NUCLEAR REACTOR WITH CONVECTOR HEAT EXCHANGER

Abstract

A nuclear reactor having a nuclear core (6), disposed on a support (6a) in the bottom of a cylindrical vessel (9) oriented about a vertical axis, filled with a heat transfer fluid, in particular water, under an upper dome of the vessel, comprising an annular primary heat exchanger (91) surrounding a stack (9a) for the installation and extraction of said core, said heat exchanger starting above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit consisting of the heat transfer fluid circulating in the core and in said primary heat exchanger by convection, for which a first part of the cylindrical wall (94) of said vessel (9) is surrounded by a first annular shell (1a) forming a cold plenum (83) connected to an inlet (91a) of a secondary circuit of said heat exchanger (91) in the lower part of said heat exchanger and extending from the bottom of the vessel to under an outlet (91b) of the heat exchanger primary,in that a second part of said cylindrical wall is surrounded by a second annular shell (1b), above said first annular shell (1a), forming a hot plenum connected to the outlet (91b) of said secondary circuit of said heat exchanger (91) in the upper part of said heat exchanger, an intermediate heat extraction circuit comprising a first pipe (81), for the inlet of cold heat transfer fluid, opening into the upper part of said first shell (1a) while a second pipe for the outlet of hot heat transfer fluid exits into the upper part of said second shell (1b). Figure 2,

Description

Title of the invention: NUCLEAR REACTOR WITH CONVECTIVE EXCHANGER Technical field

[0001] The present disclosure relates to the field of nuclear reactors and in particular low-power nuclear reactors intended for use as heat generators for heating networks. Prior art

[0002] The fight against global warming requires reducing the carbon footprint of all energy sectors. Housing is one of these, and is still heavily dependent on fossil fuels today. Public policies that provide strong incentives to move towards low-carbon solutions are being implemented across Europe.

[0003] District heating, which is highly developed in northern and eastern Europe, is experiencing significant growth but remains highly dependent on fossil fuels, particularly gas and coal. The latter is a major contributor to pollution in highly urbanized areas. Solutions, such as geothermal energy, exist, but are difficult to deploy on a large scale. The main route being considered is the use of biomass, but this limited resource is coveted by several energy sectors, such as aeronautics, and will inevitably be subject to strong pressure on the price of the fuel raw material.

[0004] Nuclear energy, which a number of countries, including France, have chosen as a means of controlling their carbon emissions in electricity production, can be considered as a potential solution in the field of urban heating.

[0005] According to the International Atomic Energy Agency, around forty nuclear reactors worldwide currently cogenerate electricity and heat for housing. Numerous projects are currently under development to use the "fatal" heat from nuclear reactors for district heating, particularly for the new generation of small reactors (SMRs) currently in development.

[0006] However, the coupling of constraints linked to electricity production and heat production does not facilitate the implementation of optimal dual solutions.

[0007] Nuclear reactors purely dedicated to district heating have been developed in the past, but none have seen concrete implementation to date.

[0008] For example, we can cite the Thermos reactor, from the French Atomic Energy Commission (CEA), designed during the 1970s. However, in addition to the problems of acceptance of this technology, the power of the reactor, 100 MW, proved to be oversized compared to the capacities of local heat networks and the economic relevance of such an achievement appeared unconvincing compared to the construction of a coal-fired power station.

[0009] A small-sized reactor is also described in documents WO2022 / 106756 A1.

[0010] The question of the safety of a reactor for district heating requires the management of emergency cooling situations in the event of a fault in the installation and in particular in the case of the shutdown of the cooling liquid circulation pumps of a secondary circuit.

[0011] Document FR2 314 560 A1 describes a nuclear reactor for a district heating network comprising so-called shutter tubes replacing valves, the primary cooling circuit being constantly in open communication with a cooling basin via an exhaust orifice and an inlet orifice in the boundary walls of the primary cooling circuit, the exhaust orifice at least being provided with a connection member provided for this purpose, in the form of a gas shutter tube.

[0012] Document US 4,363,780 relates to a steam generating reactor which comprises valves allowing, in the event of overheating, steam to be evacuated and water from a pool to enter the container of the reactor core. Technical problem

[0013] It is desirable to produce low-power reactors and therefore to improve their efficiency while producing a passive safety device suitable for cooling the core of the reactor in the event of failure of an active heat recovery circuit of said core. Statement of the invention

[0014] In view of this situation, the present disclosure proposes several improvements for a nuclear reactor installation suitable for district heating.

[0015] The present disclosure relates in particular to a reactor comprising a convective exchanger in the reactor vessel making it possible to transmit heat to an improved intermediate circuit and comprising equipment designed so that the heat transfer liquid, for example consisting of water, contained in the pool can naturally cool the reactor without any intervention by an operator, a cooling solution making it possible to cover all its operating states, normal or accidental. This reactor will therefore not require any human intervention to guarantee safety for an extended period, typically at least one week, and does not use any electrical source to ensure cooling of the core.

[0016] The present invention relates in particular to a nuclear reactor comprising a vessel provided with a nuclear core and a primary convective heat exchanger for transferring heat to an intermediate circuit comprising at least one annular envelope around a portion of the cylindrical wall of the vessel.

