METHOD AND APPARATUS FOR THE DESTRUCTION OF RADIOACTIVE SODIUM

Description

[0001] The invention generally relates to installations for the destruction of radioactive metallic sodium, typically originating from fast neutron nuclear reactors.

[0002] It is possible to arrange such an installation as illustrated in the figure 1 This installation includes a liquid metallic sodium storage tank 3, with a capacity designed for one day of operation of the installation at its nominal capacity.

[0003] The liquid metallic sodium storage tank 3 receives the sodium to be treated and feeds a sodium supply circuit 5 using an electromagnetic pump 7.

[0004] The sodium supply circuit 5 mainly comprises a charging tank 9 and a dosing pump 11. The dosing pump is placed at a level lower than the charging tank 9, and its suction is supplied by gravity from the charging tank 9.

[0005] Sodium from sodium storage tank 3 fills charge tank 9 to a spill 13, which returns excess sodium to sodium storage tank 3.

[0006] The sodium level in the charging tank is thus maintained at a substantially constant level, which guarantees a constant pressure at the suction of the dosing pump 11.

[0007] A sodium filter 15 is inserted on line 17 connecting the electromagnetic pump to the charging tank 9. It allows the cleanliness of the sodium coming from the sodium storage tank 3 to be checked.

[0008] The installation 1 also comprises a reaction tank 19 containing an aqueous sodium hydroxide solution. The sodium discharged by the metering pump 11 is injected from above into the reaction tank 19 and reacts with a jet of sodium hydroxide. The jet of sodium hydroxide comes from a nozzle 21, placed in the reaction tank 19, and supplied by a recirculation circuit 23. This arrangement guarantees a complete reaction of the sodium with the aqueous solution.

[0009] The sodium is thus converted into soda, this reaction being accompanied by a release of gaseous hydrogen. The soda accumulates in the lower part of the reaction tank, the hydrogen being discharged to a hydrogen treatment circuit 25.

[0010] A heat exchanger 27 is inserted in the recirculation circuit 23. It is connected to a cooling unit 29. The heat exchanger 27 allows the thermal energy released by the reaction of the sodium with the aqueous solution to be evacuated.

[0011] Furthermore, a demineralized water supply line 31 is connected to the recirculation circuit 23. Line 31 makes it possible to inject into the reaction tank an adequate quantity of demineralized water to keep the molarity of the soda inside the reaction tank substantially constant.

[0012] The hydrogen treatment circuit 25 is connected to the cover of the reaction vessel 19, and opens into the top of this vessel. It successively comprises a gas scrubber 33, a condenser 35, a heater 37 and a THE (Very High Efficiency) filter 39. The hydrogen treatment circuit, downstream of the THE filter, is connected to a process ventilation 41.

[0013] In process ventilation 41, the hydrogen from the reaction vessel 19 is diluted in a ventilation duct (not shown) to a level that excludes any risk of explosion. The hydrogen content in the gas from the reaction vessel is approximately 100%. In process ventilation, the hydrogen is diluted to a level of approximately 1% during normal operation. The ventilation duct in which the dilution is carried out is equipped with instrumentation to confirm the effectiveness of the dilution, by monitoring both the gas flow rate in the duct and the hydrogen content.

[0014] An inert gas supply 43 is connected to the cover of the reaction vessel 19. This supply is intended to inert the reaction vessel 19 and the hydrogen treatment circuit 25, before starting the sodium conversion reaction. In the event of a shutdown of the installation, this inert gas supply 43 ensures the evacuation of the hydrogen.

[0015] The installation also includes a liquid effluent treatment unit 45. Unit 45 includes a tank for draining and storing the aqueous solution 47. A drain line 49 connects an overflow outlet 51 of the reaction tank 19 to the drain and storage tank 47. A purger 52 is inserted in the drain line 49 immediately downstream of the outlet 51. It makes it possible to separate the hydrogen gas from the soda, the hydrogen gas being returned to the top of the reaction tank 19.

[0016] Furthermore, the drain line 49 comprises a lyre 53, forming a siphon which makes it possible to limit the transfer of hydrogen from the top of the reaction tank 19 to the drain and storage tank 47. A liquid plug is formed in the lyre, blocking the gaseous transfers between the tank 19 and the reservoir 47.

[0017] The drain and storage tank 47 has a large capacity. It can store several days of production from installation 1. It is sized to be able to receive, in addition, all the aqueous solution present in the installation, in particular in the reaction tank 19, in the washer 33 and in the various circuits filled with aqueous solution.

