a pebble fuel, most often a TRISO type fuel. The US8442182 describes such a reactor where a molten salt is used as coolant for a pebble fuel immersed in the coolant.

[0046] A coolant salt is a salt that contains no fissile material. In one embodiment, the MSR comprises a molten salt being a molten fuel salt circulating in and out of a reactor vessel, said vessel comprising pebbles of a solid moderator.

[0047] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0048] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0049] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0050] - one or more drain tanks in communication with the molten salt drain system;

[0051] - a buffer water tank executed as a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within the conjoined inner and outer tube of the tube system, said buffer water tank surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank.

[0052] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0053] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0054] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0055] - one or more drain tanks in communication with the molten salt drain system;

[0056] - a buffer water tank executed as a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within conjoined inner and outer tube of the tube system, said buffer water tank surrounding the one or more drain tanks, wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank.

[0057] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0058] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt; - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0059] - one or more drain tanks in communication with the molten salt drain system;

[0060] - a buffer water tank executed as a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within conjoined inner and outer tube of the tube system, said buffer water tank surrounding the one or more drain tanks, wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank within a rectangular or cylindrical recess, preferably a sea chest.

[0061] In one embodiment, the reactor construction comprises an upper compartment above a drain tank compartment, said upper compartment comprising:

[0062] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0063] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0064] - one or more drain tanks in communication with the molten salt drain system;

[0065] - a buffer water tank comprising an inner wall and an outer wall and buffer water in the gap between the inner wall and the outer wall of the buffer water tank, said buffer water tank surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank.

[0066] In one embodiment, the reactor construction comprises an upper compartment above a drain tank compartment, said upper compartment comprising:

[0067] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0068] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0069] - one or more drain tanks in communication with the molten salt drain system;

[0070] - a buffer water tank comprising an inner wall and an outer wall and buffer water in the gap between the inner wall and the outer wall of the buffer water tank, said buffer water tank surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank, and optionally a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank. In one embodiment, the reactor construction comprises an upper compartment above a drain tank compartment, said upper compartment comprising:

[0071] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0072] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0073] - one or more drain tanks in communication with the molten salt drain system;

[0074] - a buffer water tank comprising an inner wall and an outer wall and buffer water in the gap between the inner wall and the outer wall of the buffer water tank, said buffer water tank surrounding the one or more drain tanks, and a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the buffer water tank, said reservoir water being in thermal contact with an environment; and optionally a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the buffer water tank; and optionally a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank.

[0075] The separation of the upper compartment comprising the MSR from a drain tank compartment below the upper compartment enables a cooling of the molten salt such as a molten fuel salt to take place when the reactor is not in operation and thus to avoid parasitic heat loss during normal operation.

[0076] The reactor vessel has an inner surface made from a lining material. The reactor vessel may be made from any material, e.g., a metal, a metal alloy, a ceramic material or a combination thereof, and in the present context this material is referred to as the reactor vessel material. The inner surface may be a surface of the reactor vessel material so that the lining material is the reactor vessel material, or the reactor vessel material may be coated with a further material thus providing the lining material. For example, the reactor vessel material may be a metal alloy, e.g. a nickel based alloy, a nickel based superalloy or a Hastelloy, or nickel. In the present context, a nickel based alloy is an alloy having at least 50 %w / w nickel.

[0077] The molten salt drain system comprises a piping system connected to the reactor vessel and one or more opening means such as valves or salt plugs to enable a flow of molten salt when the opening means are open and thereby draining the molten salt from the reactor vessel to one or more drain tanks. In one embodiment, the one or more drain tanks is one or more molten fuel salt drain tanks. In one embodiment, the molten salt drain system is a molten fuel salt drain system connected to the reactor vessel and to one or more molten fuel salt drain tanks.

[0078] The piping and the valves may be made of the same material as the reactor vessel such as made of a metal alloy, e.g. a nickel based alloy. The molten fuel salt drain system comprises piping that are connected to the reactor vessel in the upper compartment and the piping is extending to the region of the drain tank compartment where it connects with the one or more drain tanks.

[0079] The opening means such as the one or more valves may be placed on the piping in the vicinity of the reactor vessel in the upper compartment or in the vicinity of the of one or more drain tanks.

[0080] In one embodiment, the buffer water tank holds buffer water in a tank that surrounds the one or more drain tanks partially, such as the along 95% of the outline around all the one or more drain tanks, such as along 80% of the outline around all the one or more drain tanks outline. In one embodiment, the buffer water tank is completely surrounding the one or more drain tanks.

[0081] In one embodiment, the water buffer tank has a substantially cylindrically inner and outer wall having a concentric relation or a substantially rectangular inner and outer wall having a concentric relation. In one embodiment, the minimum distance between the inner wall and the outer wall of the buffer water tank may be less than 3 meters, such as less than 2 meters, such as less than 1 meter.

[0082] The first piping structure is aimed at transporting steam arising from the buffer water in the buffer water tank to the heat exchanger inside the reservoir water tank and condensed water back from the heat exchanger inside the reservoir water tank to the buffer water tank. This circuit of evaporated and condensed water provides a mechanism for transferring heat away from the molten salt in the drain tanks that are heating the buffer water. In one embodiment, the first piping structure is defining a sealed circuit comprising a heat exchanger. A heat exchanger is present in the reservoir water tank to aid the cooling process and is preferably submerged partly or fully in the reservoir water.

[0083] In one embodiment, the reservoir water tank is an open tank. In one embodiment, the reservoir water tank is a closed tank.

[0084] The second piping structure is also aimed at transporting heat and steam arising from the buffer water in the buffer water tank away and sending condensed water back to the buffer water tank. In one embodiment, the second piping structure is defining a sealed circuit. The heat exchanger in thermal contact with the second piping structure exchanges heat to an environment such as the outside environment and thus air. The exchange may also be to a gas such as air in a closed volume where the heated gas in the closed volume transfers the heat to the outside environment being in thermal contact with the closed volume. This thermal contact could for example be between the casing of the closed volume and the outside environment. .

[0085] The use of seawater as the ultimate heat sink provides a diverse alternative for the ultimate heat sink for the case that the reactor construction is deployed at sea or at a barge or another maritime structure. The seawater heat exchanger is connected to a sea piping system comprising pipes in contact with the buffer water in the buffer water tank so that the buffer water can circulate from the buffer water tank to the seawater heat exchanger in the sea. In one embodiment, the sea piping system comprises a pipe through the outer wall of the buffer water tank, said pipe being connected to the seawater heat exchanger.

[0086] In one embodiment, the seawater heat exchanger is provided with an antifouling system.

[0087] In one embodiment, the first piping structure comprises: - at least one rise-pipe with a first end being an inlet for at least part of the buffer water and with a second end in contact with an inlet for a heat exchanger in the reservoir water tank; and

[0088] - at least one fall-pipe with a first end in contact with an outlet of the heat exchanger in the reservoir water tank and with a second end being an outlet for at least part of the buffer water.

[0089] The rise-pipe or the fall-pipe of the first piping system can have any cross-section shape such as circular, ellipse or rectangular and be manufactured of an alloy such as a stainless steel. The minimum cross-section size is the diameter in the case of a circular shape and the minor axis in the case of an ellipse shape.

[0090] In one embodiment, two sets of rise-pipes and fall-pipes are provided, preferably with the sets opposite each other along the outline around the one or more drain tanks.

[0091] In one embodiment, the first end of the rise-pipe and / or the second end of the fall-pipe is above the buffer water surface level,

[0092] The rise-pipe will collect or catch buffer water steam evaporated from the buffer water tank through the first end of the rise-pipe and the buffer water steam will by natural circulation rise upwards an onward through the rise-pipe. The buffer water steam may partly condense on the inner sides of the rise-pipe and in general, some of the buffer water in the rise-pipe may be in a liquid state and some may be in a gaseous state, thus buffer water steam. In the same way, some of the buffer water in the fall-pipe may be in a liquid state and some may be in a gaseous state, thus buffer water steam.