[0017] More specifically, the present disclosure proposes a nuclear reactor provided with a nuclear core, arranged on a support in a bottom of a cylindrical vessel oriented along a vertical axis, filled with a heat transfer liquid, in particular water, under an upper dome of the vessel, comprising an annular primary exchanger surrounding a chimney for installing and extracting said core, said exchanger starting above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit constituted by the heat transfer liquid circulating in the core and in said primary exchanger by convection, for which a first part of the cylindrical wall of said vessel is surrounded by a first annular envelope forming a cold plenum placed in communication with an inlet of a secondary circuit of said exchanger in the lower part of said exchanger and extending from the bottom of the vessel to under an outlet of the primary exchanger,in that a second part of said cylindrical wall is surrounded by a second annular envelope, above said first annular envelope, forming a hot plenum placed in communication with the outlet of said secondary circuit of said exchanger in the upper part of said exchanger, an intermediate heat extraction circuit comprising a first pipe, for the arrival of cold heat transfer liquid, opening into the upper part of said first envelope while a second pipe for the departure of hot heat transfer liquid exits in the upper part of said second envelope.

[0018] The use of envelopes forming a hot and cold plenum increases the efficiency of heat transfer from the primary exchanger to the intermediate circuit by recovering part of the heat from the wall of the tank.

[0019] Advantageously, the upper dome of the tank is filled with a chemically neutral gas such as nitrogen for regulating the pressure in the tank.

[0020] The tank is preferably arranged in a well made in the bottom of a swimming pool filled with the same heat transfer liquid as the intermediate circuit, said first casing and said second casing being surrounded by the heat transfer liquid contained in the swimming pool.

[0021] According to an advantageous embodiment, the first casing comprises a first opening for placing said cold plenum in the first casing in communication with the pool, said first opening being kept closed by a first gravity valve pushed back under the action of a first flow of said heat transfer liquid between the first pipe, for the inlet of cold heat transfer liquid of an intermediate circuit and the second pipe for the outlet of hot heat transfer liquid of said intermediate circuit, said first opening opening under the action of the first gravity valve in the absence of said first flow so as to allow heat transfer liquid to pass between the first casing and the pool.

[0022] Said first gravity valve may comprise a mass calibrated according to said first flow to close the first opening from a given first flow value. This makes the operation of the valve completely passive and linked to the presence or absence of the expected flow.

[0023] The second casing may advantageously comprise a second opening for connecting said hot plenum with the pool, said second opening being kept closed by a second gravity valve in the presence of said first flow leaving the reactor via a second pipe, said second opening opening under the action of the second gravity valve in the absence of said first flow so as to allow heat transfer liquid to pass between the second casing and the pool. Like the first valve, the second valve is passive and only the presence or absence of the first flow changes its position.

[0024] Said second gravity valve may comprise a mass calibrated according to said first flow to close the second opening from a given first flow value.

[0025] The volume of the pool is preferably sized so as to guarantee passive cooling of a residual power of the core in the absence of other cooling means and to maintain the heat transfer fluid of the primary circuit below a fixed temperature for a determined duration. The temperature can be defined as the boiling temperature of the pool water or a lower temperature and the duration can be a usual duration in the field to allow for troubleshooting intervention, namely approximately one week.

[0026] More particularly, the first pipe and the second pipe are part of the intermediate circuit comprising an intermediate exchanger and at least one pump. The intermediate exchanger can in particular distribute the heat from the intermediate circuit to a primary district heating distribution network supplying first buildings and substations supplying heat to second buildings. Brief description of the drawings

[0027] Other characteristics, details and advantages of the invention will appear on reading the detailed description below of non-limiting exemplary embodiments, and on analyzing the appended drawings, in which:

[0028] [Fig.l] is a schematic cross-sectional view of a nuclear facility of the present disclosure;

[0029] [Fig.2] is a sectional view of a well and an example of a reactor in its vessel;

[0030] [Fig.3] is a variant of the reactor of [Fig.2];

[0031] [Fig.4A], [Fig.4B] show a gravity inlet valve in two positions;

[0032] [Fig.5A], [Fig.5B] show a gravity outlet valve in two positions;

[0033] [Fig.6] is a top view of a reactor core in a basket;

[0034] [Fig.7] is a sectional side view of a lower part of the reactor;

[0035] [Fig.8A] is a top view of an intermediate plate of a support basket of nuclear core;

[0036] [Fig.8B] is a top view of a top plate of a nuclear core support basket;

[0037] [Fig.8C] is a top view of a bottom plate of a nuclear core support basket;

[0038] [Fig.9] shows a nuclear core in a basket in side view;

[0039] [Fig. 10] shows a top view of parts of a nuclear facility building of the present disclosure;

[0040] [Fig. 11] shows a diagram of a district heating network comprising a nuclear installation of the present disclosure. Description of embodiments

[0041] The following drawings and description contain elements which may not only serve to better understand the present invention, but also contribute to its definition, where appropriate.

[0042] [Fig. 1] is a schematic view of a nuclear installation comprising a nuclear reactor vessel 1 in which there is a nuclear reactor core with a removable cover 11. The vessel is arranged in a shaft 2 of a pool 3 filled with a heat transfer fluid for cooling and blocking radiation such as water and more particularly demineralized water. Water or heat transfer fluid will be used indifferently in the description below given that water is the simplest heat transfer fluid to use. In the pool there is a space 33 for removing the removable cover 11, one or more fuel storage racks 31 in the pool, a reloading bench 32 and a water / air exchanger 5. The pool 3 is located in a building 4 which comprises a bridge 41 and its sling 42 and a reloading machine 43.