[0018] The liquid effluent treatment unit 45 also comprises a lifting pump 50 and a line 54 provided for filling the reaction tank 19 and the drain line 49 with aqueous solution, from the drain and storage tank 47. The lifting pump 50 also makes it possible to transfer the aqueous solution out of the installation.

[0019] Such an installation is satisfactory for treating significant quantities of liquid metallic sodium. However, it has the disadvantage of being complex and large, so that the treatment cost is excessively high when the daily flow rate to be treated is limited.

[0020] Another installation for the destruction of radioactive metallic sodium is known from document EP 0 245 148 A1.

[0021] In this context, the invention aims to propose an installation which is more economical, and better adapted for a reduced treatment capacity.

[0022] To this end, the invention relates, according to a first aspect, to a facility for destroying radioactive metallic sodium, comprising: a reaction vessel containing an aqueous solution, the reaction vessel having an aqueous solution outlet; a sodium supply circuit configured to supply the reaction vessel with liquid metallic sodium; a liquid effluent treatment unit, comprising a drain tank and a drain line fluidly connecting the aqueous solution outlet to the drain tank; a gas treatment unit configured to dilute the gases and release the diluted gases into the atmosphere, the drain tank having a gas outlet fluidly connected to the gas treatment unit; an inert gas supply unit, configured to supply the drain tank with an inert gas.

[0023] Thus, the top of the drain tank is filled with inert gas.

[0024] Hydrogen from the reaction vessel flowing into the drain tank through the drain line cannot cause an explosion.

[0025] Hydrogen is continuously discharged to the gas treatment unit. This discharge is achieved by the fact that the drain tank is connected to the inert gas supply unit.

[0026] This continuously maintains an overpressure of inert gas in the drain tank and / or continuously ensures a sweep of inert gas in the top of this tank.

[0027] Consequently, it is not necessary to provide a lyre or siphon along the drain line, to constitute a plug of aqueous solution preventing the circulation of hydrogen from the reaction vessel into the drain tank.

[0028] This helps to reduce the overall height of the installation. In the installation of the figure 1 , the lyre has a height dimensioned by the internal pressure of the reaction tank. It thus has a height of several meters, and the building housing the installation must be of sufficient height to allow its installation.

[0029] The total cost of installation is reduced accordingly.

[0030] The installation may also have one or more of the following characteristics, considered individually or in all technically possible combinations: the aqueous solution outlet is by overflow; the installation comprises a control member configured to maintain an inert gas pressure in the drain tank greater than a predetermined minimum; the controller controls the inert gas supply unit to ensure a sweep of inert gas in the drain tank, with an inert gas flow rate within a predetermined range; the drain line is configured to empty entirely by gravity into the drain tank; the installation comprises a hydrogen treatment circuit connected to the reaction tank, equipped with at least one gas scrubber, the drain tank having a storage capacity of between: a minimum volume equal to a nominal volume of aqueous solution contained in the reaction tank plus a nominal volume of aqueous solution contained in the gas scrubber; and a maximum volume equal to the production of soda over a period of 6 hours;the installation comprises an aqueous solution storage tank having a storage capacity greater than one day of production of the installation at nominal treatment capacity; the installation comprises a transfer member having a suction connected to the drain tank and a discharge connected to the aqueous solution storage tank; the drain tank is at least partially located below ground level and the aqueous solution storage tank is located above ground level.;

[0031] According to a second aspect, the invention relates to a method for destroying radioactive metallic sodium, comprising the following steps: supplying liquid metallic sodium to a reaction vessel containing an aqueous solution, the reaction vessel having an aqueous solution outlet; collecting the aqueous solution from the reaction vessel in a drain tank, the aqueous solution flowing through a drain line from the aqueous solution outlet to the drain tank; supplying an inert gas to the drain tank; diluting the gases from the drain tank, and releasing the diluted gases into the atmosphere.

[0032] Other characteristics and advantages of the invention will emerge from the detailed description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which: there figure 1 is a schematic representation of an example of a sodium destruction installation not in accordance with the invention; the figure 2 is a schematic representation of the sodium destruction installation according to the invention; and the figure 3 is an enlarged representation of a detail of the figure 2 , for an alternative embodiment of the sodium destruction installation not in accordance with the invention.

[0033] Installation 55 shown on the figure 2 is intended for the destruction of radioactive sodium, typically from a fast neutron reactor such as Phénix or Super Phénix or from a test loop.

[0034] It is intended more specifically to destroy metallic sodium used as a heat transfer fluid in fast neutron reactors, by conversion into soda.