[0093] In one embodiment, the minimum cross section of the first end of rise-pipe is larger than the minimum cross section of the second end of the fall-pipe.

[0094] In one embodiment, the minimum cross section of the first end of the rise-pipe is larger than the minimum cross section of the second end of the fall-pipe and the minimum cross section of the first end of the rise-pipe is in the interval of from 1 cm to 20 cm, such as 1.5 cm to 10 cm and the minimum cross section of the second end of the fall-pipe is in the interval of from 0.5 to 15 cm, such as 1 cm to 8 cm. In one embodiment, a natural convection enhancer is comprised in the buffer water in the buffer water tank, said natural convection enhancer comprising a tank-wall in the gap between the inner and outer wall dividing the buffer water tank in an inner tank region and an outer tank region, the tank-wall extending above buffer water level into a dry wall section contacting the outer wall at a wall contact location, wherein

[0095] - the dry wall section and / or the inner wall comprises perforations to allow air circulation between the inner tank region and the outer tank region,

[0096] - the first end of the rise-pipe being allocated above the wall contact location and the second end of the fall-pipe being allocated below the wall contact location.

[0097] In one embodiment, the end of the fall-pipe being allocated below the wall contact location will have spray nozzles capable of condensing any steam in the gas volume above the buffer water surface.

[0098] The natural convection enhancer may be a wall structure placed in the buffer water, preferably concentrically around the inner wall of the buffer water tank. The wall structure is preferably made of an alloy, such as stainless steel. The dry wall section is preferably integral with the wall structure and may also be made of an alloy such as stainless steel.

[0099] In one embodiment, the tank-wall is a substantially vertical structure below the buffer water level and continuing above water into a dry wall section being a plate structure being at an angle to the substantially vertical structure, preferably said angle being between 30 to 120 degrees, such as between 60 to 100 degrees. The outer tank region has a gas volume above the buffer water surface confined by the surfaces of the dry wall section, the outer wall and the buffer water surface. The perforations in the dry wall section allows gas such as steam and air to circulate and prevent any pressure from building up on either side of the dry wall separating the inner tank region and the outer tank region. This prevents the case where a pressure builds up in the outer tank region that could thereby be hindering a free flow of water coming down through the fall-pipe into the outer tank region.

[0100] In one embodiment, there are no perforations in the tank wall or dry wall section between the inner and outer tank regions. It may be advantageous to maintain a certain high pressure to help the circulation of steam and condensed water. In one embodiment, a seawater heat exchanger is provided, said seawater heat exchanger having an inlet for buffer water through a perforation of the tank wall, and having an outlet for buffer water into the buffer water tank, wherein the outlet is situated lower than the inlet.

[0101] The inlet through the perforation of the tank wall has the effect that it is only the buffer water in the inner tank region that starts the circulation through the seawater heat exchanger. The buffer water in the inner tank region being closer to the drain tanks has a higher temperature and is advantageously cooled in the seawater heat exchanger whereby it enters the buffer water tank through the outlet into the outer tank region.

[0102] In one embodiment, the buffer tank inner wall is connected to an inner bottom plate so that inner wall and the inner bottom plate has a bowl shape.

[0103] The bowl is fully surrounding the one or more drain tanks and buffer water is fully surrounding the bowl. The bowl may have a substantial rotational symmetry around an axis extending from the upper compartment to the drain tank compartment, for example a substantially circular bowl shape surrounding the drain tank compartment with a circularly shaped periphery. The bowl may also have an open box shape where the open box surrounds the drain tank compartment with a rectangular shaped periphery.

[0104] The buffer water is outside the inner wall and inside the outer wall, thus between the inner and outer wall. The buffer water is between the inner bottom plate and a tank bottom plate of the buffer water tank.

[0105] In the event of a severe accident condition conventionally termed as a fuel salt spill incident, the bowl functions as a "fuel salt catcher". This incident may be caused by minor or extensive breaches of vessels, tanks or piping containing the fuel salt. The inner bottom plate functions as a thermal barrier to protect containment integrity while facilitating the heat transfer path to the buffer water in the buffer water tank at the same time. Preferably, the inner bottom plate is constructed with a larger thickness than the inner walls, such as a 3 times larger thickness, such as a 2 times larger thickness. This means that the 'fuel salt catcher' has two main safety functions, namely both to protect the integrity of the containment under severe accident conditions and to ensure decay heat removal capability under severe accident conditions. In one embodiment, the inner bottom plate is a multiply structure such as a double-layer structure with the layer facing the drain tanks having a larger thickness than the inner walls, such as 2 times larger thickness, such as 1.5 times larger thickness.

[0106] In one embodiment, the inner wall of the buffer water tank is a double-wall structure, wherein the double-wall structure is closed, and a gas is comprised between the two walls of the double-wall structure, or the double-wall structure is open, and the air between the two walls is in communication with the surrounding air.

[0107] The provision of a double-wall structure for the inner wall of the buffer tank enables an extra containment barrier in case there is a breach of the inner wall that would otherwise allow water to enter the drain tank compartment with possible damage to the drain tanks containing the molten fuel salt. When the double-wall structure is closed, and a gas is comprised between the two walls then a gas leak detection system can be used to measure the pressure between the two walls to alert of any sudden pressure variations indicating a wall breach. The double-wall structure may also provide some thermal stress relieve such as thermal stress relieve if contacted with molten salt during the event of a fuel salt spill accident.

[0108] In one embodiment, the molten salt drain system is a molten fuel salt drain system and comprises a salt piping system comprising at least one salt plug.

[0109] The salt plug may be a freeze valve (a "freeze plug") which is plug of salt in the salt piping system that is cooled and thereby remains a solid during cooling. When the cooling ceases, the salt melts due to the high temperature of the molten fuel salt in contact with the plug in the salt piping system. The salt piping system is part of the drain system, and the molten fuel salt will therefore drain when the salt plug has melted.

[0110] The distinctive safety characteristic of this kind of design is that the actuation in case of loss of power supply is completely passive (without the need for any operator action or electricity supply). In this kind of event, the active cooling of the freeze valve is lost, and the freeze plug will melt allowing for passive salt draining. The salt piping system is connected to other piping, such as pipi ng for a carrier gas. Parts of the piping system and the salt piping system may contain a molten fuel salt or a gas such as a carrier gas depending on the mode of operation, for example if there a draining mode or a normal power producing mode.

[0111] For other type of events, without loss of power supply, the active cooling has to be shut off by the Reactor Protection System, in a more conventional way and using safety classified instrumentation and signals. In any case, the actuation and the system can be made totally passive on demand.

[0112] In one embodiment, the reactor construction further comprises a gas supply system comprising

[0113] - a carrier gas reservoir;

[0114] - a carrier gas piping comprising at least one valve, said carrier gas piping connecting from the carrier gas reservoir to the one or more drain tanks.

[0115] In one embodiment, the gas supply system further comprises an off-gas system comprising

[0116] - an off-gas storage tank

[0117] - a carrier gas piping comprising at least one valve, said carrier gas piping connecting a chemistry control system and the reactor vessel.

[0118] In most instances during normal power operation where the reactor is producing heat from the nuclear fission process, the gas supply system supplies gas to the reactor.

[0119] The supplied gas can function as a carrier gas that continuously carries the gaseous fission products away from the reactor to an off-gas system for further processing of the gaseous fission products. The supplied gas can also alone or in addition supply gaseous reactants such as H2to the molten fuel salt to adjust for example the redox potential of the molten fuel salt. This has the purpose of lowering the corrosiveness of the molten fuel salt. The valve is open during the above-mentioned normal operation. If the molten fuel salt has been drained for example during a maintenance operation and the molten fuel salt needs to be reloaded back into the reactor, then the gas supply system can be used to press the molten fuel salt up from the one or more drain tanks via a piping system into the reactor.