[0043] The nuclear installation of the present disclosure is in particular suitable for serving as a heat generator for a heating network such as a district heating network and [Fig.l 1] proposes a schematic diagram of the installation of the nuclear installation of [Fig.l] in a heating network.

[0044] The reactor core in the vessel 9 produces heat, transferred to a primary exchanger 91 by means of a primary circuit 92, the heat is transferred to an intermediate circuit 8 provided with one or more circulation pumps 101 to an exchanger secondary 100 which supplies heat to a main distribution network 105 on which there are possibly other production installations 106 which may be of various technologies, conventional or nuclear. This organization makes it possible to guarantee a continuous supply of heat to tertiary exchangers supplying heating circuits 115, 125 for buildings 107, 108, 109. The presence of several production installations makes it possible to provide heating even during maintenance periods or in the event of an incident on one of the production components.

[0045] As will be seen later, the reactor is composed of two circuits. The primary cooling circuit 92 extracts the heat from the core where the chain reaction occurs and this heat is transferred to the intermediate cooling circuit 8, of pressure equal to or higher than the primary circuit, in order to guarantee that any possible radioactive pollution cannot propagate. It is this intermediate circuit which is, through an exchanger, in interface with the main distribution network.

[0046] In this context, the present disclosure relates to a cooling solution for a reactor immersed in a pool and having a core outlet temperature in a range of 70°-1 10°C. This reactor does not require any human intervention to guarantee its safety for an extended period of at least one week, and does not use any electrical source to ensure the cooling of the core, thus providing inherent safety and the possibility of installing the reactor near urbanized areas without risk.

[0047] Firstly, the present disclosure proposes to optimise the recovery of heat from the core. Furthermore, as unlike conventional boilers using fossil fuels, nuclear reactors continue to produce heat, a few percent of the nominal power, after the chain reaction has stopped, this residual heat must therefore be able to be extracted, whether the reactor is in a normal shutdown state or in an unexpected shutdown, following an incident or accident of internal or external origin.

[0048] As seen above, the nuclear part of the reactor, object of the invention, is immersed in a pool, a solution usually used for research reactors, as indicated in [Fig.l]. This provides a water reserve guaranteeing a large autonomy without requiring any human intervention. This pool is located in a reactor hall which has a controlled atmosphere according to nuclear criteria and protected against all external attacks always considered for nuclear installations. The reactor hall is located in a building whose other rooms contain all the equipment necessary for the operation of the installation.

[0049] [Fig.2] describes the operation of the primary circuit 9 and its interface with the intermediate circuit 8 when the reactor is in production mode.

[0050] The primary circuit is encapsulated in a cylindrical vessel 9 comprising a peripheral wall 94 and which contains in its lower part the nuclear core 6, composed of fuel assemblies containing the uranium necessary for the chain reaction. These fuel assemblies can take the form of rods, as in power water reactors, or plates, as in research reactors.

[0051] To clarify the ideas, the present disclosure relates to a reactor with a power of approximately 15 MW to 80 MW thermal comprising a tank with a height of the order of 15 m to 25 m and a diameter of the order of 3.50 m to 4 m.

[0052] In the vessel 9 there are also the control rods 601 arranged at the end of rods 7 and which can be pushed in or out of the core to regulate the chain reaction and stop it to bring the reactor into a safe state whatever the initial conditions, normal or accidental. The rods carrying the control rods are, as known in the art, moved by electric motors, for example electric motors integrated in cylindrical sleeves crossed by the rods and welded to a cover of the vessel 9, and held in position by a magnetic field in a control device 74. In the event of a lack of supply current to the motors, the rods 7 are released and the bars 601 fall into the core, stopping the chain reaction.

[0053] The tank also comprises an internal chimney 9a for guiding by convection towards the top of the tank a first heat transfer liquid, for example water, heated by the core and an annular space for the descent of the heat transfer liquid towards the bottom of the tank under the core, which creates a closed circuit for the circulation of the heat transfer fluid passing through the core 6. The annular descent space incorporates a counter-current heat exchanger or primary exchanger 91 comprising a primary circuit in which the first heat transfer liquid circulates. The exchanger 91 allows the heat generated by the core 6 to be evacuated towards the intermediate circuit 8.

[0054] The temperature of the heat transfer liquid in the tank, at the outlet of the core, is in the present application of the order of 70°C to 110°C and more precisely between 75°C and 90°C which does not require the tank to be sized for high pressures and high temperatures.

[0055] The intermediate circuit 8 is a loop composed mainly of the downstream part or secondary circuit of the exchanger 91, of a pumping system composed of one or more pumps 101 powered electrically by an external network, ensuring forced circulation in the intermediate circuit and making it possible to accommodate the different operating states of the reactor when it is in operation, of pipes 81, 82 connecting the different components and the upstream part of a secondary exchanger 100 between the intermediate circuit 8 and the main distribution network 105.

[0056] Still according to [Fig.2], a lower part of the cylindrical wall 94 of the tank 9 is surrounded by a first annular envelope forming a cold plenum 83 of the intermediate circuit 8 which is placed in communication with an inlet 91a of the secondary circuit of the primary exchanger 91 in the lower part of said exchanger.

[0057] The first annular envelope extends from the bottom of the tank to under an outlet 91b of the primary exchanger 91 in the upper part of this exchanger.

[0058] An upper part of the cylindrical wall 94 of the tank is surrounded by a second annular envelope 1b, above said first annular envelope 1a. This second annular envelope 1b encompasses the outlet 91b of the secondary circuit of the primary exchanger 91.