[0035] This installation uses the NOAH process, which was developed by the Atomic Energy Commission.

[0036] The principle of this process is to inject small quantities of liquid sodium into a high-flow stream of aqueous solution, this operation being carried out inside a sealed tank. Because the reaction is highly exothermic, the aqueous solution is continuously cooled, its temperature being maintained at approximately 40°C.

[0037] The reaction of sodium with water in the aqueous solution produces sodium hydroxide and hydrogen. The sodium hydroxide is rapidly dispersed in the aqueous solution without causing a violent reaction. The sodium hydroxide concentration in the reaction vessel is kept approximately constant by injecting demineralized water. This concentration is continuously adjusted, for example, to ten moles / liter.

[0038] The hydrogen generated is released into the atmosphere, after dilution and purification.

[0039] Installation 55 includes a tank 57 for storing liquid metallic sodium.

[0040] The installation 55 comprises a reaction tank 59 containing an aqueous solution.

[0041] The installation 55 also comprises a sodium supply circuit 61, configured to supply the reaction vessel 59 with liquid metallic sodium.

[0042] The sodium supply circuit 61 comprises a sodium circulation member 63.

[0043] The sodium 63 circulation organ is a pump, and more precisely a metering pump.

[0044] It is suitable for any type. For example, it is a diaphragm pump.

[0045] The circulation member 63 has a suction 65 in fluid communication with the sodium storage tank 57 and a discharge 67 in fluid communication with the reaction vessel 59.

[0046] More specifically, the sodium supply circuit 61 comprises a suction line 69 connecting the suction 65 of the sodium circulation member to the sodium storage tank 57. The circuit 61 also comprises a discharge line 71 connecting the discharge 67 of the sodium circulation member 63 to the reaction tank 59.

[0047] The sodium supply circuit 61 also comprises a sodium return line 73 fluidly connecting the discharge line 71 to the sodium storage tank 57.

[0048] The installation 55 further comprises an inert gas supply unit 75, configured to supply the sodium storage tank 57 with an inert gas.

[0049] The inert gas is typically nitrogen.

[0050] The sodium storage tank 57 is located at a first level above ground.

[0051] As described below, the installation 55 preferably comprises a single level, the equipment resting on a base 77. The ground level considered here corresponds, for example, to the level of the base 77.

[0052] The circulation member 63 is located at a second level relative to the ground, higher than the first level, that is to say higher than the level of the reservoir 57.

[0053] This means that the suction 65 of the circulation member 63 is located higher than the storage tank 57, and in particular higher than the free surface of the liquid metallic sodium stored in the storage tank 57 when the latter is filled to its maximum level.

[0054] The installation 55 advantageously comprises a controller 79, controlling the inert gas supply unit 75 to control an inert gas pressure in the sodium storage tank 57, such that a pressure at the suction 65 of the sodium circulation member 63 is maintained within a predetermined range.

[0055] Maintaining the pressure at the suction of the sodium circulating member within a predetermined range helps ensure that the flow rate of sodium delivered by the circulating member 63 to the reaction vessel 59 is precisely controlled. This is essential to control the reaction of sodium with the aqueous solution and avoid runaway of this reaction.

[0056] The predetermined pressure range is preferably [0, 100] mbar effective and the maximum pressure must not induce siphoning transfer to the reaction vessel, with the pump stopped.

[0057] To do this, the inert gas supply unit 75 comprises an inert gas source 81 fluidly connected by an inert gas line 83 to the storage tank 57.

[0058] It also includes a discharge line 84, fluidly connecting the top of the storage tank 57 to a gas treatment unit not shown.

[0059] The inert gas source 81 is, for example, an inert gas distribution network. Alternatively, it is a high-pressure inert gas storage cylinder.

[0060] The inert gas source 81 supplies the inert gas to the line 83 at a pressure greater than the predetermined pressure range, for example 400 mbar effective. This pressure typically corresponds to the pressure at the suction of the metering pump 63, plus the manometric height between the sodium level in the reservoir 57 and the suction 65 of the circulation member 63.

[0061] The inert gas supply line 83 is connected to a tapping 87 carried by the upper part of the storage tank 57 and opening into the tank roof. A sodium vapor trap 89 is installed on the line 83.

[0062] The discharge line 84 is connected to a tapping 90 carried by the upper part of the storage tank 57 and opening into the tank ceiling. A sodium vapor trap 88 is installed on the line 84. The gas treatment unit allows the gases from the tank 57 to be discharged into the environment, after purification.