[0120] This provides a procedure of transferring the molten fuel salt without the use of molten fuel salt pumps and instead using the existing gas supply system for the operation of reloading the molten fuel salt.

[0121] In one embodiment, the molten fuel salt is having a composition selected from the group consisting of compositions comprising: sodium fluoride + potassium fluoride + uranium fluoride; lithium fluoride + thorium fluoride + plutonium fluoride; lithium fluoride + thorium fluoride + uranium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride + zirconium fluoride; sodium fluoride + rubidium fluoride + uranium fluoride; sodium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride + zirconium fluoride; potassium chloride + plutonium chloride + uranium chloride; sodium chloride + plutonium chloride; sodium chloride + plutonium chloride + uranium chloride.

[0122] When using a molten fuel salt, the fissile material is dissolved in the molten salt comprising carrier salts and the carrier salts are preferably fluoride- or chloridebased salts. Fluoride salts comprises F-19 which is the sole naturally occurring isotope for fluorine and has a low neutron capture probability. Fluoride salts are therefore especially suited for thermal MSRs with a tight neutron economy. The fissile material may be a fluoride salt or a chloride salt comprising a fissile isotope such as U-235, U-233 or Pu-239.

[0123] In one embodiment, the molten fuel salt is a fluoride fuel salt, or a chloride fuel salt based on one or more fluoride compounds or chloride compounds, respectively forming a fuel salt composition. The fuel salt comprising U-235 may be of various enrichment levels such as SEU (<2% U-235), LEU (typically 3-5% U-235), HALEU (5-20% U-235) or even in a grade of naturally occurring uranium.

[0124] In one embodiment, the MSR further comprises a moderator based on a material selected from the group of a graphite material, a Be compound containing material, a molten salt of a metal hydroxide wherein when the moderator is molten salt of a metal hydroxide, the reactor construction further comprises a molten salt of a metal hydroxide drain system including molten salt of a metal hydroxide drain tanks.

[0125] In one embodiment, the MSR comprises a molten salt of a metal hydroxide or a metal deuteroxide.

[0126] In one embodiment, the molten salt reactor (MSR), comprising a reactor vessel comprises a molten fuel salt and a moderator being a molten salt of a metal hydroxide or a metal deuteroxide. Such reactors are known from EP3639279B1 where the molten fuel salt is present in an inner tubing in the reactor vessel and the molten salt of a metal hydroxide or a metal deuteroxide is in the reactor vessel surrounding the inner tubing.

[0127] When a molten moderator salt is used such as a molten salt of a metal hydroxide or a metal deuteroxide then the upper compartment further comprises a molten moderator salt drain system connected to the reactor vessel and to one or more molten moderator salt drain tanks.

[0128] Other moderators used may be water, deuterated water (D2O), Be-compounds or hydrides such as zirconium hydrides.

[0129] In one embodiment, the reactor construction comprises a compartment separator comprising: a grid structure forming a substantially horizontal surface separation between the upper compartment and the drain tank compartment, said grid structure comprising a metal grid and a thermally insulating layer and one or more funnels penetrating the grid structure, each funnels comprising a plug made of a sacrificial material. The upper compartment comprising the reactor and the drain tank compartment comprising drain tanks for the molten salt may in general be situated in the same volume of space in the reactor construction. The upper compartment and the drain tank compartment may be separated by a physical barrier. Such a compartment separator has the advantage of maintaining a relatively low temperature in the upper compartment where reactor instrumentation needs protection from high temperatures from the drain tank compartment where the drain tanks may be pre-heated at a constant high temperature. The wish for preheating of the drain tanks is because a quick draining of the molten salt in an emergency event could cause a thermal shock of the drain tank material with the risk of rupture and causing a salt spill incident if the drain tanks are not at a sufficiently high temperature. Furthermore, the compartment separator should preferably provide a walkable surface for MTSI (Maintenance, Testing, Surveillance, and Inspection) in the upper compartment.

[0130] A salt spill accident in the upper compartment for example in relation to the reactor piping must however also be mitigated by allowing the spilled salt to reach the fuel salt catcher installed at the lowest position in the drain tank compartment. The compartment separator solves this problem by the provision of the grid structure comprising funnels. The metal grid provides a load bearing characteristic in the event that the compartment separator should provide a walkable surface.

[0131] In one embodiment, a solid metal sheet is applied to the grid structure to enhance the construction strength of the compartment separator, said solid metal sheet, the thermally insulating layer and the metal grid forming a sandwich structure.

[0132] In one embodiment, the plug comprises a disc, preferably having one or more insulation discs below the disc, said one or more insulation discs facing the drain tank compartment.

[0133] The disc is made of a sacrificial material in the sense that the material decomposes for example by melting upon heat influence at a temperature below the prevailing temperature of the molten salt. The disc material may be a low melting metal alloy or a polymer such as carbon and fluoride containing polymer. The polymer may be made of a polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) or ethylene tetrafluoroethylene (EFTE).

[0134] The insulation disc may comprise an insulation material selected from the list of glass wool, aerogel, polymer foam.

[0135] The thermally insulating layer may comprise an insulation material selected from the list of glass wool, stone wool, ceramic wool, aerogel, silicate composites.

[0136] In one embodiment, the number of funnels penetrating the grid structure is at least 1 funnel per 5 m2, such as 2 funnels per 5 m2, 3 funnels per 5 m2of grid structure surface area. The provision of several funnels in the compartment separator is a construction for draining the molten salt that is lower in the vertical direction compared to one or a few large funnels for collecting and draining the molten salt. The reason is that the size of the funnel inclination needed for the molten salt to flow freely and at a sufficient speed would require a high construction if a few funnels were used to cover a major part of the compartment separator surface area.

[0137] In one embodiment, the reactor construction is located on a marine structure, preferably a barge.

[0138] The marine structure may be a barge that comprises a power generation structure comprising one or more MSRs and steam turbines that can generate electricity.

[0139] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0140] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0141] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising: - one or more drain tanks in communication with the molten salt drain system;

[0142] - a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within the conjoined inner and outer tube of the tube system, said tube system surrounding the one or more drain tanks, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the tube system, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the tube system and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the tube system.

[0143] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0144] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0145] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0146] - one or more drain tanks in communication with the molten salt drain system;

[0147] - a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within the conjoined inner and outer tube of the tube system, said tube system surrounding the one or more drain tanks, wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein a first piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with a reservoir water tank, said reservoir water tank being at a level above the tube system, said reservoir water being in thermal contact with an environment and / or a second piping structure is defining a circuit for at least part of the buffer water and comprising a heat exchanger in thermal contact with an environment, said heat exchanger being at a level above the tube system and / or a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the tube system.

[0148] In accordance with an aspect of the invention, there is provided a reactor construction comprising an upper compartment above a drain tank compartment, said upper compartment comprising:

[0149] - a molten salt reactor (MSR), comprising a reactor vessel comprising a molten fuel salt;

[0150] - a molten salt drain system connected to the reactor vessel; said drain tank compartment comprising:

[0151] - one or more drain tanks in communication with the molten salt drain system;

[0152] - a tube system comprising an inner tube and an outer tube, wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within conjoined inner and outer tube of the tube system, said tube system surrounding the one or more drain tanks, wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein a seawater heat exchanger in thermal contact with the buffer water and seawater, said seawater heat exchanger being located below seawater level outside the outer wall of the tube system within a rectangular or cylindrical recess, preferably a sea chest.