[0059] The radial distance between the external wall of the first and second annular envelopes and the wall of the tank 9 is given to achieve an exchange volume adapted to the heat of the tank and the expected flow rate and is for example of the order of 20 cm to 30 cm for a reactor as defined above.

[0060] This second annular envelope forms a hot plenum of the intermediate circuit 8 in communication with the outlet 91b of said secondary circuit of the primary exchanger 91 in the upper part of said exchanger.

[0061] The intermediate circuit further comprises a first pipe 81, for the arrival of a cold heat transfer liquid, for example cold water, opening into the first casing 1a in the upper part of said first casing 1a while a second pipe for the departure of hot heat transfer liquid exits in the upper part of said second casing 1b.

[0062] In operation, the intermediate circuit is designed to operate with a temperature difference of the order of 20°C to 30°C between the cold heat transfer liquid inlet pipe and the hot heat transfer liquid outlet pipe.

[0063] The first and second annular envelopes constitute a cylindrical interface box, which envelops a large part of the primary tank 9, at the level of the main exchanger and which is organized in the following manner:

[0064] - The inlet pipes 81 and outlet pipes 82 of the intermediate circuit are connected to this box. a. - The inlet plenum or cold plenum 83 guides the water from the intermediate circuit towards inlets in the lower part of the exchanger 91 integrated in the primary tank 9, b. - In the exchanger 91, regularly distributed plates ensure the distribution of the flow, the efficiency of the heat exchange and the vibratory maintenance of the exchanger tubes, c. - In the upper part, the hot water leaves the exchanger 91 to be injected into the upper plenum from where it joins the hot pipes of the inter circuit mediator.

[0065] This housing also recovers heat leaks from the primary tank to inject them into the useful heat production system.

[0066] The intermediate circuit and the pool use the same heat transfer fluid, for example water which is used to cool the reactor when it is in operation and when it is shut down.

[0067] [Fig. 3] describes an aspect of the present disclosure according to which valves 85, 86 are incorporated in the envelopes 1a and 1b between said envelopes and the pool 3.

[0068] These valves are configured to isolate the intermediate circuit of the swimming pool when the pump(s) 101 are in operation but configured to put the hot and cold plenums in communication with the heat transfer liquid contained in the swimming pool when the pump(s) 101 of the intermediate circuit are stopped, which allows on the one hand the entry of the water from the swimming pool into the cold plenum of the interface box and on the other hand the exit of the heated water from the hot plenum towards the swimming pool.

[0069] Indeed, with the pump stopped, for example in the case of a loss of electrical power to the pump(s) or by decision of the operator, for example in the absence of a heat demand or during a maintenance operation, the device is designed so that, on the cold plenum side, the cold water from the pool descends to the inlet vent 91a in the primary exchanger 91, where it rises while heating up. The difference in density between the descending and rising water provides sufficient driving force for the establishment of natural circulation.

[0070] It should be emphasized that when circulation in the primary circuit stops, the reactor is designed so that the control rods descend into the core and so that the residual power of the core is only a few percent of its nominal power. In the shutdown configuration of the pump(s) 101, the flow rate of the natural circulation between the pool and the exchanger will be only a small fraction of the nominal flow rate, pumping through the intermediate circuit, leading to very significantly lower water velocities.

[0071] [Fig.4A] shows the inlet valve 85 in a position where the water inlet from the pool is closed and the water inlet from the intermediate circuit is open while [Fig.4B] shows the inlet valve 85 in the position where the water inlet from the pool is open while the water inlet from the intermediate circuit is closed. According to the example, the valve is a gravity valve comprising a flap 85 for closing an opening 83a for connecting the cold plenum 83 with the pool above the inlet of the pipe 81 in the cold plenum. This valve comprises a counterweighted vane 86 at the level of the arrival of the pipe 81 in the casing forming the cold plenum 83. In [Fig.4A] the flow of the heat transfer liquid Fl circulating in the intermediate circuit 8 and arriving via the inlet pipe 81 pushes the counterweighted vane which pushes the flap 85 and closes the opening 83a. In [Fig.4B], the heat transfer fluid no longer circulates in the intermediate circuit 8, the counterweight descends under the action of gravity which moves the flap 85 away from the opening 83a and allows a flow F3 of water from the pool to enter the cold plenum while the outlet of the pipe 81 in the cold plenum is closed.

[0072] [Fig.5A] shows the outlet valve 87 in the closed position while [Fig.5B] shows the outlet valve 87 in the open position.

[0073] In [Fig.5A], the flow F1 leaving the primary exchanger is sucked in by the conduit 82 under the action of the pump of the intermediate circuit. This flow of liquid forces the valve 87 into the closed position which does not allow the water from the hot plenum to exit into the pool 3.

[0074] In [Fig.5B], the flow F1 is stopped when the pump 101 of [Fig.2] stops, the valve opens under the action of its counterweight 88 and the water heated in the exchanger 91 escapes into the swimming pool according to the flow F3.

[0075] The outlet valve does not need to close the outlet to the intermediate circuit which is further preferred to avoid problems

[0076] As previously indicated, when the pump(s) are stopped, the natural circulation, which takes place in the interface box, is only a very small fraction of the nominal circulation with the pumps. In this configuration, the water velocities are low and insufficient to prevent the valves from opening by gravity, the latter being ballasted with sufficient weight to be kept open by the dynamic pressure and closed when the latter is reduced. When the pumps restart, the force of the jet is sufficient to push the valve back into the closed position and maintain it in this position.