[0063] The inert gas supply unit 75 includes a measurement 91 of the gas pressure in the headspace of the storage tank 57. This measurement is of any suitable type. The pressure measurement 91 informs the controller 79 and transmits the measured pressure values ​​to it.

[0064] The sodium storage tank 57 is advantageously equipped with a sodium level measurement 92. This measurement is of any suitable type. The sodium level measurement 92 informs the controller 79 and transmits the measured sodium level values ​​to it.

[0065] Furthermore, the inert gas supply line 83 comprises a control member 93, controlled by the controller 79. The control member 93 is inserted on the line 83, and controls the flow of inert gas delivered to the storage tank 57 by the line 83. Similarly, a control member 94, controlled by the controller 79, is inserted on the discharge line 84.

[0066] For example, control organs 93 and 94 are controlled valves.

[0067] The control organs 93 and 94 are controlled by the controller 79 using the measurement 91 of the gas pressure in the sky of the storage tank 57 and possibly the measurement of the sodium level 92.

[0068] More precisely, the controller 79 controls the control members 93 and 94 so as to maintain the gas pressure in the tank headspace within a determined gas pressure range.

[0069] The controller 79 chooses this gas pressure range as being equal to the pressure range at the suction of the circulation member plus the manometric height between the sodium level in the reservoir 57 and the suction of the circulation member 63.

[0070] If the sodium level in the tank is substantially constant or changes slightly, the controller 79 will consider a substantially constant pressure head to determine the gas pressure range.

[0071] Otherwise, the controller 79 uses the sodium level measurement 92 to determine the manometric height between the sodium level in the reservoir 57 and the suction of the circulation member 63.

[0072] Indeed, the manometric height between the sodium level in the reservoir 57 and the suction of the circulation member 63 varies with the sodium level in the reservoir 57.

[0073] The sodium storage tank 57 is sealed against the inert gas and all the lines opening into the top of the tank 57 are equipped with a cut-off device to prevent leaks of inert gas towards these lines when the circulation device 63 is in operation.

[0074] Advantageously, the suction line 69 is configured to empty entirely by gravity into the sodium storage tank 57.

[0075] In other words, the supply line 69 does not include a siphon, or U-shaped portion, in which the liquid metallic sodium could stagnate when the circulation member 63 is stopped.

[0076] On the contrary, when the circulation device 63 is stopped, all the sodium contained in the line 69 returns by gravity to the storage tank 57.

[0077] Similarly, return line 73 is configured to empty entirely by gravity into sodium storage tank 57.

[0078] In particular, the suction line 69 includes an end portion 95 oriented so as to immerse into the liquid metallic sodium contained in the sodium storage tank 57 through a free surface 97 of the liquid metallic sodium.

[0079] In other words, the end portion 95 of the suction line is a tapping 99 connected to the upper portion of the tank 57 and dipping into the liquid metallic sodium. This tapping 99 is typically substantially vertical.

[0080] The exit of the liquid metallic sodium to the circulation member 63 is therefore via the top of the storage tank 57 and not via the bottom, which helps to reduce the risk of leakage from the tank.

[0081] Installation 55 also includes a line 101 for filling storage tank 57 with sodium.

[0082] This line 101 is connected to a large-capacity storage facility containing the stock of liquid metallic sodium to be destroyed or to a small-capacity emptying facility.

[0083] Typically, the sodium storage tank 57 has a storage capacity greater than or equal to a minimum value equal to one day of treatment capacity of the installation plus the volume of sodium likely to be contained in the supply circuit 61.

[0084] The sodium storage tank 57 preferably has a storage capacity less than or equal to two days of treatment capacity of the installation.

[0085] Advantageously, the sodium storage tank 57 is at least partially located below ground level, which contributes to reducing the overall height of the installation 55.

[0086] The sodium storage tank 57 is a horizontal axis tank, typically a horizontal axis cylindrical tank.

[0087] For example, it has an axial length of around 2.5 m and a section perpendicular to its axis of around 0.5 m 2< .

[0088] Arranging such a tank with its axis horizontal is more advantageous than placing the same tank with its axis vertical.

[0089] In fact, when a given volume of sodium is taken from this reservoir, the level difference is less if the reservoir is arranged with its axis horizontal than if it is arranged with its axis vertical.

[0090] In other words, the section of the tank, taken perpendicular to the vertical direction, is larger when this tank has its horizontal axis than when it has its vertical axis, as long as the volume of liquid sodium stored is greater than a minimum volume.

