Method and system for treating an effluent gas of a molten salt nuclear reactor

Abstract

Disclosed are a system and method for purifying gases to be treated originating from a nuclear reactor core (1) using a liquid primary fluid as fuel, the device comprising a first cooling vessel (2), a second scrubbing vessel (3) for trapping chemical compounds in molten scrubbing salts, the scrubbing vessel delimiting an internal volume (V3), and receiving the cooled gases to be treated (F3) coming from the first vessel, the scrubbing vessel comprising a diffuser (5) separating the internal volume into a lower compartment (3A) and an upper compartment containing the molten salts, with a recirculation system (4) for recirculating the gases, the recirculated gases being introduced into the lower compartment and diffused through the diffuser into the molten scrubbing salts, the cooled gases to be treated (F3) coming from the first vessel being introduced into the upper compartment above the diffuser.

Description

[0001] DESCRIPTION

[0002] TITLE OF THE INVENTION: Method and system for treating effluent gas from a molten salt nuclear reactor

[0003]

[0001] The present invention relates to a method and a system for treating a gas laden with gaseous effluents from a molten salt nuclear reactor.

[0004]

[0002] We are interested here in a nuclear fission reactor, the core of which uses a fuel salt at substantially atmospheric pressure. In the jargon, this is referred to as a 4th generation fast neutron molten salt nuclear reactor.

[0005] The term "fuel salt" here refers to a composition comprising at least one carrier salt, for example a fluoride such as LiF or NaF or a chloride such as NaCl, which carrier salt is in a solid state (crystals) at room temperature but becomes liquid above a certain temperature that allows the reactor to start, and a fuel based on heavy nuclei. These heavy nuclei include fissile isotopes, for example Uranium-233 and / or Uranium-235 and / or Plutonium-239 or a mixture thereof.

[0006]

[0004] In molten salt reactors, the core produces volatile compounds that must be removed from the core. The present invention focuses on the treatment of the gases that are the effluents from the reactor core.

[0007] These effluent gases from the reactor core are loaded with components and contaminants, some of which are radioactive.

[0008] It is desirable to secure the contaminants, including radioactive ones, contained in the effluent gases from the core.

[0009]

[0007] In practice, it is known to install a gas circuit, based on the circulation of a carrier gas, such as helium, to convey the gaseous contaminants effluents from the core to a washing and purification circuit.

[0010] In such a washing and purification circuit, a trapping process using molten trapping salts is employed. This trapping / capture process relies on condensation for suspended droplets and on the principle of physical absorption of vapors and volatile compounds for other contaminants. The molten trapping salts irreversibly absorb volatile compounds and other contaminants; this is known as a "lost-load" trap.

[0011]

[0009] In the known art, sodium or potassium hydroxides, or even ternary carbonates which will be discussed later, are used as trapping molten salts.

[0012]

[0010] In known solutions, droplets of these molten salts are sprayed from above using spray nozzles into a washing chamber where the gas to be purified resides. The spraying is continuous and may utilize a molten salt recirculation circuit. As the process progresses, the molten salts become saturated with the chemical compounds targeted for capture. The molten salts are periodically replenished in these known solutions.

[0013]

[0011] These known solutions involve a significant consumption of molten trapping salts.

[0012] Furthermore, the gas scrubbing chambers for these known solutions generally have a considerable height.

[0014]

[0013] It is in this context that the inventors came to propose the particularly advantageous solution set out below.

[0015]

[0014] To this end, a device is proposed for purifying gases to be treated from a nuclear reactor core using as fuel a liquid primary fluid of the molten salt type (core molten salts), the device comprising:

[0016] - a cooling tank, receiving a flow of gas to be treated and capable of cooling the gas to be treated, coming from the reactor core,

[0017] - a trapping tank, comprising trapping molten salts, configured to trap chemical compounds to be captured in the trapping molten salts, the trapping tank delimiting an internal volume, the trapping tank receiving the cooled gases to be treated from the cooling tank, the trapping tank comprising a diffuser separating the internal volume into two compartments, with a lower compartment not containing a substantial quantity of trapping molten salt, and a main upper compartment containing the trapping molten salts, the trapping tank being equipped with a system for recirculating the gases being purified, the recirculated gases being introduced into the lower compartment below the diffuser, and diffused upwards through the diffuser into the trapping molten salts,The cooled gases to be treated, coming from the cooling tank, are introduced into the main upper compartment above the diffuser.

[0018]

[0015] The cooling tank, forming a first tank, condenses the suspended elements that are easiest to condense and captures part of the solid dust.