[0077] It should be noted that since the heat transfer liquid and the pool liquid are identical, the valves 85, 87 do not necessarily require qualified sealing in the closed position, a slight tolerance on leaks being acceptable.

[0078] Due to the heat released by the core in the tank 9, the heat transfer fluid circulating in this tank expands and creates a slight overpressure which is stabilized by the presence of a gas such as nitrogen in a dome 93 of the tank in the upper part of the tank.

[0079] The volume of the pool 3 is in particular dimensioned so as to guarantee passive cooling of the residual power of the core 6 in the event of a problem with the electrical supply of the pump 101 and to maintain the heat transfer liquid of the primary circuit below a fixed temperature for a determined duration.

[0080] This fixed temperature may in particular be a boiling temperature of the heat transfer liquid possibly less a safety margin of 10°C to 20°C and the determined duration may for example be 5 to 10 days and in particular one week.

[0081] The confinement of radioactive materials, as is customary in nuclear reactors, is ensured both statically and dynamically. Static confinement relies on the physical barriers interposed between the nuclear materials and the environment: the fuel cladding, the primary circuit and the building in which the reactor is located.

[0082] These three barriers are watertight and designed to maintain this watertightness even in the event of possible internal and external attacks on the installation. In particular, the third barrier, the building, must maintain sufficient integrity in all situations to confine all the nuclear materials it contains.

[0083] Returning to [Fig. 1], the radioactive materials present in the reactor installation described here will be located in the pool 3 which contains the primary vessel of the reactor 1, the fuel reloading bench 32 of the reactor, the racks 31 for storing new or spent fuel assemblies, the core reloading zone 33. The pool as shown in [Fig. 10] is located in a reactor hall 4 of a building 10.

[0084] The nuclear part of the reactor is thus immersed in the pool, a solution usually used for research reactors. This provides a water reserve guaranteeing a large autonomy without requiring any human intervention. This pool is located in the reactor hall which has a controlled atmosphere according to nuclear criteria and protected against all external attacks always considered for nuclear installations. Reactor hall 4 is located in building 10, the other rooms of which contain all the equipment necessary for the operation of the installation.

[0085] Building 10 comprises structures - walls, roof and access airlock - which are watertight with reinforced concrete walls to ensure their resistance in the event of internal or external aggression. According to the present disclosure, dynamic confinement is ensured by a nuclear ventilation system according to which the parts of the installation containing nuclear materials, the coolant or other possibly contaminated fluids are subject to specific ventilation, of nuclear quality. A nuclear ventilation system conventionally fulfills two missions: ensuring ambient conditions allowing human activities, where necessary, and the proper functioning of the equipment on the one hand, and on the other hand ensuring the filtration of the air in the installation before its release when it may be contaminated by radioactive bodies. To do this and with reference to [Fig.10], the outside air is sucked in by calibrated outside vents 201a, dedicated and protected at the level of a suction module 201. Extraction fans 202, powered by an external electrical network, maintain in normal operation a slight depression in the room(s) likely to contain fission products in gaseous or aerosol form and then the air is discharged. through an atmospheric chimney 210 after passing through adsorption or filtration devices 203 which retain these possible fission products in gaseous or aerosol form. To ensure that the radioactive products which could be present in the air of the installation can only exit through the chimney 210, after inlet filtration in the inlet filtration room 201a, a slight depression is maintained in the rooms where nuclear materials are present by means of one or more extractors 202 which suck in the air contained in the reactor hall 4. Thus, the releases of radioactivity in gaseous form, through small leaks such as at door seals, are therefore excluded because the depression guarantees an inlet flow and not an outlet flow. Any few residues which have escaped filtration are discharged upwards through the chimney 210 and are therefore extremely diluted, thus avoiding any stress on the surrounding population.

[0086] Usually in nuclear reactors, in the event of an accident, the rooms likely to contain radioactive elements are isolated and maintain their depression for a certain time, depending on the tightness of the isolation means. In the present case, an important point of the present disclosure is to ensure that a depression is maintained over time in the rooms likely to contain radioactive isotopes and in the reactor hall without requiring any recourse to actions by an operator or the use of active means, such as electric fans, the aim being to guarantee the maintenance of the filtration of gaseous discharges over a period consistent with the 7 days typically considered for the passive cooling of the reactor after stopping the circulation of the heat transfer liquid of the intermediate circuit 8.

[0087] The safety of the facility of the present disclosure includes a nuclear ventilation system that ensures the containment of radioactive gases and particles that may be emitted in the facility. To allow personnel activity, it cleans the air, regulates the temperature and humidity within specified ranges. It therefore protects the public, the environment and personnel.

[0088] The ventilation system comprises a water / air exchanger 5, installed in the pool of the reactor hall 4 on the ventilation circuit, upstream of the extractor(s) 202.

[0089] In [Fig. 10], the arrows indicate the air movements between the different rooms: a. fresh air is continuously introduced into the installation through one or more air inlets 201a each comprising a calibrated opening; b. this air is purified and treated (heated or cooled) in an inlet filtration room 201 before being distributed to the various rooms requiring nuclear ventilation.

[0090] A first circuit comprises an air outlet 201b in the reactor hall 4 comprising the reactor 1 in a cooling pool 3 of said reactor.