[0091] This contributes to obtaining a substantially constant pressure at the suction of the circulation member 63.

[0092] The reaction vessel 59 includes a sodium injection nozzle 102, configured to eject sodium downward. This nozzle is connected to the discharge line 71.

[0093] The aqueous solution contained in the reaction tank 59 is typically sodium hydroxide.

[0094] The reaction vessel 59 comprises an aqueous solution ejection nozzle 103, placed substantially below the sodium injection nozzle 102. The aqueous solution ejection nozzle 103 ejects the aqueous solution upwards, such that the jet of aqueous solution meets the sodium injected by the sodium injection nozzle 102.

[0095] The nozzle 103 is supplied by a recirculation and cooling circuit 105. The circuit 105 comprises a conduit 107, one upstream end of which is tapped onto a side wall of the reaction tank 59. The downstream end of the conduit 107 is connected to the nozzle 103. The circuit 105 also comprises a recirculation pump 109 discharging the aqueous solution towards the nozzle 103.

[0096] The circuit 105 also includes an exchanger 111, one side of which is inserted into the conduit 107. The other side is connected to a cooling unit 113, thus making it possible to cool the aqueous solution circulating in the recirculation and cooling circuit 105.

[0097] A demineralized water supply line 115 is connected to the conduit 107. It is connected to a demineralized water distribution network or to a demineralized water reserve. It makes it possible to dilute the aqueous solution contained in the reaction tank 59, so as to maintain the concentration of the soda at a predetermined value. The demineralized water supplied by the line 115 is injected into the conduit 107 and mixed with the aqueous solution circulating in the recirculation and cooling circuit 105.

[0098] The installation 55 further comprises a liquid effluent treatment unit 117, with a drain tank 119 and a drain line 120 fluidly connecting an aqueous solution outlet 121 from the reaction tank 59 to the tank 119.

[0099] The outlet of aqueous solution 121 is typically by overflow.

[0100] In other words, the drain line 120 is connected to an overflow of the reaction vessel 59, the aqueous solution contained in the reaction vessel 59 flowing through the overflow into the line 120 when the level of aqueous solution in the reaction vessel exceeds the level of the overflow 121.

[0101] A purge 123 is interposed along the drain line 120, immediately downstream of the outlet 121.

[0102] Advantageously, the drain line 120 is configured to empty entirely by gravity into the drain tank 119.

[0103] In other words, there is no lyre or siphon along the drain line 120, in which a plug of aqueous solution could form.

[0104] Installation 55 also includes a hydrogen treatment circuit 125, connected to reaction tank 59.

[0105] The hydrogen treatment circuit 125 comprises a hydrogen filter 127, placed inside the reaction tank 59.

[0106] The hydrogen treatment circuit 125 further comprises a gas scrubber 129. A gas inlet of the gas scrubber 129 is fluidically connected to the hydrogen filter 127.

[0107] The hydrogen treatment circuit 125 further comprises a condenser 131, placed downstream of the gas scrubber 129. A gas outlet of the scrubber 129 is connected to a gas inlet of the condenser 131. The condenser 131 is equipped with a coil 132, in which refrigerated water circulates.

[0108] The hydrogen treatment circuit 125 comprises a THE (Very High Efficiency) filter 133, placed downstream of the condenser 131. A gas outlet of the condenser 131 is connected to a gas inlet of the THE filter 133.

[0109] The washer 129 contains a volume of aqueous solution. The washer 129 has an aqueous solution outlet by overflow, connected to the drain line 120 by an outlet line 135 on which a purger 137 is interposed.

[0110] The condenser 131 has at its low point an outlet for the aqueous solution, connected by an outlet line 139 to the drain line 120. A purger is interposed on the outlet line 139.

[0111] Plant 55 also includes a gas treatment unit 141, configured to dilute the gases and release the diluted gases into the atmosphere.

[0112] An outlet of the THE filter is connected to the gas treatment unit 141.

[0113] The gas treatment unit 141 comprises a sheath (not shown) in which the gases from the hydrogen treatment circuit 125 are diluted such that the hydrogen content in the diluted gas is less than a predetermined content excluding any risk of explosion.

[0114] In practice, the gases from the hydrogen treatment circuit 125 comprise, during operation of the installation, approximately 100% hydrogen. In the gas treatment unit 141, these gases are diluted to a hydrogen content of approximately 1%.

[0115] Due to the absence of a lyre in the drain line 120, it is possible for hydrogen to exit the reaction vessel 59 through the aqueous solution outlet 121 and flow to the drain tank 119.