[0153] In accordance with an aspect of the invention, there is provided a method of transferring heat from a molten salt comprises the steps of

[0154] - providing a molten fuel salt in one or more drain tanks for molten fuel salt;

[0155] - surrounding the one or more drain tanks for molten fuel salt with a buffer water tank comprising buffer water in a gap between an inner wall and an outer wall of the buffer water tank, said molten fuel salt transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall to heat the buffer water into steam;

[0156] - providing a first piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a heat exchanger in the reservoir water in a reservoir water tank above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the heat exchanger in the reservoir water in the reservoir water tank to the buffer water tank and / or

[0157] - providing a second piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a gas heat exchanger in thermal contact with an environment comprising gas, said gas heat exchanger being above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the gas heat exchanger to the buffer water tank and / or

[0158] - providing a seawater heat exchanger in thermal contact with the buffer water, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank, wherein buffer water circulates through the seawater heat exchanger. In one embodiment, a method of transferring heat from a molten salt comprises the steps of

[0159] - providing a molten fuel salt in one or more drain tanks for molten fuel salt;

[0160] - surrounding the one or more drain tanks for molten fuel salt with a buffer water tank comprising buffer water in a gap between an inner wall and an outer wall of the buffer water tank, said molten fuel salt transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall to heat the buffer water into steam;

[0161] - providing a first piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a heat exchanger in the reservoir water in a reservoir water tank above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the heat exchanger in the reservoir water in the reservoir water tank to the buffer water tank and / or

[0162] - providing a second piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a gas heat exchanger in thermal contact with an environment comprising gas, said gas heat exchanger being above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the gas heat exchanger to the buffer water tank and / or

[0163] - providing a seawater heat exchanger in thermal contact with the buffer water, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank, wherein buffer water circulates through the seawater heat exchanger.

[0164] In one embodiment, a method of transferring heat from a molten salt comprises the steps of

[0165] - providing a molten fuel salt in one or more drain tanks for molten fuel salt; - surrounding the one or more drain tanks for molten fuel salt with a buffer water tank comprising buffer water in a gap between an inner wall and an outer wall of the buffer water tank, said molten fuel salt transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall to heat the buffer water into steam;

[0166] - providing a first piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a heat exchanger in the reservoir water in a reservoir water tank above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the heat exchanger in the reservoir water in the reservoir water tank to the buffer water tank and / or

[0167] - providing a second piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a gas heat exchanger in thermal contact with an environment comprising gas, said gas heat exchanger being above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the gas heat exchanger to the buffer water tank and, optionally

[0168] - providing a seawater heat exchanger in thermal contact with the buffer water, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank, wherein buffer water circulates through the seawater heat exchanger.

[0169] The method of transferring heat from a molten salt is applicable to the molten salt irrespective of which kind of container the molten salt is contained in.

[0170] In one embodiment, the molten salt is a molten fuel salt.

[0171] The method may transfer heat arising from the decay of fission products in a molten fuel salt. The heat may be transferred through at least one of heat radiation, heat conduction or heat convection from the drain tanks for molten fuel salt to the inner wall to heat the buffer water to buffer water steam. The rise-pipe will collect or catch buffer water steam evaporated from the buffer water tank through the first end of the rise-pipe and the buffer water steam will by natural circulation rise upwards an onward through the rise-pipe. The buffer water steam may partly condense on the inner sides of the rise-pipe and in general, some of the buffer water in the rise-pipe may be in a liquid state and some may be in a gaseous state, thus buffer water steam. In one embodiment, the rise-pipe is at least partly insulated, such as with a pipe-section insulation. This may alleviate any problems with condensation in the rise-pipe. Eventually, the condensed water from the inner sides of the rise-pipe will re-evaporate and the formed buffer water steam will rise upwards. In the same way, some of the buffer water in the fall-pipe may be in a liquid state and some may be in a gaseous state, thus buffer water steam. Eventually, the buffer water steam in the fall-pipe will condense and be led downwards in the fall-pipe.

[0172] The method is well suited for transferring heat from a molten fuel salt contained in one or more drain tanks as discussed above.

[0173] In one embodiment, a method of transferring heat from a molten salt comprises the steps of

[0174] - providing a molten fuel salt in a reactor vessel;

[0175] - surrounding the reactor vessel with a buffer water tank comprising buffer water in a gap between an inner wall and an outer wall of the buffer water tank, said molten fuel salt transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall to heat the buffer water into steam;

[0176] - providing a first piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a heat exchanger in the reservoir water in a reservoir water tank above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the heat exchanger in the reservoir water in the reservoir water tank to the buffer water tank and / or

[0177] - providing a second piping structure comprising at least one rise-pipe above at least part of the buffer water, said rise-pipe collecting steam and directing the steam to a gas heat exchanger in thermal contact with an environment comprising gas, said gas heat exchanger being above the buffer water tank to condense the steam into water; at least one fall-pipe to direct water from the gas heat exchanger to the buffer water tank and, optionally

[0178] - providing a seawater heat exchanger in thermal contact with the buffer water, said seawater heat exchanger being located below seawater level outside the outer wall of the buffer water tank, wherein buffer water circulates through the seawater heat exchanger.

[0179] The method of transferring heat from a molten fuel salt may alternatively be used for an MSR comprising a reactor vessel comprising molten fuel salt in an operational state producing heat from a fission process or an operational state being an anticipated operational occurrence. The method of transferring heat from a molten fuel salt may also be for a temporarily or permanently shut-down MSR. The method of transferring heat from a molten fuel salt may also be for an accident condition.

[0180] When the MSR is operational then there is a trade-off that a certain parasitic heat loss is allowed in exchange for a passive heat removal system that enables an improved safety case over an active heat removal system.

[0181] In one embodiment, the method of transferring heat from a molten salt furthermore comprises the steps of transferring heat from a molten salt of a metal hydroxide or a metal deuteroxide used as a moderator, wherein the reactor compartment further comprises a molten moderator salt drain system connected to the reactor vessel and to one or more molten moderator salt drain tanks.

[0182] It may be an advantage to use a seawater heat exchanger for transferring heat to the ultimate sink of the seawater when the one or more drain tanks for molten fuel salt is located in proximity to the sea. This is especially advantageous when the one or more drain tanks for molten fuel salt is localised below sea level. Other ultimate heat sinks based on water may be a lake, a fjord, a river or a pond and a heat exchanger immersed into the water may equivalently work as a seawater heat exchanger. Such a heat exchanger being located below water level outside the outer wall of the buffer water tank will transfer heat from the buffer water.

[0183] In one embodiment, there is a transfer of heat from the reservoir water tank to an environment, such as an outside environment in thermal contact with the reservoir water tank.

[0184] In one embodiment, the transfer of heat from the reservoir water tank is at least partly through the evaporation of the reservoir water to the outside environment.

[0185] The heat exchanger in the reservoir water tank transfers the heat from the buffer water to the reservoir water in the reservoir water tank. The heat is further transferred to an environment such as the outside environment and thus air. The reservoir water tank may be a closed volume where the heated reservoir water in the closed volume transfers the heat to the outside environment being in thermal contact with the closed volume. This thermal contact could for example be between the casing of the closed volume and the outside environment.

[0186] The reservoir water tank may also be an at least partly open tank where the heated reservoir water transfers the heat to the outside environment by evaporation of the reservoir water. The evaporation of reservoir water provides a very efficient way of transferring heat. The reservoir water is in thermal contact but not direct contact with the buffer water in the heat exchanger and although the buffer water may be activated by radiation from the molten salt in the drain tanks, the reservoir water is not activated and can therefore evaporate into the outside environment without being a safety issue.

[0187] In one embodiment, the heat transfer is a passive heat transfer method.