[0091] In reactor hall 4, the air passes through an air / water heat exchanger 5 placed in the swimming pool. It heats up there because the pool water is, in normal operation, maintained at 50°C by a circuit 220 dedicated to the management of the pool water located in the auxiliary systems room, this circuit, having in addition a filtration function, being configured to recover the heat from the pool water by heat pumps to reinject it into the production system. An electrically powered air extractor 202, downstream of the exchanger, ensures that the reactor hall is kept under negative pressure.

[0092] The other rooms of building 10, likely to contain radioactive elements, such as here auxiliary systems room 12, a heat transfer room 13 and loading / unloading hall 14 are supplied with air from the inlet filtration room 201 by an air duct 201c and each provided with a third air outlet 203a, 203b, 203c towards the reactor hall 4 so that the air extracted from these rooms follows the same exit path as the air from the reactor hall.

[0093] The air extracted by the extractor 202 then passes through filters of different mesh sizes and activated carbon traps or equivalent in a filtration device 203 before being evacuated by a chimney 210. The exchanger 5, when the reactor is in normal operation, can contribute to the cooling of the pool water, heated by the thermal leaks from the primary vessel and by any spent fuel assemblies present in the storage racks 31. It thus relieves a possible auxiliary system for regulating the water temperature which maintains it at 50°C. But its main function, according to the present disclosure, is its contribution to maintaining dynamic confinement in an accidental situation where the electrical supply of the reactor by the network is no longer ensured, involving the loss of the extraction fan 202 and the auxiliary circuit for regulating the temperature of the pool.In such an accident sequence, the reactor is immediately shut down by the fall of control rods into the core. In addition, the absence of electrical power causes a shutter to fall or a diaphragm to close at the air inlet(s) 201a.

[0094] Indeed, in normal operation, the shutter or diaphragm is held open by an electromagnetic field and said air inlet 201a in the wall of the building 10 is configured in the maximum open position and, when said extractor stops, the released shutter or diaphragm reduces the surface area of ​​the air inlet of the nuclear ventilation in order to adapt it to a configuration where the electric extractors are no longer in operation.

[0095] In the core of the reactor, the fission reactions are stopped, but the residues of the previous fissions continue to emit heat at a low level: at a few percent of the nominal power in the first hours, then at a few percent in the first days. This residual heat from the fuel is evacuated in the pool which gradually rises in temperature, beyond 50°C, its cooling circuit being non-operational. The mass of water in the pool is sized for this temperature remains below 80°C after a week of reactor insulation. At the level of exchanger 5, the air heated by the pool water is filtered and has only the chimney as an outlet, in particular because the air inlets have been reduced to increase the depression in the reactor hall. The exchanger then ensures the chimney draft, the outside air having a temperature lower than the air heated by the pool water. The radioactive elements present in the air in the reactor hall are therefore, almost 100%, trapped in the outlet filtration device 203 and the discharges therefore no longer present a risk to the environment.

[0096] In the same logic, the present disclosure concerns means intended to prevent the control of the reactivity of the core from being guaranteed, which contributes to the inherent safety of the installation and reinforces the possibility of installing the reactor in urbanized areas by eliminating a whole family of risks.

[0097] In the reactor, all the core reactivity variation requirements are met by the control rods 601 which, as seen above, under the impulse of the control rods 7 operated by traditional type electromagnetic mechanisms, move vertically in the core.

[0098] One of the most severe accidents that can occur in a nuclear reactor is an uncontrolled runaway chain reaction that would cause an exponential power excursion. To avoid such a situation, prevention and mitigation measures are conventionally implemented in reactors to avoid what is called prompt criticality. They reduce the consequences of such an accident to an acceptable level.

[0099] In the present low-power installation, for example from 15 to 80 MWth, the very low pressure difference between the reactor vessel and the surrounding pool makes it possible to exclude control rod ejection type accidents, which are penalizing on most power reactors. Furthermore, the maximum speed of extraction of the rods by the control mechanisms is very slow, rapid power variations not being necessary in district heating given the high thermal inertia of the networks. The increase in power due to an unexpected withdrawal of control rods will leave a large delay for the protection system to react and cause all the rods to drop.

[0100] The core of a nuclear reactor is the site of a chain reaction which must be able to be maintained over time. It therefore has an excess of reactivity which is usually represented by the neutron multiplication factor between two successive generations (keff). This excess is compensated, at the beginning of the core's life, by neutron absorbers that maintain this factor equal to 1. These absorbers are gradually removed to compensate for the increase in neutron capture of the created bodies and residues of nuclear fission, until the reactivity reserve is no longer sufficient to maintain the critical core. At this point, worn assemblies are removed from the core and replaced with new ones.

[0101] Nuclear regulations impose under-reactivity criteria when the reactor is shut down, whether the fuel assemblies are still in the core, being moved for loading and unloading operations, or in storage at the reactor site. The usual criterion is to guarantee in all cold shutdown configurations keff < 0.95.

[0102] In the proposed solution, the shutdown, reloading - unloading or storage configurations are defined in such a way that a return to criticality is physically impossible and that the regulatory criteria are naturally respected, making it possible to exclude any uncontrolled reactivity excursion.

[0103] When the reactivity reserve becomes insufficient, at the end of the fuel cycle, all of the control rods are inserted into the core, stopping the chain reaction and providing the regulatory anti-reactivity margin.

[0104] [Fig.6] gives the example of a core 6, in top view, equipped with a plate fuel 600, but the invention is just as relevant when it is a rod fuel, similar to that of pressurized water power reactors. In the case presented, the core comprises 45 assemblies and 32 control rods 601 in the shape of a cross and regularly distributed in the core. The neutron absorbing material of the rods can be boron carbide, hafnium or an alloy of particular metals containing silver, indium and cadmium, according to the state of the art. These 32 rods 601 are organized into several groups, each managed autonomously by means of control rods 7 shown diagrammatically in particular in [Fig.2].