[0019]

[0016] Depending on the design of the nuclear reactor and its operating mode, the temperature of the gases to be treated from the reactor core can range from 500 °C to 900 °C. In one embodiment, the cooling vessel allows the temperature of the gases to be treated to be lowered by several degrees, up to several hundred degrees, until reaching the target temperature. In one embodiment, this target cooling temperature is between 50 °C and 500 °C. In one embodiment, a means is provided for maintaining the temperature of the cooling vessel, enabling the heat removed from the gas to be treated to be dissipated.

[0020]

[0017] Advantageously, in the second trapping tank, the recirculated gases, introduced under the diffuser, cause a bubbling phenomenon and maintain agitation, which makes it possible to limit the accumulation of precipitates on the diffuser, and to increase the homogeneity of the liquid / bu mixture.

[0021]

[0018] The diffuser allows gases to pass through but does not allow molten salts to pass through, and the lower compartment serves as a plenum to distribute the recirculated gases evenly over the entire surface of the diffuser, and thus the bubbling phenomenon occurs substantially throughout the entire volume of the molten salts, thereby maximizing the capture of the chemical compounds to be captured.

[0022]

[0019] Thanks to these arrangements, the trapping molten salts can accumulate a significant amount of contaminants, and consequently, the consumption of trapping molten salts is lower than in the known art. Furthermore, the gas recirculation forms a much more compact solution in the vertical direction than the spray solution of the molten salts. A very advantageous vertical compactness can be achieved.

[0023]

[0021] In this document, the terms "lower", "below", "upper" and "above" are to be interpreted with respect to the local vertical defined by Earth's gravity.

[0024]

[0022] It should be noted from the outset that the first passage of the gases to be treated does not pass through the diffuser because the injection is carried out above the diffuser.

[0025]

[0023] According to one embodiment, the trapping tank includes at least one inlet port for admitting the gases to be treated cooled above the diffuser, at least one outlet port to allow the gases to exit from the trapping tank for recirculation and at least one recirculation inlet port arranged under the diffuser.

[0026] According to one design, the gases to be treated have a helium base.

[0027] Advantageously, helium is an inert gas with respect to other chemical components carried along in the gas stream. Helium is a neutral carrier gas that does not interact with other chemical species.

[0028] According to an alternative implementation, another carrier gas could be used, such as argon, krypton, nitrogen, or another inert gas.

[0029] According to one embodiment, the diffuser includes through passages, and the cross-section of the through passages is between 0.1 micrometers 2 and 500 micrometers 2 Depending on one possible option, the cross-section of the through passages can be between 1 micrometer 2 and 100 micrometers 2 .

[0030]

[0028] As a result, the recirculated gas flow can pass through the through-passages and ascend via said passages to the main upper compartment. Conversely, the molten salts in the main upper compartment cannot pass through the through-passages and descend to the lower compartment. The lower compartment therefore remains free of molten salts.

[0031] Depending on the option chosen, the diffuser is produced by sintering or a similar process, using either a metallic or ceramic material. This is a well-established and precise manufacturing solution.

[0032]

[0030] In general, any solution allowing the production of a microporous material with passages of homogeneous cross-section and within a desired interval can be suitable for manufacturing the gas diffuser.

[0033]

[0031] According to one embodiment, the diffuser is presented as a flat disc, of diameter D5 and of thickness E5.

[0034]

[0032] According to a particular embodiment, the molten trapping salts are based on hydroxide salts, e.g. NaOH and / or KOH. A mixture of NaOH and KOH hydroxide salts in substantially equal quantities may be chosen.

[0035]

[0033] For example, hydroxide trapping salts may contain in molar composition 51% of NaOH and 49% of KOH, this mixture having a melting point of 172°C.

[0036]

[0034] The temperature range involved for hydroxide trapping salts is around 200°C to 250°C. We generally work with a margin of a few tens of degrees above the melting temperature, to avoid any risk of unwanted crystallization.

[0037]

[0035] According to another particular embodiment, the molten trapping salts are based on carbonate salts, e.g., Li2CO3 and / or Na2CO3 and / or K2CO3.

[0036] The temperature range involved for the carbonate trapping salts can be between 450 °C and 550 °C. According to one option, the temperature involved can be around 500 °C.

[0038]

[0037] According to one embodiment, a system is provided for maintaining the temperature of the trapping tank. This system makes it possible, on the one hand, to prevent the crystallization of the salts and, on the other hand, to prevent overheating of the trapping tank.

[0039]

[0038] The temperature maintenance system in question may include a heating system, in particular to melt the salts if they are introduced in the form of crystals, and may include a cooling system to remove the heat produced by the residual energy contained in the gas stream to be treated.

[0040]

[0039] According to one embodiment, a single auxiliary fluid circuit with thermal coupling by exchanger can be used to perform both the cooling and heating functions.