[0188] The method of transferring heat from a molten salt may be carried out without pumps or other active mechanisms that may also require human intervention to activate. The transfer of heat with steam from heated buffer water relies on the transfer of the steam carrying the heat evaporating naturally upwards through a permanently installed rise-pipe to the heat exchanger in thermal contact with the colder reservoir water where the steam will condense. The condensed water will thereafter be transferred naturally downwards due to gravity through the permanently installed fall-pipe back into the water buffer tank. Water molecules, being lighter than the surrounding air molecules N2and O2will acquire a higher speed at a given temperature and preferentially flow faster than air molecules upwards in the rise-pipe.

[0189] In one embodiment, there is provided a natural convection enhancer comprising a tank-wall in the gap between the inner and outer wall dividing the buffer water tank in an inner tank region and an outer tank region

[0190] - causing the buffer water in the inner tank region to boil off, and

[0191] - providing an upwards flow path for steam.

[0192] The tank-wall of the natural convection enhancer separates the water close to the heated inner wall from the remaining water in the buffer water tank causing the water close to the inner wall will heat to boil locally. The natural convection enhancer also acts as a thermal barrier to the water closer to the outer wall. The difference in temperature and density encourages buoyantly driven flow. Furthermore, the natural convection enhancer provides a flow path upwards for the steam.

[0193] In one embodiment, the tank-wall extends above buffer water level into a dry wall section contacting the outer wall at a wall contact location, wherein the dry wall section and / or the inner wall comprises perforations to allow air circulation between the inner tank region and the outer tank region, and wherein the first end of the rise-pipe is being allocated above the wall contact location and the second end of the fall-pipe being allocated below the wall contact location.

[0194] The outer tank region has a gas volume above the buffer water surface confined by the surfaces of the dry wall section, the outer wall and the buffer water surface. The perforations in the dry wall section allows gas such as steam and air to circulate and prevent any pressure from building up on either side of the dry wall separating the inner tank region and the outer tank region. This prevents the case where a pressure builds up in the outer tank region that could thereby be hindering a free flow of water coming down through the fall-pipe into the outer tank region. In one embodiment, the buffer water in the inner tank region is having a higher temperature that buffer water in the outer tank region enters an inlet to the seawater heat exchanger through a perforation of the tank wall, where after the buffer water is cooled in the seawater heat exchanger and enters an outlet for buffer water into the buffer water tank, wherein the outlet is situated lower than the inlet.

[0195] In one embodiment, there are no perforations in the dry wall section and the tank wall. In this embodiment, the inner gas space pressure will be increased by steam generation and the steam and condensate water circulation can be enhanced.

[0196] Brief description of the drawings

[0197] In the following the invention will be explained in greater detail with the aid of examples and with reference to the schematic drawings, in which:

[0198] Fig. 1 shows an embodiment of the reactor construction where an MSR is in an operational state.

[0199] Fig. 2 shows an embodiment of the reactor construction where an MSR is in an accident state.

[0200] Fig. 3 shows a reactor construction installed onboard a barge.

[0201] Fig. 4 shows a reactor construction wherein the buffer water tank is executed as a tube system.

[0202] Fig. 5 shows a reactor construction wherein the buffer water tank is executed as a tube system, wherein the tube system comprises a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses.

[0203] Fig. 6 shows a reactor construction wherein the buffer water tank is executed as a tube system, wherein the tube system comprises a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein the buffer water and sea water are in thermal contact below seawater level . Reference signs list

[0204] 1 A reactor construction

[0205] 2 An upper compartment

[0206] 3 A drain tank compartment

[0207] 4 A molten salt reactor (MSR)

[0208] 5 A reactor vessel

[0209] 6 A molten fuel salt

[0210] 7 A fuel salt pump

[0211] 8 A primary heat exchanger

[0212] 9 A drain system

[0213] 10 A drain tank

[0214] 11, 110, 111, 112 A valve

[0215] 12 A carrier gas piping

[0216] 13 A salt plug

[0217] 14 A salt cooling pump

[0218] 15 A buffer water tank

[0219] 16 An inner wall

[0220] 17 An outer wall

[0221] 18 An inner bottom plate

[0222] 19 A tank bottom plate

[0223] 20 A fuel salt catcher

[0224] 21 A buffer water

[0225] 22 A first piping structure

[0226] 23 A heat exchanger in thermal contact with reservoir water

[0227] 24 A second piping structure

[0228] 25 A gas heat exchanger in thermal contact with an environment

[0229] 26 A seawater heat exchanger

[0230] 27 A rise-pipe

[0231] 28 A first end of a rise-pipe

[0232] 29 A second end of a rise-pipe

[0233] 30 A reservoir water tank

[0234] 31 A fall-pipe

[0235] 32 A first end of a fall-pipe 33 A second end of a fall-pipe

[0236] 34 A tank wall

[0237] 35 An inner tank region

[0238] 36 An outer tank region

[0239] 37 A dry wall section

[0240] 38 A wall contact location

[0241] 39 A dry wall section perforation

[0242] 40 A barge

[0243] 41 A double walled enclosure

[0244] 42 An upper deck

[0245] 43 An outlet for reservoir water steam

[0246] 44 A heat exchanger structure

[0247] 45 An opening for hot air

[0248] 46 A first wall

[0249] 47 A second wall

[0250] 48 An opening for hot seawater

[0251] 49 A Reservoir water tank

[0252] 50 A Condenser

[0253] 51 A Cold wall outer tube

[0254] 52 A Hot wall inner tube

[0255] 53 A Backup tank

[0256] 54 A Sea water level

[0257] 55 Condensation part of a tube system

[0258] 56 Adiabatic part of a tube system

[0259] 57 Evaporation part of a tube system

[0260] 58 A sea chest

[0261] The invention is not limited to the embodiment / s illustrated in the drawings. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims. The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include the term "comprising", other features besides the features prefaced by this term in each statement can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in a similar manner.

[0262] Detailed Description

[0263] The present invention will now be illustrated with reference to the accompanying drawings and in the following non-limiting examples.

[0264] A reactor construction 1 of the invention is shown in Figure 1 having an upper compartment 2 above a drain tank compartment 3. Figure 1 shows an embodiment of the reactor construction 1 where an MSR 4 is in an operational state.

[0265] The entire reactor construction 1 including the upper compartment 2 and the drain tank compartment 3 is surrounded by a wall construction. The molten salt reactor (MSR) 4 is situated in the upper compartment 2 and is shown as having a reactor vessel 5 supported by a reactor floor (not shown). The fission process taking place in the molten fuel salt 6 in the reactor core is producing heat in the core and the resulting heated fuel salt is circulated by means of a fuel salt pump 7 out of the core to a primary heat exchanger 8. The primary heat exchanger 8 may be connected directly or via other (not shown) heat exchangers to a steam generator (not shown) for producing electricity. The gaseous fission products collect and are treated with an off-gas system to separate and store the gaseous fission products. A gas supply system is capable of providing a carrier gas such as He to the off-gas system for carrying the gaseous fission products away from the reactor core. The gas supply system is also capable of supplying gas to a chemistry control unit where reactants are supplied to the molten salt 6 to maintain a targeted chemical composition of the molten salt. The gas supply system comprises gas piping 12 to lead a carrier gas to the various installations described here above but also has a piping system to the drain tanks in the drain tank compartment. The piping system to the drain tanks can be opened or closed with a valve 11, 110 on the carrier gas piping system.