[0116] According to the invention, the drain tank 119 has a gas outlet 143 fluidly connected to the gas treatment unit 141 by a line 155.

[0117] The gas outlet 143 is provided in the upper part of the tank 119 and opens into the roof of the tank 119.

[0118] Furthermore, the inert gas supply unit 75 is configured to supply the drain tank 119 with inert gas.

[0119] The installation comprises a control member 152 configured to maintain an inert gas pressure in the drain tank 119 above a predetermined minimum.

[0120] The control device 152 is a relief valve installed on the line 155 to maintain the top of the tank 119 under inert gas pressure.

[0121] The relief valve 152 is a valve device that regulates the fluid pressure upstream of said relief valve. In other words, the relief valve is a restriction controlled by the upstream pressure level.

[0122] The overflow valve 152 is calibrated to maintain a pressure in the tank 119 greater than said minimum. Said minimum is between 10 and 100 mbar effective, preferably between 20 and 60 mbar effective and is for example 40 mbar effective.

[0123] Furthermore, the controller 79 is configured to control the inert gas supply unit 75 to ensure a sweep of inert gas inside the tank 119, with an inert gas flow rate within a predetermined range.

[0124] The drain tank 119 has for this purpose an inert gas inlet 145, arranged in the upper part of the tank. This inlet 145 is connected to the inert gas source 81 by a line 147 on which an adjustment member 149 is inserted.

[0125] The regulating member 149 controls the flow of inert gas delivered to the drain tank 119 via the line 147. It is controlled by the controller 79.

[0126] The adjustment member 149 is for example an adjustable valve.

[0127] Furthermore, the drain tank 119 is equipped with a gas pressure measurement 151. The measurement 151 provides the measured pressure values ​​to the controller 79.

[0128] Furthermore, the liquid effluent treatment unit 117 is preferably equipped with a flow meter 153 configured to measure the flow rate of gas circulating in the ceiling of the tank 119.

[0129] The controller 79 controls the control member 149 as a function of the flow rate values ​​measured by the flow meter 153, so as to maintain the gas flow rate within a predetermined range.

[0130] The predetermined gas flow rate range is for example [0.1; 10] Nm3 / h, preferably [0.2; 5] Nm3 / h, more preferably [0.5; 1] Nm3 / h.

[0131] The controller 79 is preferably programmed to adapt the gas flow rate to the flow rate of soda transfer to the soda storage tank 157, which will be described later. The gas flow rate is thus increased to maintain the pressure in the tank head by compensating for the increase in gas volume linked to the soda transfer.

[0132] According to the variant illustrated on the figure 2 , the flow meter is installed on line 147. In this case, the controller 79 is programmed to control the control member 149 not only using the measurement provided by the flow meter 153 but also using the pressure measurement 151.

[0133] For example, the degree of opening O of the control member 149 is determined using the following equation: O = K 1 * P mes − P ref + K 2 * Q mes − Q ref + K 3

[0134] Where K1, K2 and K3 are predetermined constants, P ref is a reference value for the gas pressure in the tank 119, Q ref is a reference value for the gas flow rate entering the head of the tank 119, P mes is the value provided by the gas pressure measurement 151, Q mes is the value provided by the flow meter 153.

[0135] Alternatively, the flow meter 153 is installed so as to measure the gas flow rate in the line 155 connecting the gas outlet 143 of the tank to the gas treatment unit 141.

[0136] In this configuration, the controller 79 controls the control member 149 using the measurement provided by the value provided by the flow meter 153, without using the pressure measurement 151.

[0137] Thus, the top of the drain tank is filled with inert gas.

[0138] Hydrogen from the reaction vessel 59 and flowing in an incident situation into the drain tank 119 via the drain line 120 cannot cause an explosion.

[0139] It is continuously evacuated to the gas treatment unit 141. This evacuation is obtained because the drain tank 119 is connected to the inert gas supply unit 75. The latter continuously maintains an overpressure of inert gas in the drain tank 119 and continuously ensures a sweep of inert gas in the ceiling of this tank.

[0140] Keeping the drain tank 119 pressurized helps reduce the risk of air entering the tank.

[0141] The drain tank 119 has a storage capacity of between: a minimum volume equal to a nominal volume of aqueous solution contained in the reaction tank 59, plus a nominal volume of aqueous solution contained in the gas scrubber 129; and a maximum volume equal to the production of soda over a period of 6 hours.