[0041]

[0040] According to one embodiment, the trapping tank is generally of revolution around an axis and has an internal diameter D3 taken at mid-height, the diffuser being formed as a disc of external diameter D5, and D5 being between 70% and 96% of D3. D5 can be between 85% and 96% of D3.

[0042]

[0041] The area covered by the through passages of the diffuser is therefore very large in relation to the area occupied by the horizontal section of the main upper compartment, and yet it is possible to move the diffuser from bottom to top by a lifting operation to extract it from the tank as will be seen later.

[0043]

[0042] According to one embodiment, the trapping tank comprises a tank body and a lid, the tank body having an upwardly open mouth with a mouth diameter D1 greater than the outside diameter D5 of the diffuser.

[0044]

[0043] Removing the cover allows all necessary maintenance operations to be carried out, including operations planned during a reactor shutdown and refueling sequence.

[0045]

[0044] According to one embodiment, the diffuser includes a central attachment element, and a vertical lifting system is provided for lifting the diffuser and removing it from the tank.

[0046] At the end of the cycle, the trapping tank can be emptied and the diffuser removed through the tank opening in order to replace it with a new one to start a new production cycle.

[0047] According to one embodiment, the main upper compartment contains the molten trapping salts with a filling rate between 60% and 85% of the internal volume (V3).

[0048]

[0047] This characteristic represents an optimum capture rate of the volatile compounds that one wishes to trap. It should be noted that a substantial gaseous space remains above the liquid phase, as the gas is drawn off by the recirculation system from this gaseous space without drawing in any liquid.

[0049] According to one embodiment, the lower compartment (3A) occupies less than 5% of the internal volume (V3).

[0050] In one embodiment, the cooled gases to be treated from the cooling tank are introduced into the main upper compartment, above the diffuser, via a plurality of orifices formed on one or more bubbler tubes. In one embodiment, each bubbler orifice is directed radially towards the tank axis or downwards, or, in an intermediate manner, obliquely downwards and inwards.

[0051]

[0051] According to one embodiment, the orifices are regularly distributed around the tank body.

[0052]

[0052] Thus, the first pass of the gas to be treated does not pass through the diffuser, and consequently, the risk of clogging the diffuser's through-passages by dust or components that could cause blockage is eliminated. The first pass through the molten salt bath traps the coarsest elements in an initial pass. Subsequent passes provided by the recirculation system increase the capture rate of the most volatile compounds.

[0053]

[0053] The present invention also relates to a method for purifying contaminated gases from a nuclear reactor core using a liquid primary fluid of the molten salt type as fuel, the method comprising:

[0054] - to provide a trapping tank, configured to trap, in trapping molten salts, chemical compounds to be captured present in the gas to be purified, the trapping tank delimiting an internal volume, the trapping tank comprising a diffuser separating the internal volume into two compartments, with a lower compartment not containing a substantial quantity of molten salt, and a main upper compartment containing the trapping molten salts, the trapping tank being equipped with a system for recirculating the gases being purified,

[0055] - introduce the gases to be treated into the main upper compartment above the diffuser,

[0056] - introduce the gases recirculated by the recirculation system into the lower compartment, and diffuse them through the diffuser into the molten trapping salts.

[0057]

[0054] As already indicated above, the recirculated gases introduced under the diffuser cause a bubbling phenomenon and maintain agitation, which limits the accumulation of precipitates on the diffuser, and increases the homogeneity of the liquid / bubbles mixture.

[0058]

[0055] In addition, the first pass of the gases does not pollute the diffuser and the risk of clogging the passages of the diffuser during the initial pass is eliminated.

[0059]

[0056] According to one embodiment, a portion of the recirculation flow is taken to form an outlet flow (F6) derived from the recirculation system

[0060]

[0057] This outlet flow compensates on average for the gas generated by the reactor core, so that the pressure in the gas circuit remains substantially constant over time. It should be noted that this outlet flow then undergoes further treatment to complete the purification and to allow the carrier gas (helium or another carrier gas) to be reused for reinjection into the primary circuit. The carrier gas circuit thus forms a closed loop.

[0061]

[0058] According to one embodiment, the fraction representing the outlet flow is between 5% and 50% of the recirculation flow. According to one option, the fraction representing the outlet flow is between 5% and 25% of the recirculation flow. According to another option, the fraction representing the outlet flow is between 5% and 15% of the recirculation flow. According to a particular embodiment, the portion of the outlet flow is preferably close to 10%. As a result, the gas being treated passes through the molten salt bath about ten times. This increases the purification and reduces the level of volatile components that are not captured by the process proposed here.

[0062] According to one option, the gases to be treated can undergo a cooling step upstream of the trapping tank.