[0266] The reactor 4 is shown in an operational mode where the reactor vessel 5 contains molten fuel salt 6, and the drain tanks 10 are empty. The salt piping system is shown with a salt plug 13 that is actively cooled with a salt cooling system 14. The part of the salt piping system below the salt plug 13 towards the drain tanks 10 situated lower than the upper compartment 2 does not contain fuel salt 6 and is shown as void of molten salt 6 but containing a He gas. The drain tank compartment 3 shows the drain tanks 10 to be entirely surrounded with a rectangular buffer water tank 15 shown in cross-section. The buffer water tank 15 contains buffer water 21 between an inner wall 16 and an outer wall 17, both walls 16, 17 seen in cross-section. The buffer water tank 15 also surrounds the drain tanks 10 from beneath with buffer water 21 where an inner bottom plate 18 is facing the drain tanks 10 on one side and the buffer water 21 on the other side of the inner bottom plate 18. The inner bottom plate 18 is shown as a two-layer plate having a shallow bowl-shape and where the plate facing the drain tanks 10 of the two-layer plate can function as fuel salt catcher. The tank bottom plate 19 is shown at the lowest location of the buffer water tank 15. A natural convection enhancer comprising a tank wall 34 is shown in cross-section. The tank wall 34 is supported at its lower part by a tank wall support (not shown) allowing circulation and convection of buffer water 21 under the lowest edge of the tank wall 34. The tank wall 34 divides the buffer water tank 15 into an inner tank region 35 facing the drain tanks 10 and an outer tank region 36. A dry wall section 37 connects the tank wall 34 with the outer wall 17 where the dry wall section 37 is a rectangular ring-shaped plate with a ring width corresponding to approximately the width of the outer tank region 36. The assembly of the outer wall 17 together with the rectangular ring-shaped plate connected thereto forms a closed barrier between the inner 35 and outer tank region 36 except for the above-mentioned gap under the lowest edge of the tank wall 34 and for perforations 39 in the dry wall section 37 and / or the tank wall 34 of the natural convection enhancer. The perforations 39 are shown in the tank wall 34 above the water level of the buffer water 21 in the buffer water tank 15.

[0267] The first piping structure 22 is shown and is described in the following. A risepipe 27 is shown with its first end 28 of the pipe inside the inner tank region 35 above the ring-shaped dry wall section 37. The rise-pipe first end 28 is above the water level of the buffer water 21 and the steam (not shown) in the inner tank region 35 can enter the first end 28 of the rise-pipe thus being an inlet for the buffer water steam. During normal operation there will only be a very limited evaporation of buffer water 21. The rise-pipe 27 extends upwards from its first end 28 and the rise-pipe 27 is shown as entering a reservoir water tank 30 around the height level of the MSR 4 above the drain tank compartment 3. The second end 29 of the rise-pipe is connected to a heat exchanger 23 surrounded by and in thermal contact with reservoir water in the reservoir water tank 30. Any buffer water steam condenses at least partly in the heat exchanger 23 and the buffer water 21 leaves the heat exchanger 23 at the outlet end of the heat exchanger 23 into a first end 32 of a fall-pipe. As mentioned above, the amount of evaporated and condensed buffer water 21 will be very limited during normal operation. The fall-pipe 31 extends downwards from its first end 32 and enters the outer tank region 36 of the buffer water tank 15 where buffer water 21 can be led through the second end 33 of the fall-pipe into the buffer water 21 in the outer tank region 36. The second end 33 of the fall-pipe is below the ring-shaped dry wall section 37 and above the buffer water level.

[0268] The second piping structure 24 is shown and is described in the following.

[0269] A rise-pipe 27 is shown with its first end 28 of the pipe inside the inner tank region 35 above the rectangular ring-shaped dry wall section 37. The rise-pipe first end 28 is above the water level of the buffer water 21 and any steam (not shown) in the inner tank region 35 enters the rise-pipe first end 27 thus being an inlet for the buffer water steam. As stated above for the first piping: During normal operation there will only be a very limited evaporation of the buffer water 21. The rise-pipe 27 extends upwards from its first end 28 and the rise-pipe 27 is shown as connected to a gas heat exchanger 25 around the height level of the MSR 4 above the drain tank compartment 3. The second end 29 of the rise-pipe is connected to the gas heat exchanger 25 surrounded by and in thermal contact with the outside environment. Any buffer water steam will condense at least partly in the heat exchanger 25 and the buffer water 21 leaves the gas heat exchanger 25 at the outlet end of the gas heat exchanger 25 into a first end 32 of a fall-pipe. The fall-pipe 31 extends downwards from its first end 32 and enters the outer tank region 36 of the buffer water tank 15 where buffer water 21 is led through the second end 33 of the fall-pipe into the buffer water 21 in the outer tank region 36. The second end 33 of the fall-pipe is below the ringshaped dry wall section 37 and above the buffer water level.

[0270] A sea water heat exchanger 26 is shown outside the buffer water tank 15 and immersed into the seawater below the seawater level. The sea water heat exchanger 26 has an inlet and an outlet for buffer water 21 where the inlet is situated higher than the outlet. The inlet for buffer water 21 to the seawater heat exchanger 26 ends with its open end in the inner tank region 35 where the buffer water 21 has a higher temperature than in the outer tank region 36. During normal operation the temperature difference between the buffer water 21 in the inner tank region 35 and the outer tank region 36 will be very limited. The buffer water 21 entering the inlet and be cooled by the seawater having a lower temperature than the buffer water 21 in the inner tank region 35 whereupon the buffer water 21 is circulated back into the lower part of the buffer water tank 15. This circulation of buffer water 21 through the seawater heat exchanger 26 takes place as a natural circulation from a hot region to a cold region and is a passive circulation without the need of actively pumping the water through the seawater heat exchanger 26.

[0271] Figure 2 shows an embodiment of the reactor construction 1 where an MSR 4 is in an accident state.

[0272] The entire reactor construction 1 including the upper compartment 2 and the drain tank compartment 3 is shown as in figure 1. The molten fuel salt 6 has been drained from the reactor core and no fission process is taking place in the molten fuel salt 6 in the reactor core. The draining was initiated because of an external loss of power. This caused the cooling system 14 to shut off the active cooling of the salt plug 13 which again caused the salt plug 13 to melt and thereby enabling a free passage of the molten fuel salt 6 from the MSR 4 through the piping system to the drain tanks 10. The drain tanks 10 situated lower than the upper compartment 2 is shown as containing fuel salt 6 giving off heat from the decay of the fission products. The drain tank compartment 3 shows the drain tanks 10 to be entirely surrounded with a rectangular buffer water tank 15 as described in figure 1. The tank wall 34 divides the buffer water tank 15 into an inner tank region 35 facing the drain tanks 10 and an outer tank region 36 and the buffer water 21 in the inner tank region 35 acquires heat because it is in contact with the inner wall 16 facing the drain tanks 10. A dry wall section 37 connects the tank wall 34 with the outer wall 17 as described in figure 1.

[0273] The first piping structure 22 is as described in figure 1. The rise-pipe first end 28 is above the water level of the buffer water 21 and the steam (not shown) in the inner tank region 35 enters the first end 28 of the rise-pipe thus being an inlet for the buffer water steam. There may be an extensive evaporation of buffer water 21 especially the buffer water 21 in the inner tank region 35 immediately after the draining of the molten fuel salt 6. The steam pressure may be controlled to be below a certain value by the steam escaping through perforations 39 in the tank wall 34. The rise-pipe 27 extends upwards from its first end 28 and the risepipe 27 is shown including evaporating steam as entering a reservoir water tank 30 around the height level of the MSR 4 above the drain tank compartment 3. The second end 29 of the rise-pipe is connected to a heat exchanger 23 surrounded by and in thermal contact with reservoir water in the reservoir water tank 30. The buffer water steam condenses at least partly in the heat exchanger 23 and the buffer water 21 leaves the heat exchanger 23 at the outlet end of the heat exchanger 23 into a first end 32 of a fall-pipe. The fall-pipe 31 extends downwards from its first end 32 and enters the outer tank region 36 of the buffer water tank 15 where the buffer water 21 is led through the second end 33 of the fall-pipe into the buffer water 21 in the outer tank region 36. The second end 33 of the fall-pipe is below the ring-shaped dry wall section 37 and above the buffer water level. The above circulation of buffer steam and buffer water 21 in the first piping structure 22 is supplemented by a circulation by natural convection of buffer water 21 under the tank wall 34 from the outer tank region 36 to the inner tank region 35.

