Patent Publication Number: US-2023139794-A1

Title: Process and installation for the destruction of radioactive sodium

Description:
The present disclosure relates in general to facilities for the destruction of radioactive metallic sodium, typically from fast breeder nuclear reactors. 
     BACKGROUND 
     Such a facility can be arranged as shown in  FIG.  1   . This facility comprises a storage tank for liquid metallic sodium  3 , with a capacity designed for one days operation of the facility at its nominal capacity. 
     The liquid metallic sodium storage tank  3  receives the sodium to be treated and feeds it into a sodium feed circuit  5  by means of an electromagnetic pump  7 . 
     The sodium feed circuit  5  mainly comprises a charge tank  9  and a dosing pump  11 . The dosing pump is placed at a lower level than the charge tank  9 , and its suction is fed by gravity from the charge tank  9 . 
     Sodium from the sodium storage tank  3  fills the charge tank  9  up to an overflow  13 , which returns the excess sodium to the sodium storage tank  3 . 
     The level of sodium in the charge tank is thus maintained at a substantially constant level, which ensures a constant pressure at the suction of the dosing pump  11 . 
     A sodium filter  15  is interposed on the line  17  connecting the electromagnetic pump to the charge tank  9 . It is used to monitor the cleanliness of sodium from the sodium storage tank  3 . 
     The facility  1  further comprises a reaction vessel  19  containing an aqueous soda ash solution. The sodium delivered by the dosing pump  11  is injected from above into the reaction vessel  19  and reacts with a stream of soda ash. The soda ash jet comes from a nozzle  21 , placed in the reaction vessel  19 , and fed by a recirculation circuit  23 . This arrangement ensures a complete reaction of the sodium with the aqueous solution. 
     The sodium is thus converted into soda ash, with the release of hydrogen gas. The soda ash accumulates in the lower part of the reaction vessel, the hydrogen being discharged to a hydrogen processing circuit  25 . 
     A heat exchanger  27  is interposed on 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 removed. 
     In addition, a demineralised water supply line  31  is connected to the recirculation circuit  23 . The line  31  allows an adequate amount of demineralised water to be injected into the reaction vessel to keep the molarity of the soda ash inside the reaction vessel substantially constant. 
     The hydrogen processing circuit  25  is connected to the lid of the reaction vessel  19 , and opens into the head of this vessel. It comprises successively a gas scrubber  33 , a condenser  35 , a heater  37  and a VHE (Very High Efficiency) filter  39 . The hydrogen processing circuit, downstream of the VHE filter, is connected to a process ventilation  41 . 
     In the process ventilation  41 , the hydrogen from the reaction vessel  19  is diluted in a ventilation shaft (not shown) to a level that excludes the risk of explosion. The hydrogen content in the gas from the reaction vessel is approximately 100%. In the process ventilation, the hydrogen is diluted to about 1% in normal operation. The ventilation shaft in which the dilution is carried out is equipped with instrumentation to confirm the effectiveness of the dilution, by monitoring both the gas flow in the shaft and the hydrogen content. 
     An inert gas supply  43  is connected to the lid of the reaction vessel  19 . This supply is provided to inert the reaction vessel  19  and the hydrogen processing circuit  25 , prior to starting the sodium conversion reaction. In the event of a shutdown, this inert gas supply  43  ensures that the hydrogen is removed. 
     The facility further comprises a liquid effluent treatment unit  45 . The unit  45  has a tank for draining and storing the aqueous solution  47 . A drain line  49  connects an overflow outlet  51  of the reaction vessel  19  to the drain and storage tank  47 . A drain valve  52  is interposed in the drain line  49  immediately downstream of the outlet  51 . This separates the hydrogen gas from the soda ash, with the hydrogen gas being returned to the reaction vessel  19 . 
     In addition, the drain line  49  includes a lyre shape  53 , forming a siphon which allows the transfer of hydrogen from the head of the reaction vessel  19  to the drain and storage tank  47  to be limited. A liquid plug is formed in the lyre shape, blocking gas transfers between the vessel  19  and the tank  47 . 
     The drain and storage tank  47  is of large capacity. It can store several days of production from the facility  1 . It is sized to accommodate, in addition, all the aqueous solution present in the facility, particularly in the reaction vessel  19 , in the scrubber  33  and in the various circuits filled with aqueous solution. 
     The liquid effluent treatment unit  45  further comprises a lift pump  50  and a line  54  for filling the reaction vessel  19  and the drain line  49  with aqueous solution from the discharge and storage tank  47 . The lift pump  50  also allows the aqueous solution to be transferred out of the facility. 
     Such a facility is satisfactory for processing 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 to be treated is limited. 
     SUMMARY 
     In this context, the present disclosure aims to provide a more economical facility, better adapted to a reduced treatment capacity. 
     To this end, the present disclosure provides a radioactive sodium destruction facility, the facility comprising: a liquid metallic sodium storage tank, located at a first level with respect to the ground; a reaction vessel containing an aqueous solution; a sodium feed circuit, comprising a sodium circulation member located at a second level with respect to the ground higher than the first level, the circulation member having a suction in fluid communication with the sodium storage tank and a discharge in fluid communication with the reaction vessel; an inert gas supply unit, configured to supply the sodium storage tank with inert gas; a controller, driving the inert gas supply unit to control a gas pressure in the sodium storage tank such that a suction pressure of the sodium circulating member is maintained within a predetermined range. 
     Thus, the circulation member is fed directly from the liquid metallic sodium storage tank, which is located at a lower level than the sodium circulation member. The suction pressure of the sodium circulation member is kept substantially constant by controlling the inert gas pressure inside the sodium storage tank. 
     This makes it possible to do away with the charge tank, which in the system shown in  FIG.  1    was used to supply the suction side of the dosing pump by gravity. As a result, the sodium feed circuit can be considerably simplified and the volume of the facility is reduced. The total cost of the facility is reduced accordingly. 
     The facility may furthermore exhibit one or more of the following features, taken individually or in any combination that is technically possible:
         the predetermined pressure range is [0; 100] mbar rms;   the sodium circulation member is a dosing pump;   the sodium feed circuit comprises a suction line connecting the suction of the sodium circulation member to the sodium storage tank, the suction line being configured to drain entirely by gravity into the sodium storage tank;   the sodium feed circuit comprises a discharge line connecting the discharge of the sodium circulation member to the reaction vessel and a sodium return line fluidly connecting the discharge line to the sodium storage tank, the sodium return line being configured to drain entirely by gravity into the sodium storage tank;   the suction line comprises an end portion oriented to dip into the liquid metallic sodium contained in the sodium storage tank through a free surface of the liquid metallic sodium;   the sodium storage tank has a horizontal axis;   the sodium storage tank is at least partially below ground level;   the sodium storage tank has a storage capacity greater than or equal to a minimum value equal to one day&#39;s processing capacity of the facility plus a volume of sodium likely to be contained in the sodium feed circuit.       