[0063] The present invention also relates to a nuclear reactor using as fuel a liquid primary fluid of the molten core salt type and comprising at least one device as described above.

[0064] The present invention also relates to a nuclear reactor using as fuel a liquid primary fluid of the molten core salt type, implementing a process as described above.

[0065] According to another aspect, whereby the gases to be treated already arrive at a suitable temperature, a device is proposed for purifying gases to be treated originating from a nuclear reactor core using a liquid primary fluid of the molten salt type (core molten salts) as fuel, the device comprising:

[0066] - a trapping tank, comprising trapping molten salts, configured to trap chemical compounds to be captured in the trapping molten salts, the trapping tank delimiting an internal volume, the trapping tank receiving the gases to be treated, the trapping tank comprising a diffuser separating the internal volume into two compartments, with a lower compartment not containing a substantial quantity of trapping molten salt, and a main upper compartment containing the trapping molten salts, the trapping tank being equipped with a system for recirculating the gases being purified, the recirculated gases being introduced into the lower compartment below the diffuser, and diffused upwards through the diffuser into the trapping molten salts, the gases to be treated being introduced into the main upper compartment above the diffuser.

[0067]

[0064] The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the attached figures illustrating variants of the invention.

[0068] Figure 1 shows a general schematic diagram of a molten salt type nuclear reactor in which the present invention can be implemented.

[0069] Figure 2 schematically illustrates a vertical cross-sectional view of the cooling tank.

[0070] Figure 3 schematically illustrates a vertical cross-sectional view of the trapping tank.

[0071] Figure 4 schematically illustrates a horizontal cross-sectional view of the trapping tank.

[0072] Figure 5 schematically illustrates an operation to empty the trapping tank.

[0073] Figure 6 schematically illustrates an operation to replace the diffuser of the trapping tank.

[0074] Figure 7 schematically illustrates an example of a method promoted by the present invention

[0075] Figure 8 shows a detail of an example of a separator pot.

[0073] In the various figures, the same reference numerals designate identical or similar elements. For clarity, some elements are not necessarily shown to scale.

[0076]

[0074] With reference to the figures, a molten salt type nuclear fission reactor is now described with auxiliary equipment for purifying a carrier gas charged with gaseous flows produced by the primary circuit of the reactor.

[0077]

[0075] This concerns an industrial electricity or heat production facility based on a compact nuclear reactor. This facility, designated 100, can be, depending on an option, a transportable unit. For example, the target installed power can be between 50 megawatts and 100 megawatts.

[0078]

[0076] The core of the molten salt fuel reactor is noted as 1. It is hermetically sealed in a vessel 10 lined with a containment structure.

[0079]

[0077] The core of reactor 1 is not particularly pressurized; it is operated substantially at local atmospheric pressure.

[0080]

[0078] The heavy nuclei forming the fissile fuel are mixed with the molten salts. A passive drain tank is provided below to interrupt the chain reaction process in case of an incident. Control rods may also be provided.

[0081]

[0079] The reactor core is considered known in itself, therefore not described in further detail here.

[0082]

[0080] A heat exchanger 12 is provided between a coil of the primary circuit 13 and a secondary circuit 16 also based on molten salts. The downstream part of the power circuit, namely for example the electro-generation units and the general condenser circuits, are not shown in Figure 1.

[0083]

[0081] In other reactor core configurations, not shown in the figure, the secondary circuit fluid can penetrate more intimately into the reactor core and extract heat directly from the contact with the plurality of primary flow channels.

[0084] The primary circuit is equipped with at least one primary circulation pump marked 15.

[0085] It is at this circulation pump 15 that a carrier gas flow is injected, which in the illustrated example is helium. This gas injection flow is denoted F1.

[0086]

[0084] The carrier gas is injected by means of a circulation pump 47, which injects the carrier gas with a pressure close to 3 bar (max 6 bar).

[0087]

[0085] An arrangement is provided at the location of the primary circulation pump 15 that allows the carrier gas to be injected at that location. This may consist of gas conduit elements 14 with associated sealing means around the primary circulation pump 15.

[0088]

[0086] At the outlet of the gas purification device noted 90, there is an outlet flow of purified carrier gas noted F6 which continues its progression in a complementary purification circuit 72, not described in detail here.

[0089]

[0087] The carrier gas circuit thus forms a loop. The carrier gas return loop 18 supplies the circulation pump 47.

[0090]

[0088] The carrier gas circuit arriving in the purification device 90 is noted 11. It can optionally pass through a delay line marked 71. At this point, the carrier gas circuit is substantially at atmospheric pressure or slightly lower.