[0105] Control bars can be organized into groups: a. - A group dedicated to regulating the reactor power. b. - A group dedicated to compensating for wear of assemblies combustibles. c. - Two shutdown groups, each having the capacity to shut down the reactor and provide the regulatory anti-reactivity margin to carry out unloading - reloading operations.

[0106] The movement of each of the bars is ensured by traditional electromagnetic mechanisms acting on the control rods, and, in the absence of electric current, the bars fall into the core, according to the usual principles in nuclear reactors: an exit of a parameter from the normal operating range induces the cutting of the electrical power supply to the mechanisms, and the bars stop the chain reaction and ensure regulatory subcriticality.

[0107] The heart is shown diagrammatically in [Fig.7] in cross-sectional side view.

[0108] To allow the extraction of the core, the latter is installed in a chimney surrounded by the annular exchanger 91. The control rods are composed of cross-shaped plates 601 also visible from above in [Fig.8A] and are inserted between the fuel assemblies 600. The control rods are attached to rods 7 as seen previously.

[0109] The core 6 is particularly small, compared to that of a power reactor, which leads to very significant peripheral and axial neutron leaks. Usually, in research reactors, the neutron economy is preserved by surrounding the core with light materials, which capture neutrons poorly, such as heavy water, beryllium or graphite, which reflect the neutrons towards the fissile material, and increase the performance, in particular that on the neutron flux, an essential parameter for this type of reactor. In the present disclosure, the core is surrounded according to FIGS. 6 and 7 by four corner reflectors 62 and four peripheral reflectors 61 under the exchanger 91.

[0110] According to an important aspect, the core is arranged in a basket 60. The basket shown more precisely in [Fig.9] comprises, according to the example, an openwork bottom supporting the fuel assemblies and two removable openwork plates: an intermediate plate 70 covering the fuel assemblies 600, comprising cross-shaped slots 71 for the passage of the control rods and water passage holes 72 shown in [Fig.8A], an upper plate 73 provided with holes 74 for the passage of the control rods 7 and water passage holes 72 shown in [Fig.8B] and a lower plate 75 comprising water passage openings 76 also making it possible to locate the fuel assemblies. The control rods are mechanically disconnectable from the control rods to allow their removal while maintaining the rods in the core and therefore in the basket.

[0111] This basket comprises lateral uprights provided with angles 60a for passing the corner reflectors 62 and is a metallic structure which can be made of steel and / or a material transparent to neutrons such as zircaloy or aluminum.

[0112] The basket 60 allows the core to be completely removed from its vessel and placed on a reloading bench 32 in the pool by means of an overhead crane 41 and for example a sling 42 or a spreader connected to the overhead crane and itself comprising slings. Once in the reloading bench, the walls of which are for example composed of neutron-absorbing materials (boron steel, hafnium, cadmium, etc.), the upper internals (plates 70, 73 and bars 601) of the basket are removed to allow the unloading-reloading operations. A reloading machine positioned above the pool can ensure the removal of assemblies used and the insertion of new assemblies into the core and the storage of the assemblies in the adjacent racks 31.

[0113] Once the core has been reconfigured for a new cycle, with the control rods inserted, the reverse operations to those for unloading are carried out and the basket containing the core is repositioned in the reactor by the overhead crane, the internals put in place and the vessel 11 closed.

[0114] The unloaded fuel assemblies are temporarily stored in racks before being transferred by the reloading machine into transport casks which will be removed from the reactor hall by truck.

[0115] The core exhibits its highest intrinsic reactivity when new assemblies have just been introduced into it, at the start of the cycle. Minimizing this reactivity translates directly into the available antireactivity margin when the core is in its various operational configurations, in or out of the reactor, and therefore facilitates the achievement of regulatory antireactivity margins.

[0116] The first way to reduce this intrinsic reactivity, while maintaining the energy produced during the cycle, can be the use of consumable poisons. To do this, bodies whose nuclei capture neutrons are introduced into the fuel, creating new isotopes that are themselves non-absorbent. They are in common use in the nuclear industry; among the solutions that can be considered for the present disclosure, we can cite cadmium wires or gadolinium oxide, integrated into the fuel part of the assemblies.

[0117] The use of the reflectors 61, 62 also makes it possible to significantly reduce the number of new assemblies to be introduced into the core at each reloading, while maintaining the same energy produced during the fuel cycle. This therefore makes it possible to lower the initial intrinsic reactivity of the core, when it is no longer swaddled by its reflector covers.

[0118] This minimization of the initial reactivity, bare core, without control rods, allows a greater margin of anti-reactivity when these are inserted, and therefore facilitates compliance with regulatory criteria. During unloading - reloading operations, the core, in its entirety, is extracted from its reflective environment with the bars inserted and therefore sees its margin of anti-reactivity increased until it is deposited in the reloading bench.

[0119] The reactor can be recharged every year, during the summer period, when the demand for heat is lowest, for a period of a few days during which additional means of production on the heat network are activated. This is also the time when maintenance is carried out and the systems tested.