     The present disclosure also provides a method for the destruction of radioactive sodium, typically from a fast neutron reactor, the method comprising the following steps: transferring the liquid metallic sodium to be treated into a liquid metallic sodium storage tank located at a first level with respect to the ground; feeding sodium to a reaction vessel containing an aqueous solution via a sodium feed circuit, the sodium feed circuit comprising a sodium circulating member located at a second level with respect to the ground higher than the first level, the sodium circulating member having a suction in fluid communication with the sodium storage tank and a discharge in fluid communication with the reaction vessel; supplying the sodium storage tank with inert gas; maintaining a pressure at the suction of the sodium circulation member within a predetermined range by controlling a gas pressure in the sodium storage tank. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       Further features and advantages of the present disclosure will be apparent from the detailed description given below, by way of indication and not in any way limiting, with reference to the appended figures, among which: 
         FIG.  1    is a diagram of an example sodium destruction facility not according to the present disclosure; 
         FIG.  2    is a diagram of an embodiment of the sodium destruction facility according to the present disclosure; and 
         FIG.  3    is an enlarged depiction of a detail of  FIG.  2   , for a variant embodiment of the sodium destruction facility. 
     
    
    
     DETAILED DESCRIPTION 
     The facility  55  shown in  FIG.  2    is designed for the destruction of radioactive sodium, typically from a fast neutron reactor such as Phenix or Super Phenix or from a test loop. 
     It is intended more specifically to destroy the metallic sodium used as a coolant in fast neutron reactors, by conversion into soda ash. 
     This facility uses the NOAH process, which was developed by the French Atomic Energy Commission. 
     The principle of this process is to inject small quantities of liquid sodium into a stream of high flow rate aqueous solution, this operation being carried out inside a sealed tank. Because the reaction is highly exothermic, the aqueous solution is cooled continuously, its temperature being kept at about 40° C. 
     The reaction of sodium with the water in the aqueous solution produces soda ash and hydrogen. The soda ash is rapidly dispersed in the aqueous solution, without causing a violent reaction. The concentration of soda ash in the reaction vessel is kept substantially constant by injection of demineralised water. This concentration is continuously adjusted to e.g. ten moles/litre. 
     The hydrogen generated is released into the atmosphere, after dilution and purification. 
     The facility  55  comprises a storage tank  57  for liquid metallic sodium and a reaction vessel  59  containing an aqueous solution. 
     The facility  55  further comprises a sodium feed circuit  61 , comprising a sodium circulation member  63 . 
     The sodium circulation member  63  is a pump, more precisely a dosing pump. 
     It is of any suitable type. For example, it is a membrane pump. 
     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 . 
     More specifically, the sodium feed 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  further comprises a discharge line  71  connecting the discharge  67  of the sodium circulation member  63  to the reaction vessel  59 . 
     The sodium feed circuit  61  further comprises a sodium return line  73  fluidly connecting the discharge line  71  to the sodium storage tank  57 . 
     The facility  55  further comprises an inert gas supply unit  75 , configured to supply the sodium storage tank  57  with an inert gas. 
     The inert gas is typically nitrogen. 
     The sodium storage tank  57  is located at a first level above the ground. 
     As described below, the facility  55  is preferably single-level, with the equipment resting on a floor slab  77 . The ground level considered here is, for example, the level of the floor slab  77 . 
     The circulation member  63  is located at a second level above the ground, higher than the first level, i.e. higher than the level of the tank  57 . 
     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 it is filled to its maximum level. 
     Advantageously, the facility  55  comprises a controller  79 , driving 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. 
     Maintaining the suction pressure of the sodium circulation member within a predetermined range helps to ensure that the flow rate of sodium delivered by the circulation member  63  to the reaction vessel  59  is accurately controlled. This is essential to control the reaction of the sodium with the aqueous solution and to prevent the reaction from getting out of control. 
     The predetermined pressure range is preferably [0, 100] mbar rms and the maximum pressure should not induce siphoning to the reaction vessel when the pump is off. 