[0091] In Figure 1, the gas purification device 90 is schematically represented to the right of the reactor core.

[0092] As will be seen below, the gas purification device 90 is designed to purify (also called 'wash' or 'treat') the carrier gas loaded with contaminants taken from the reactor's primary circuit.

[0093]

[0091] The gas purification device includes at least a first tank, called cooling tank 2, for cooling the gases to be treated.

[0094] The first tank receives the gases to be treated from the reactor core, this flow is noted F2, and delivers cooled gases to be treated (flow noted F3) to a second tank.

[0095] The gas purification system includes at least one second tank, referred to as trapping tank 3.

[0096] The trapping tank receives the cooled gases to be treated from the first tank, known as the cooling tank.

[0097] Optionally, an enclosure is provided to enclose the first tank 2 and the second tank 3.

[0098]

[0096] The trapping tank 3 is equipped with a recirculation system 4 for the gases being purified.

[0099]

[0097] The outlet 41 of the trapping tank forms a flow F4 which divides into two parts, on the one hand a recirculated flow denoted F5 and on the other hand an outlet flow denoted F6

[0100] Cooling tank the first tank, called cooling tank 2. Cooling tank 2 delimits an internal volume noted V2.

[0101] The cooling tank has a general shape of revolution around the axis marked X2.

[0102] The cooling tank 2 is generally or partially conical. It has a tangential inlet at the top and an outlet from the bottom via a sump on the axis. The gas generally advances along a cyclonic path 27. This path allows for the capture of condensates 23 in a liquid phase at the base of the tank and the capture of dust particles aggregated on the walls at points 24 and 25.

[0103] The cooling tank is of the double-walled type. The cooling tank comprises an outer wall 20 and an inner wall 21.

[0104] A system for rinsing the cooling tank is planned, using the same trapping salt as the second tank, by siphoning. The tank is at least partially filled with molten rinsing salt via a rinsing pipe 81. The liquid contents of the tank are then siphoned via a siphon line 82.

[0105] Cooling tank 2 is equipped with a temperature maintenance system with heating elements 28 adjacent to the tank, designed to maintain the temperature of the molten salts for rinsing a few tens of degrees above their precipitation temperature. Alternatively, the contents of the cooling tank can be emptied from the bottom via a drain hatch, normally hermetically sealed and possibly double-walled.

[0106] The outlet of the cooling tank is marked 22.

[0107] It is noted that, with regard to gas pressure, the cooling tank 2 does not cause any significant pressure loss, nor does the previously mentioned delay line 71.

[0108] The gases to be treated (flow F2) arrive in the cooling tank at a temperature between 500°C and 900°C. According to a particular design, the gases to be treated (flow F2) arrive at a temperature between 600°C and 700°C.

[0109] Figure 3 illustrates the second tank, called trapping tank 3, which receives the gases to be treated, cooled from the cooling tank via pipe 22.

[0110] The trapping tank has a general shape of revolution around the axis denoted X3. The trapping tank 3 delimits an internal volume denoted V3.

[0111] The trapping tank 3 comprises a tank body 31 and a removable lid 32. The lid 32 is removably mounted on the tank body. Under normal operating conditions, a sealing system 39 ensures a seal between the lid and the tank body. When energy production is stopped, the lid 32 can be lifted or pivoted to allow access for robotic arms to operate inside the tank body.

[0112] The trapping tank is of the double-walled type. In which case, the trapping tank comprises an outer wall 30 and an inner wall 31.

[0113] According to one example of implementation, the trapping tank is made of nickel-based alloy.

[0114] The trapping tank 3 has an internal diameter D3 taken at mid-height. According to a particularly relevant example, D3 < 1 m.

[0115] Trapping tank 3 has a height (excluding lid) noted as H3.

[0116] In general H3 < 2 m, in particular we can choose H3 < 1.5 m.

[0117] According to one example of implementation, the geometry of the trapping tank is such that its internal volume V3 is less than 200 liters.

[0118] In one particular example, the diameter D3 is between 0.4 m and 0.6 m and the height is between 0.8 m and 1.2 m.

[0119] Trapping vessel 3 contains trapping molten salts, labeled 8, the composition of which will be discussed later. It should be noted that these trapping molten salts 8 are distinct from the molten salts present in the reactor core (which can be referred to as core molten salts).

[0120]

[0122] It is noted that the salt filling level is between 60% and 85% of the total volume V3 of the tank. Above the liquid salts 8 is a gaseous headspace of sufficient volume to ensure that the outlet 41 of the trapping tank contains only gas and no liquid. At least one outlet port 53 is provided for the gas to exit the trapping tank.