[0142] In other words, the drain tank 119 is sized to receive the volume of aqueous solution contained in the reaction tank 59, and the volume of aqueous solution contained in the recirculation and cooling circuit 105, in the drain line 120 and in the hydrogen treatment circuit 125. It is not sized, however, to store the volume of aqueous solution resulting from the operation of the installation 55 for a significant period of time, for example a day.

[0143] For this purpose, the installation 55 comprises an aqueous solution storage tank 157, having a storage capacity greater than one day of production of the installation at nominal treatment capacity.

[0144] For example, aqueous solution storage tank 157 has a storage capacity of one week of production at nominal processing capacity.

[0145] If necessary, the installation includes several aqueous solution storage tanks.

[0146] For example, the drain tank 119 has a storage capacity of the order of 1 m 3< of aqueous solution. The installation 55 also comprises three aqueous solution storage tanks 157, each with a volume of 30 m 3<.

[0147] The storage tank 157 has an air inlet 158, through which the sky of the storage tank 157 communicates with the atmosphere of the room where the tank 157 is located. The storage tank 157 also has a gas outlet 159 opening into the sky of the tank 157. This gas outlet 159 is connected to the ventilation system 160 of the building. This ventilation system is of the conventional type. It is different from the gas treatment unit 141 to which the gases from the process, containing hydrogen, are directed.

[0148] The drain tank 119 is at least partially located below ground level. The or each aqueous solution storage tank 157 is, on the other hand, located above ground level.

[0149] The installation 55 further comprises a transfer member 161 having a suction fluidly connected to the drain tank 119 and a discharge fluidly connected to the aqueous solution storage tank 157.

[0150] Using a small capacity drain tank, at least partially buried, and a large capacity aqueous solution storage tank, located above ground, helps to reduce the overall height of the installation.

[0151] It then becomes possible to arrange the 55 installation on a single level.

[0152] In the installation illustrated on the figure 1 , the equipment is arranged on two levels. The sodium storage tank 3 and the aqueous solution drain and storage tank 47 are arranged at ground level, with the reaction tank 19 located at an upper level.

[0153] As a result, the cost of Civil Engineering for the sodium destruction facility 55 of the figure 2 is reduced.

[0154] The inert gas supply unit 75 comprises a line 162 connected to the top of the reaction tank 59. This line is intended to inert the reaction tank and the hydrogen treatment circuit, before starting the installation.

[0155] Installation 55 also includes a circuit 163 supplying coil 132 with chilled water. Circuit 163 is connected to a cooling unit 165.

[0156] A variant of installation 55 will now be described, with reference to the figure 3 . Only the points through which the installation of the figure 3 differs from the installation of the figure 2 will be detailed below.

[0157] Identical elements or those providing the same function will be designated by the same references.

[0158] In the variant of the figure 3 , not in accordance with the invention, the aqueous solution outlet 121 from the reaction tank 59 is not an overflow outlet. The aqueous solution outlet 121 is submerged, and opens below a nominal level of aqueous solution in the reaction tank 59.

[0159] Typically, the aqueous solution outlet 121 is a tapping, provided in a side wall 173 of the reaction tank 59, below the nominal level of aqueous solution.

[0160] In this case, the reaction tank 59 advantageously comprises an aqueous solution level measurement 175. This level measurement is of any suitable type. It measures the level of aqueous solution in the reaction tank 59. It transmits the measured values ​​to the controller 79.

[0161] The drain line 120 also includes a regulating shut-off member 177.

[0162] The shut-off member 177 is inserted on the drain line 120, and makes it possible to modulate the passage section offered to the aqueous solution circulating in the drain line 120.

[0163] The shut-off member 177 is, for example, a regulating valve.

[0164] The controller 79 controls the shut-off member 177 using the level values ​​measured by the level measurement 175. The controller 79 controls the shut-off member 177 to maintain the level of aqueous solution in the reaction vessel 59 within a predetermined range.

[0165] When the aqueous solution level is within the predetermined range, the aqueous solution outlet 121 is flooded.

[0166] Accordingly, it is not necessary to connect the drain tank 119 to the inert gas supply unit 75. It is also not necessary to connect the drain tank 119 to the gas treatment unit 141.

[0167] Indeed, because the outlet 121 is constantly flooded, there is no hydrogen circulating from the reaction vessel 59 to the drain tank 119 via the drain line 120.

[0168] The drain tank 119, on the other hand, comprises a gas inlet 179 putting the top of the storage tank 119 into fluid communication with the atmosphere 181 of the room in which the tank 119 is located.

[0169] The drain tank 119 also includes a gas outlet 183 connecting the roof of the tank 119 with the building ventilation 160.