[0274] The second piping structure 24 is as described in figure 1. The rise-pipe first end

[0275] 28 is above the water level of the buffer water 21 and steam (not shown) in the inner tank region 35 enters the rise-pipe first end 28 thus being an inlet for the buffer water steam. The rise-pipe 27 extends upwards from its first end 28 and the rise-pipe 27 is shown as connected to a gas heat exchanger 25 around the height level of the MSR 4 above the drain tank compartment 3. The second end

[0276] 29 of the rise-pipe is connected to the gas heat exchanger 25 surrounded by and in thermal contact with the outside environment. The buffer water steam condenses at least partly in the heat exchanger 25 and the buffer water 21 leaves the gas heat exchanger 25 at the outlet end of the gas heat exchanger 25 into a first end of a fall-pipe 32. The fall-pipe 31 extends downwards from its first end 32 and enters the outer tank region 36 of the buffer water tank 15 where buffer water 21 is led through the second end 33 of the fall-pipe into the buffer water 21 in the outer tank region 36. The second end 33 of the fall-pipe is below the ring-shaped dry wall section 37 and above the buffer water level.

[0277] A seawater heat exchanger 26 is shown outside the buffer water tank 15 and immersed into the seawater below the seawater level. The seawater heat exchanger 26 has an inlet and an outlet for buffer water where the inlet is situated higher than the outlet. The inlet for buffer water 21 to the seawater heat exchanger 26 ends with its open end in the inner tank region 35 where the buffer water 21 has a higher temperature due to the decay heat on the other side of the inner tank wall 16 as described above. The temperature difference between the buffer water 21 in the inner tank region 35 and the outer tank region 36 will give rise to a natural circulation of the buffer water 21. The buffer water 21 enters the inlet for buffer water 21 to the seawater heat exchanger 26 and is cooled by the seawater having a lower temperature than the buffer water 21 in the inner tank region 35 whereupon the buffer water 21 is circulated back into the lower part of the buffer water tank 15. This circulation of buffer water 21 through the seawater heat exchanger 26 takes place as a natural circulation from a hot region to a cold region and is a passive circulation without the need of actively pumping the water through the seawater heat exchanger 26.

[0278] In figure 3, a reactor construction 1 is shown as installed onboard a barge 40. The reactor construction 1 is illustrated where the MSR 4 is in an operational state and the molten fuel salt 6 is in the reactor core and reference is made to the description of figure 1 above. The upper compartment 2 and the drain tank compartment 3 are surrounded by a double walled enclosure 41 that may constitute a structure required for hull integrity or alternatively a biological shielding for the reactor construction.

[0279] The first piping structure 22 and the second piping structure 24 is shown. The first piping structure 22 is shown with an outlet for reservoir water steam through the upper deck 42 of the barge 40. The second piping structure 24 is shown with the rise-pipe 27 ending and the fall-pipe 31 starting outside the upper deck 42 of the barge 40. A gas heat exchanger 25 is situated outside the upper deck 42 of the barge 40 and surrounded by a heat exchanger 44 structure having openings 45 for the hot air arising from the heat exchanger 44. The hot air is led out into the atmosphere through these openings 45. The seawater heat exchanger 26 is located in the seawater outside a first wall 46 of the hull structure. A second wall 47 of the hull structure is shown with an opening 48 for the hot seawater arising from the heat exchange with buffer water 21.

[0280] Fig. 4 shows a reactor construction, wherein the buffer water tank is executed as a tube system. The reactor construction is similar to that shown in Fig. 1. However, the buffer water tank 15 is executed as a tube system comprising an inner tube with a hot wall 52 and the outer tube with a cold wall 51, wherein the inner and outer tube are conjoined, wherein the buffer water from the reservoir water tank 49 is located within the tube within the conjoined inner and outer tube of the tube system, which is also connected to the backup tank 53. The reservoir water tank 49 houses a condenser 50, which facilitates the condensation of evaporated cooling water. The reservoir water tank 49 is located above sea water level 54.

[0281] Fig. 5 shows a reactor construction, wherein the buffer water tank is executed as a tube system, wherein the tube system comprises a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses. The reactor construction is similar to that shown in Fig. 4. However, the tube system comprising an inner tube with a hot wall 52 and the outer tube with a cold wall 51 each comprise a lower evaporation part 57, a middle adiabatic part 56 and an upper condensation part

[0282] 55. In the evaporation part 57, the buffer water evaporates. In the adiabatic part

[0283] 56, the no heat is exchanged between the buffer water and the environment. In the condensation part 55, the buffer water condenses.

[0284] Fig. 6 shows a reactor construction, wherein the buffer water tank is executed as a tube system, wherein the tube system comprises a section in which the buffer water evaporates, a section in which the buffer water is adiabatic, and a section in which the buffer water condenses, wherein the buffer water and sea water are in thermal contact below seawater level . The reactor construction is similar to that shown in Fig. 4. However, instead of the tube system being in contact with a reservoir water tank 49 comprising a condenser 50, the tube system is in contact with sea water below sea water level via a sea chest 58.

Claims

C L A I M S1. A reactor construction (1) comprising an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising:- a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6);- a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising:- one or more drain tanks (10) in communication with the molten salt drain system;- a buffer water tank (15) comprising an inner wall (16) and an outer wall (17) and buffer water (21) in the gap between the inner wall (16) and the outer wall (17) of the buffer water tank (15), said buffer water tank ( 15) surrounding the one or more drain tanks (10), wherein a first piping structure (22) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (23) in thermal contact with a reservoir water tank (30), said reservoir water tank (30) being at a level above the buffer water tank (15), said reservoir water being in thermal contact with an environment and / or a second piping structure (24) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (25) in thermal contact with an environment, said heat exchanger (25) being at a level above the buffer water tank (15) and / or a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15).

2. The reactor construction (1) according to claim 1, wherein the reactor construction (1) comprises an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising:- a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6);- a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising:- one or more drain tanks (10) in communication with the molten salt drain system;- a buffer water tank (15) executed as a tube system comprising an inner tube (52) and an outer tube (51), wherein the inner and outer tube are conjoined, wherein the buffer water is located within the tube within the conjoined inner and outer tube of the tube system, said buffer water tank (15) surrounding the one or more drain tanks (10), wherein a first piping structure (22) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (23) in thermal contact with a reservoir water tank (30), said reservoir water tank (30) being at a level above the buffer water tank (15), said reservoir water being in thermal contact with an environment and / or a second piping structure (24) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (25) in thermal contact with an environment, said heat exchanger (25) being at a level above the buffer water tank (15) and / or a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15).

3. The reactor construction (1) according to claim 1 or 2, wherein the reactor construction (1) comprises an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising:- a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6);- a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising:- one or more drain tanks (10) in communication with the molten salt drain system;- a buffer water tank (15) executed as a tube system comprising an inner tube (52) and an outer tube (51), wherein the inner (52) and outer (51) tube are conjoined, wherein the buffer water is located within the tube within conjoined inner and outer tube of the tube system, said buffer water tank (15) surrounding the one or more drain tanks (10), wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates (57), a section in which the buffer water is adiabatic (56), and a section in which the buffer water condenses (55), wherein a first piping structure (22) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (23) in thermal contact with a reservoir water tank (30), said reservoir water tank (30) being at a level above the buffer water tank (15), said reservoir water being in thermal contact with an environment and / or a second piping structure (24) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (25) in thermal contact with an environment, said heat exchanger (25) being at a level above the buffer water tank (15) and / or a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15).