[0123] The introduction of the cooled gases to be treated is carried out via two bubbling tubes 69. Each bubbling tube 69 is a hollow, curved, arc-shaped tube extending almost in a semicircle within the internal volume of the tank, against the inner wall of the tank. The two bubbling tubes 69 are positioned in the lower part of the tank, just above the diffuser 5. The cooled gases to be treated enter the trapping tank via at least one inlet port 52.

[0121] The hollow tube includes orifices 68 oriented towards the X3 axis or downwards, or obliquely and radially towards the interior of the tank. The orifices 68 can allow small dust particles to pass through and therefore typically have a diameter between 0.1 mm and 1 mm. The two aeration tubes 69 are supplied by an external manifold 67, which connects to the aeration tubes on either side of the tank, at diametrically opposite positions, as shown in Figure 4.

[0122] Of course, other configurations and arrangements for the introduction of gas by bubbling can be considered.

[0123] Diffuser and compartments includes a diffuser 5 which separates the internal volume V3 into two compartments, respectively named lower and upper.

[0124] The lower compartment 3A does not contain a substantial amount of molten salt.

[0125] Furthermore, the trapping tank includes a main upper compartment 3B containing the molten trapping salts.

[0126] The diffuser is presented as a flat disc, with an outer diameter of D5 and a thickness of E5.

[0127] The diameter of the diffuser D5 is between 70% and 98% of said internal diameter D3, preferably between 85% and 96% and even more preferably between 90% and 96% of said internal diameter D3. The diffusion area covers a large part of the volume of the trapping molten salts or even substantially all of the molten salts.

[0128]

[0132] The volume of the lower compartment 3A represents a small percentage of the total volume, for example less than 5% of the total volume V3, and even preferably around 3% of the total volume V3. It is noted that the bottom of the tank is very slightly conical or rounded.

[0129]

[0133] The thickness E5 can be between 1 mm and 10 mm, for example between 3 mm and 10 mm.

[0130] The diffuser includes 56 through-passages. The cross-section of the through-passages is, for example, between 0.1 micrometers 2 and 500 micrometers 2 Depending on one option, the cross-section of the through passages can be between 1 micrometer 2 and 250 micrometers 2 The through passages can be formed as an arrangement of pores in a microporous structure. The cross-section of the passages is chosen according to the maturity of the trapping molten salts.

[0135] The molten salts are loaded into the upper compartment 3B by means of a molten salt inlet pipe 33.

[0131]

[0136] The diffuser rests on a circular shoulder 55 formed in the lower part of the tank.

[0132] Diffuser 5 can be manufactured by sintering from a metallic material (nickel-based steel, stainless steel, etc.). Diffuser 5 can also be manufactured from a ceramic material (boron-, alumina-, or zirconia-based, for example) and produced by agglomerating ceramic powders rather than by sintering. Diffuser 5 can also be coated with a deposit or coating that gives the diffuser impermeability to molten salts (equivalent to "hydrophobic") while also resisting corrosion. This coating allows for minimizing the size of the passages without risking clogging the diffuser.

[0133]

[0138] According to a reference embodiment, the diffuser 5 will be made of inconel (same material as the tank) with a pore / passage size on the order of a micrometer, with a coating making the diffuser impermeable to molten salt.

[0134] Recirculation system

[0135] The trapping tank 3 is equipped with a recirculation system 4 for the gases being purified. This recirculation concerns the gases to be treated; it does not concern the molten trapping salts 8, which remain in the main upper compartment 3B of the trapping tank 3. The recirculation system 4 includes a pump 6 capable of providing the desired gas recirculation. The recirculation pump 6 must compensate, at a minimum, for the hydrostatic pressure induced by the liquid level in the tank and for the pressure drop due to the gases passing through the diffuser passages.

[0136] However, pump 6 must not create excessive pressure which would be detrimental to the arrival of gases from the cooling tank 2 through the inlet pipe 22.

[0137]

[0142] The pressure in the gaseous headspace of the trapping tank can be between 500 mbar and 1200 mbar.

[0138]

[0143] The recirculation system 4 includes a separator pot 42 which acts as a condenser. The recirculation system 4 includes a buffer volume 44.

[0139]

[0144] As shown in Figure 8, the separator pot 42 acts as a demister, preventing any vapors or droplets of molten salt from the trapping system arriving through outlet 41 from progressing downstream beyond the trapping tank. The separator pot 42 collects the condensed vapors and droplets, which are returned to the trapping tank 3 by gravity flow through the dedicated conduit 51. Only the gaseous fraction continues its path to the buffer volume 44.

[0140]

[0145] The recirculation system 4 includes a filter 45.