[0170] According to a second aspect, the invention relates to a method for destroying radioactive sodium, typically originating from a fast neutron reactor.

[0171] This process is specially adapted to the installation 55 described above.

[0172] Conversely, installation 55 is specially designed for the implementation of the process which will be described.

[0173] The process involves the following steps: Supplying liquid metallic sodium to a reaction vessel 59 containing an aqueous solution, the reaction vessel having an aqueous solution outlet 121; Collecting the aqueous solution from the reaction vessel 59 in a drain tank 119, the aqueous solution flowing through a drain line 120 from the aqueous solution outlet 121 to the drain tank 119; Supplying an inert gas to the drain tank 119; Diluting the gases from the drain tank 119 and releasing the diluted gases into the atmosphere.

[0174] The reaction vessel 59 is as described above with reference to the figure 2 .

[0175] The outlet of aqueous solution 121 is by overflow, or in other words by overflow. This outlet was described above with reference to the figure 2 .

[0176] The drain tank 119 is as described above, with reference to the figure 2 . The drain line 120 is as described above with reference to the figure 2 .

[0177] The drain tank 119 is supplied with inert gas as described above.

[0178] The gases from the drain tank 119 are directed to a gas treatment unit 141, of the type described above.

[0179] The gases from the drain tank 119 are likely to contain hydrogen, because the drain tank 119 is connected to a liquid outlet by overflow from the reaction tank 59. In the gas treatment unit 141, the gases from the drain tank 119 are diluted with a dilution rate ensuring that the hydrogen content after dilution is excluded from any risk of explosion.

Claims

1. A radioactive metal sodium destruction installation, the installation (55) comprising: - a reaction vessel (59) containing an aqueous solution, the reaction vessel (59) having an aqueous solution outlet (121); - a sodium feed circuit (61) configured for feeding liquid metallic sodium into the reaction vessel (59); - a liquid effluent treatment unit (117), comprising a drain tank (119) and a drain line (120) fluidically connecting the aqueous solution outlet (121) to the drain tank (119); - a gas treatment unit (141) and an inert gas feed unit (75); characterized in that: - the gas treatment unit (141) is configured for diluting the gases and releasing the diluted gases into the atmosphere, the drain tank (119) having a gas outlet (143) fluidically connected to the gas treatment unit (141); - the inert gas feed unit (75) is configured for feeding an inert gas into the drain tank (119).

2. The installation according to claim 1, wherein the aqueous solution outlet (121) is an overflow outlet.

3. The installation according to claim 1 or 2, wherein the installation comprises a control member (152) configured for maintaining an inert gas pressure in the drain tank (119) above a predetermined minimum.

4. The installation according to any preceding claim, wherein a controller (79) controls the inert gas feed unit (75) so as to provide an inert gas sweeping in the drain tank (119) with an inert gas flow-rate within a predetermined range.

5. The installation according to any of claims 1 to 4, wherein the drain line (120) is configured for emptying completely into the drain tank (119), by gravity.

6. The installation according to any of claims 1 to 5, wherein the installation comprises a hydrogen treatment circuit (125) connected to the reaction vessel (59), equipped with at least one gas scrubber (129), the drain tank (119) having a storage capacity comprised between: - a minimum volume equal to a nominal volume of aqueous solution contained in the reaction vessel (59) plus a nominal volume of aqueous solution contained in the gas scrubber (129); and - a maximum volume equal to the production of sodium hydroxide over a period of 6 hours.

7. The installation according to any of claims 1 to 6, wherein the installation comprises an aqueous solution storage tank (157) having a storage capacity greater than one production day of the installation at nominal treatment capacity.

8. The installation according to claim 7, wherein the installation comprises a transfer member (161) having an intake connected to the drain tank (119) and a discharge connected to the aqueous solution storage tank (157).

9. The installation according to claim 8, wherein the drain tank (119) is at least partially located below ground level and the aqueous solution storage tank (157) is located above ground level.

10. A process for the destruction of sodium in a fast-neutron nuclear reactor, the process comprising the following steps: - feeding liquid metallic sodium into a reaction vessel (59) containing an aqueous solution, the reaction vessel (59) having an aqueous solution outlet (121); - collecting the aqueous solution from the reaction vessel (59) in a drain tank (119), the aqueous solution flowing through a drain line (120) from the aqueous solution outlet (121) right to the drain tank (119); - feeding an inert gas into the drain tank (119); - diluting the gases coming from the drain tank (119), and releasing the diluted gases into the atmosphere.