[0120] The stopping and unloading operations can be carried out according to the following sequence: a. - Shutdown of the reactor and insertion of all 601 control rods into the core; b. - Equalization of reactor and pool pressures; c. - Opening the primary tank in the pool, removing and removing the cover 11 in a dedicated area 33; d. - Dismantling and removal of the upper internals of the vessel, leaving the control rods inserted in the core; e. - movement, by an overhead crane 41 from the reactor hall 4 of the basket containing the core with the control rods inserted between the bottom of the vessel 9 and a reloading bench 32 in the pool 3 containing the vessel 9.

[0121] The core, in the reloading bench, can be surrounded by neutron-absorbing materials (boron steel for example, or other neutron-absorbing materials), which provide sufficient anti-reactivity to manipulate the control rods and insert new assemblies.

[0122] Important features are as follows:

[0123] - A set of measurements are taken on the reactor, when it is in operation to prohibit a rapid excursion of reactivity.

[0124] - The core, in operation, is surrounded by neutron-reflecting materials, allowing to limit their leakage, very important given its small size and reduce as much as possible its intrinsic reactivity.

[0125] Consumable poisons, which gradually disappear during the fuel cycle, also contribute to the reduction of the intrinsic reactivity of the core.

[0126] - The heart is always installed in a basket which allows its extraction and its insertion into the reactor while maintaining the presence of all control and shutdown rods.

[0127] - For spent fuel extraction and fuel insertion operations new, the basket containing the core is placed in a reloading bench composed of neutron-absorbing materials, thus allowing the handling of the control and stop rods.

[0128] - The heart thus respects the regulatory anti-reactivity margins even in the event of mechanical failure or operator error. Industrial application

[0129] The invention is not limited to the examples described above, only by way of example, but it encompasses all the variants that may be envisaged by those skilled in the art within the framework of the protection sought. The configuration described allows the installation of the nuclear installation presented in urbanized areas.

Claims

Claims

1. Nuclear reactor provided with a nuclear core (6), arranged on a support (6a) in a bottom of a cylindrical vessel (9) oriented along a vertical axis, filled with a heat transfer liquid, in particular water, under an upper dome of the vessel, comprising an annular primary exchanger (91) surrounding a chimney (9a) for installing and extracting said core, said exchanger starting above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit constituted by the heat transfer liquid circulating in the core and in said primary exchanger by convection,characterized in that a first part of the cylindrical wall (94) of said tank (9) is surrounded by a first annular casing (la) forming a cold plenum (83) placed in communication with an inlet (91a) of a secondary circuit of said exchanger (91) in the lower part of said exchanger and extending from the bottom of the tank to under an outlet (91b) of the primary exchanger, in that a second part of said cylindrical wall is surrounded by a second annular casing (1b), above said first annular casing (la), forming a hot plenum placed in communication with the outlet (91b) of said secondary circuit of said exchanger (91) in the upper part of said exchanger, an intermediate heat extraction circuit comprising a first pipe (81), for the arrival of cold heat transfer liquid,opening into the upper part of said first casing (la) while a second hot heat transfer liquid outlet pipe exits into the upper part of said second casing (1b).,

2. Nuclear reactor according to claim 1 for which the upper dome of the vessel is filled with a chemically neutral gas (93) for regulating pressure in the vessel.

3. Nuclear installation comprising a reactor according to claim 1 or 2 for which the tank is arranged in a well (2) formed in the bottom of a pool (3) filled with the same heat transfer liquid as the intermediate circuit, said first envelope and said second envelope being surrounded by the heat transfer liquid contained in the pool.

4. Nuclear installation according to claim 3 for which the first envelope (la) comprises a first opening (83a) for connecting said cold plenum (83) in the first envelope (la) with the pool (3), said first opening (83a) being kept closed by a first gravity valve (85) pushed back under the action of a first flow of said heat transfer liquid (Fl) between the first pipe (81), for the arrival of cold heat transfer liquid from an intermediate circuit (8) and the second pipe (82) for the departure of hot heat transfer liquid from said intermediate circuit, said first opening opening under the action of the first gravity valve in the absence of said first flow (Fl) so as to allow heat transfer liquid to pass between the first casing and the pool (3).

5. Nuclear installation according to claim 4 for which said first gravity valve comprises a mass (86) calibrated according to said first flow (Fl) to close the first opening from a given first flow value (Fl).

6. Nuclear installation according to claim 4 or 5 for which the second casing (1b) comprises a second opening (84a) for placing said hot plenum in communication with the pool, said second opening being kept closed by a second gravity valve (87) in the presence of said first flow (F1) leaving the reactor via a second pipe (82), said second opening opening under the action of the second gravity valve in the absence of said first flow (F1) so as to allow heat transfer liquid to pass between the second casing (1b) and the pool (3).

7. Nuclear installation according to claim 6 for which said second gravity valve comprises a mass (88) calibrated according to said first flow (Fl) to close the second opening from a given first flow value (Fl).

8. Nuclear installation according to any one of claims 3 to 7 for which the volume of the pool (3) is sized so as to guarantee passive cooling of a residual power of the core (6) in the absence of other cooling means and to maintain the heat transfer liquid of the primary circuit below a fixed temperature for a determined duration.

9. Nuclear installation according to any one of the preceding claims for which the first pipe (81) and the second pipe (82) are part of the intermediate circuit (8) comprising an intermediate exchanger (100) and at least one pump (101).

10. Nuclear installation according to claim 9 for which said intermediate exchanger distributes the heat from the intermediate circuit to a primary distribution network (105) for district heating supplying first buildings (106) and substations (110, 120) supplying heat to second buildings (107, 108, 109).

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