[0141]

[0146] A flow control valve, designated 9, is provided. At the outlet of the buffer volume, the circuit 43 forms a Y with a recirculation branch on one side and an outlet branch on the other. The outlet valve 9 allows a small fraction of the flow, approximately 10%, to pass through, the remainder of the flow exiting into the recirculation branch. In addition, a control valve 49 is provided downstream of the filter and upstream of the pump 6.

[0142]

[0148] The control of the regulating valve 49 and the outlet flow valve 9 allows for good control of the gas recirculation rate by the flow F5 and the gas outlet rate from the circuit by the flow F6.

[0143] Generally, the fraction representing the outlet flow is between 5% and 50% of the recirculation flow. Depending on the option, the fraction representing the outlet flow can be between 5% and 25% of the recirculation flow, or even between 5% and 15% of the recirculation flow.

[0144]

[0150] The recirculated gases (arriving via the line 46) are introduced into the lower compartment 3A, via at least one recirculation inlet port 54, to be diffused through the diffuser 5 into the trapping molten salts. The recirculated gases pass through the diffuser from bottom to top.

[0145] The recirculated gas flow F5 passes through the through passages 56 and ascends via the passages to the main upper compartment 3B. Conversely, the molten salts 8 in the upper compartment 3B cannot pass through the through passages 56 and descend to the lower compartment 3A.

[0146]

[0152] The trapping tank 3 is equipped with a temperature maintenance system. A thermal coupling system to another fluid via a heat exchanger 37 is provided. This thermal coupling system may, for example, involve the circulation of a fluid in the available space between the inner tank 31 and the outer tank 30. This thermal coupling system is configured to cool the trapping tank and its contents; this system may also be used under certain circumstances to heat the trapping tank and its contents.

[0147]

[0153] A specific heating system based on resistances or by means of induction heating 38 may also be provided.

[0148]

[0154] The trapping tank 3 is equipped with one or more temperature sensors 92 and pressure sensors. Other sensors may be installed, for example chemical composition sensors, etc.

[0149] The sensors are arranged so as not to interfere with the lifting operations of the diffuser 5, either on the sides of the tank, or plunging from the cover 32 and therefore carried with the cover 32 when the latter is opened.

[0150] Figure 5 depicts a draining operation by siphoning the molten salts containing trapped volatile components. A draining system with a suction pipe, labeled 84, is shown. This type of operation is known in itself and therefore not described in detail.

[0151]

[0157] Figure 6 shows an extraction operation by lifting the diffuser 5. A lifting system for the diffuser 5 is provided. For this purpose, the diffuser includes a central hooking element 58.

[0152]

[0158] In addition, a vertical lifting system 57,59 is provided for lifting the diffuser.

[0153] The tank opening has a diameter D1 larger than the diffuser's diameter D5, allowing the diffuser to be completely removed from the tank. Therefore, diffuser 5 is a component that can be replaced between production cycles. Note the diameter hierarchy: D5 < D4 < D3 < D1.

[0154] Figure 7 illustrates steps a, b, c, d of the process as expressed in the present invention: a- providing the trapping tank 3, configured to hold trapping molten salts 8, with a diffuser s separating the internal volume into a lower compartment 3A and an upper compartment 3B, with the aforementioned recirculation system 4,

[0155]

[0163] b- introduce the cooled gases to be treated F3 into the main upper compartment 3B above the diffuser 5, through the bubbling tubes 69, c- introduce the recirculated gases F5 through the recirculation system into the lower compartment 3A, and diffuse them through the diffuser into the trapping molten salts 8.

[0156] Step d- consists of taking a portion of the recirculation flow to form an output flow F6 derived from the recirculation system 4.

[0157] The recirculation rate can be controlled by the pump 6 speed and the valves 49 and 9. The fraction representing the outlet flow is between 5% and 50% of the recirculation flow. Ideally, the outlet flow portion F6 should be close to 20% of the recirculation flow F5 or close to 10% of the recirculation flow.

[0158]

[0167] Regarding the trapping salts 8, according to one embodiment, they are based on NaOH and KOH (hydroxide salts), for example a mixture of the two in substantially equal proportions.

[0159]

[0168] It is understood that the composition of the hydroxide salts is modified as the volatile components to be captured are trapped and can affect the melting temperature of the mixture.

[0160] For example, hydroxide trapping salts may contain a molar mixture of 51% NaOH and 49% KOH. This mixture has a melting point of 172°C. We generally work within a margin of a few tens of degrees relative to the melting point, and consequently the temperature range involved in the proposed application for hydroxide trapping salts is preferably between 200°C and 250°C, given that

[0170] According to an alternative configuration, the trapping salts are based on U2CO3 and / or Na2COs and / or K2CO3 (carbonate salts).