4. The reactor construction (1) according any one of the above claims, wherein the reactor construction (1) comprises an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising:- a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6);- a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising:- one or more drain tanks (10) in communication with the molten salt drain system;- a buffer water tank (15) executed as a tube system comprising an inner tube (52) and an outer tube (51), wherein the inner (52) and outer (51) tube are conjoined, wherein the buffer water is located within the tube within conjoined inner and outer tube of the tube system, said buffer water tank (15) surrounding the one or more drain tanks (10), wherein the conjoined inner and outer tube of the tube system comprise a section in which the buffer water evaporates (57), a section in which the buffer water is adiabatic (56), and a section in which the buffer water condenses (55), wherein a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15) within a rectangular or cylindrical recess, preferably a sea chest (58).

5. The reactor construction (1) according any one of the above claims, wherein the reactor construction (1) comprises an upper compartment (2) above a drain tank compartment (3), said upper compartment (2) comprising:- a molten salt reactor (MSR) (4), comprising a reactor vessel (5) comprising a molten fuel salt (6);- a molten salt drain system connected to the reactor vessel (5); said drain tank compartment (3) comprising:- one or more drain tanks (10) in communication with the molten salt drain system;- a buffer water tank (15) comprising an inner wall (16) and an outer wall (17) and buffer water (21) in the gap between the inner wall (16) and the outer wall (17) of the buffer water tank (15), said buffer water tank (15) surrounding the one or more drain tanks (10), and a first piping structure (22) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (23) in thermal contact with a reservoir water tank (30), said reservoir water tank (30) being at a level above the buffer water tank (15), said reservoir water being in thermal contact with an environment; and optionally a second piping structure (24) is defining a circuit for at least part of the buffer water (21) and comprising a heat exchanger (25) in thermal contact with an environment, said heat exchanger (25) being at a level above the buffer water tank (15); and optionally a seawater heat exchanger (26) in thermal contact with the buffer water (21) and seawater, said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15).

6. The reactor construction (1) according to any one of the above claims, wherein the first piping structure (22) comprises:- at least one rise-pipe (27) with a first end (28) being an inlet for at least part of the buffer water (21) and with a second end (29) in contact with an inlet for a heat exchanger (23) in the reservoir water tank (30); and- at least one fall-pipe (31) with a first end (32) in contact with an outlet of the heat exchanger (23) in the reservoir water tank (30) and with a second end (33) being an outlet for at least part of the buffer water (21).

7. The reactor construction (1) according to claim 6, wherein the minimum cross section of the first end (28) of rise-pipe is larger than the minimum cross section of the second end (33) of the fall-pipe.

8. The reactor construction (1) according to any one of the above claims, wherein a natural convection enhancer is comprised in the buffer water (21) in the buffer water tank (15), said natural convection enhancer comprising a tankwall (34) in the gap between the inner (16) and outer wall (17) dividing the buffer water tank (15) in an inner tank region (35) and an outer tank region (36), the tank-wall (34) extending above buffer water level into a dry wall section (37) contacting the outer wall (17) at a wall contact location (38), wherein- the dry wall section (37) and / or the inner wall (16) comprises perforations (39) to allow air circulation between the inner tank region (35) and the outer tank region (36),- the first end (28) of the rise-pipe being allocated above the wall contact location (38) and the second end (33) of the fall-pipe being allocated below the wall contact location (38).

9. The reactor construction (1) according to any one of the above claims, wherein the buffer tank inner wall (16) is connected to an inner bottom plate (18) so that inner wall (16) and the inner bottom plate (18) has a bowl shape.

10. The reactor construction (1) according to any one of the above claims, wherein the molten salt drain system is a molten fuel salt drain system and comprises a salt piping system comprising at least one salt plug (13).

11. The reactor construction (1) according to any one of the above claims, further comprising a gas supply system comprising- a carrier gas reservoir;- a carrier gas piping (12) comprising at least one valve (11, 110, 111, 112), said carrier gas piping (12) connecting from the carrier gas reservoir to the one or more drain tanks (10).

12. The reactor construction (1) according to any one of the above claims, wherein the molten fuel salt (6) having a composition selected from the group consisting of compositions comprising: sodium fluoride + potassium fluoride + uranium fluoride;lithium fluoride + thorium fluoride + plutonium fluoride; lithium fluoride + thorium fluoride + uranium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride; lithium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride + zirconium fluoride; sodium fluoride + rubidium fluoride + uranium fluoride; sodium fluoride + beryllium fluoride + uranium fluoride + thorium fluoride + zirconium fluoride; potassium chloride + plutonium chloride + uranium chloride; sodium chloride + plutonium chloride; sodium chloride + plutonium chloride + uranium chloride.

13. The reactor construction according to any one of the above claims, wherein the MSR further comprises a moderator based on a material selected from the group of a graphite material, a Be compound containing material, a molten salt of a metal hydroxide wherein when the moderator is molten salt of a metal hydroxide, the reactor construction further comprises a molten salt of a metal hydroxide drain system including molten salt of a metal hydroxide drain tanks.

14. The reactor construction according to any one of the above claims, wherein the reactor construction comprises a compartment separator comprising: a grid structure forming a substantially horizontal surface separation between the upper compartment and the drain tank compartment, said grid structure comprising a metal grid and a thermally insulating layer and one or more funnels penetrating the grid structure, each funnels comprising a plug made of a sacrificial material.

15. The reactor construction according to any one of the above claims, wherein the MSR (4) is located on a marine structure, preferably a barge (40).

16. A method of transferring heat from a molten salt comprising the steps of- providing a molten fuel salt (6) in one or more drain tanks (10) for molten fuel salt (6);- surrounding the one or more drain tanks (10) for molten fuel salt (6) with a buffer water tank (15) comprising buffer water (21) in a gap between an inner wall (16) and an outer wall (17) of the buffer water tank (15), said molten fuel salt (6) transferring heat through at least one of heat radiation, heat conduction or heat convection with the inner wall (16) to heat the buffer water (21) into steam;- providing a first piping structure (22) comprising at least one rise-pipe (27) above at least part of the buffer water (21), said risepipe (27) collecting steam and directing the steam to a heat exchanger (23) in the reservoir water in a reservoir water tank (30) above the buffer water tank (15) to condense the steam into water; at least one fall-pipe (31) to direct water from the heat exchanger (23) in the reservoir water in the reservoir water tank (30) to the buffer water tank (15) and / or- providing a second piping structure (24) comprising at least one rise-pipe (27) above at least part of the buffer water (21), said risepipe (27) collecting steam and directing the steam to a gas heat exchanger (25) in thermal contact with an environment comprising gas, said gas heat exchanger (25) being above the buffer water tank (15) to condense the steam into water; at least one fall-pipe (31) to direct water from the gas heat exchanger (25) to the buffer water tank (15) and / or- providing a seawater heat exchanger (26) in thermal contact with the buffer water (21), said seawater heat exchanger (26) being located below seawater level outside the outer wall (17) of the buffer water tank (15), wherein buffer water (21) circulates through the seawater heat exchanger (26).

17. The method of transferring heat from a molten salt according to claim 16 furthermore comprising a transfer of heat from the reservoir water tank (30) to an environment, such as an outside environment in thermal contact with the reservoir water tank (30).

18. The method of transferring heat from a molten salt according to claim 17 where the transfer of heat from the reservoir water tank (30) is at least partly through the evaporation of the reservoir water to the outside environment.

19. The method of transferring heat from a molten salt according to any of claims 16 to 18, wherein the heat transfer is a passive heat transfer method.

20. The method of transferring heat from a molten salt according to any of claims 16 to 19 wherein a natural convection enhancer comprising a tank wall(34) in the gap between the inner (16) and outer wall (17) divides the buffer water tank (15) in an inner tank region (35) and an outer tank region (36)- causing the buffer water (21) in the inner tank region (35) to boil off, and- providing an upwards flow path for steam.

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