[0161] For example, we can choose a composition like the following, in molar mixture: I₂CO₃ at 43.5%; Na₂CO₃ at 31.5% and K₂CO₃ at 25%. The melting point of this composition is 397 °C.

[0162]

[0172] In this case, the average temperature for maintaining the trapping tank is chosen between 450°C and 550°C.

Claims

DEMANDS 1. Device (90) for purifying gases to be treated from a nuclear reactor core (1) using as fuel a liquid primary fluid of the molten salt type, the device comprising: - a cooling tank (2), receiving a flow of gas to be treated (F2) and capable of cooling the gases to be treated coming from the reactor core, - a trapping tank (3) comprising trapping molten salts, configured to trap, in the trapping molten salts (8), chemical compounds to be captured, the trapping tank delimiting an internal volume (V3), the trapping tank receiving a flow of cooled gases to be treated (F3) from the cooling tank, the trapping tank comprising a diffuser (5) separating the internal volume into two compartments, with a lower compartment (3A) not containing a substantial quantity of trapping molten salt, and a main upper compartment (3B) containing the trapping molten salts, the trapping tank (3) being equipped with a recirculation system (4) for gases being purified, the recirculated gases being introduced into the lower compartment below the diffuser, and diffused upwards into the trapping molten salts through the diffuser,The cooled gases to be treated (F3) from the cooling tank are introduced into the main upper compartment above the diffuser (5).

2. Device according to claim 1, wherein the trapping tank (3) comprises at least one inlet port (52) for admitting the gases to be treated cooled above the diffuser, at least one outlet port (53) for allowing the gases to exit from the trapping tank for recirculation and at least one recirculation inlet port (54) arranged under the diffuser.

3. A device according to any one of claims 1 to 2, wherein the diffuser comprises through passages (56), the cross-section of the through passages being between 0.1 micrometers 2 and 500 micrometers 2 .

4. Device according to any one of claims 1 to 3, wherein the molten trapping salts are based on hydroxide salts, e.g. NaOH and / or KOH.

5. Device according to any one of claims 1 to 3, wherein the molten trapping salts are based on carbonate salts, e.g. Li2COs and / or Na2COs and / or K2CO3 6. Device according to any one of claims 1 to 5, wherein a system for maintaining the temperature of the trapping tank is provided.

7. A device according to any one of claims 1 to 6, wherein the trapping chamber (3) is generally of revolution about an axis (X3) and has an inner diameter D3 taken at mid-height, the diffuser (5) being formed as a disc of outer diameter D5, and D5 being between 70% and 96% of D3 8. Device according to any one of claims 1 to 7, wherein the trapping tank comprises a tank body (31) and a lid (32), the tank body having an upwardly open mouth with a mouth diameter D1 greater than the outside diameter D5 of the diffuser.

9. Device according to any one of claims 1 to 8, wherein the diffuser comprises a central attachment element (58), and wherein a vertical lifting system (57, 59) is provided for lifting the diffuser (5) and removing it from the tank.

10. Device according to any one of claims 1 to 9, wherein the main upper compartment contains the trapping molten salts (8) with a filling rate between 60% and 85% of the internal volume (V3).

11. Device according to any one of claims 1 to 10, wherein the cooled gases to be treated (F3) from the cooling tank are introduced into the main upper compartment, above the diffuser (5) via a plurality of orifices (68) formed on one or more bubbling rods (69).

12. A process for purifying gases to be purified from a nuclear reactor core using a liquid primary fluid of the molten salt type as fuel, the process comprising: a- providing a trapping vessel (3), configured to trap, in trapping molten salts (8), chemical compounds to be captured present in the gas to be purified, the trapping vessel delimiting an internal volume (V3), the trapping vessel comprising a diffuser (5) separating the internal volume into two compartments, with a lower compartment (3A) not containing a substantial quantity of molten salt, and a main upper compartment (3B) containing the trapping molten salts, the trapping vessel (3) being equipped with a recirculation system (4) for the gases being purified, b- introducing the gases to be treated (F3) into the main upper compartment above the diffuser, c- introducing the recirculated gases (F5) by the recirculation system into the lower compartment (3A),and diffuse them through the diffuser into the molten trapping salts.

13. A method according to claim 12, wherein a portion of the recirculation flow is taken to form an outlet flow (F6) derived from the system of recirculation.

14. Method according to claim 13, wherein the fraction representing the outlet flow is between 5% and 50% of the recirculation flow (F5), preferably the portion of the outlet flow (F6) is close to 10% of the recirculation flow (F5).

15. Nuclear reactor using as fuel a liquid primary fluid of the molten core salt type and comprising at least one device according to one of claims 1 to 11 and / or implementing a method according to one of claims 12 to 14.

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