     For this purpose, 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 . 
     It further comprises a disposal line  84 , fluidly connecting the head of the storage tank  57  to a gas treatment unit not shown. 
     The inert gas source  81  is for example an inert gas distribution network. Alternatively, it is a high-pressure inert gas storage cylinder. 
     The inert gas source  81  supplies the inert gas to the line  83  at a pressure above the predetermined pressure range, for example 400 mbar rms. This pressure typically corresponds to the suction pressure of the dosing pump  63 , plus the head between the sodium level in the tank  57  and the suction  65  of the circulation member  63 . 
     The inert gas supply line  83  is connected to a spigot  87  carried by the upper part of the storage tank  57  and opening into the tank head. A sodium vapour trap  89  is installed on line  83 . 
     The disposal line  84  is connected to a spigot  90  carried by the upper part of the storage tank  57  and opening into the tank head. A sodium vapour trap  88  is installed on line  84 . The gas treatment unit allows the gases from tank  57  to be released into the environment after purification. 
     The inert gas supply unit  75  includes a gauge  91  measuring the gas pressure in the headspace of the storage tank  57 . This gauge is of any suitable type. The pressure measurement  91  informs the controller  79  and transmits the measured pressure values to it. 
     The sodium storage tank  57  is advantageously equipped with a sodium level gauge  92 . This gauge is of any suitable type. The sodium level gauge  92  informs the controller  79  and transmits the measured pressure values to it. 
     In addition, the inert gas supply line  83  includes a control member  93 , driven by the controller  79 . The control unit  93  is interposed on the line  83 , and controls the flow of inert gas delivered to the storage tank  57  by the line  83 . Similarly, a control unit  94 , driven by the controller  79 , is interposed on the disposal line  84 . 
     For example, the control members  93  and  94  are controlled valves. 
     The control members  93  and  94  are driven by the controller  79  using the gas pressure measurement  91  in the headspace of the storage tank  57  and possibly the sodium level gauge  92 . 
     Specifically, the controller  79  drives the control members  93  and  94  to maintain the gas pressure in the tank head within a specified gas pressure range. 
     The controller  79  selects this gas pressure range as being equal to the pressure range at the suction of the circulation member plus the head between the sodium level in the tank  57  and the suction of the circulation member  63 . 
     If the sodium level in the tank is essentially constant or changes slightly, the controller  79  will assume an essentially constant head to determine the gas pressure range. 
     Otherwise, the controller  79  uses the sodium level gauge  92  to determine the head between the sodium level in the tank  57  and the suction of the circulation member  63 . 
     Indeed, the head between the level of sodium in the tank  57  and the suction of the circulation member  63  varies with the level of sodium in the tank  57 . 
     The sodium storage tank  57  is inert gas-tight and all the lines leading into the head of the tank  57  are equipped with a shut-off device to prevent leakage of inert gas into these lines when the circulation member  63  is in operation. 
     Advantageously, the suction line  69  is configured to empty entirely by gravity into the sodium storage tank  57 . 
     In other words, the feed line  69  does not have a siphon, or U-shaped portion, in which the liquid metallic sodium could stagnate when the circulation member  63  is stopped. 
     On the contrary, when the circulation member  63  is stopped, all the sodium contained in the line  69  returns by gravity to the storage tank  57 . 
     Likewise, the return line  73  is configured to empty entirely by gravity into the sodium storage tank  57 . 
     In particular, the suction line  69  comprises an end portion  95  oriented to dip into the liquid metallic sodium contained in the sodium storage tank  57  through a free surface  97  of the liquid metallic sodium. 
     In other words, the end portion  95  of the suction line is a spigot  99  connected to the top of the tank  57  and dipping into the liquid metallic sodium. This spigot  99  is typically substantially vertical. 
     The liquid metallic sodium therefore exits to the circulation member  63  from the top of the storage tank  57  and not from the bottom, which helps to reduce the risk of the tank leaking. 
     The facility  55  further comprises a line  101  for filling the storage tank  57  with sodium. 
     This line  101  is connected to a large capacity storage containing the liquid metallic sodium stock to be destroyed or to a small capacity-draining facility. 
     Typically, the sodium storage tank  57  has a storage capacity greater than or equal to a minimum value equal to one day&#39;s processing capacity of the facility plus the volume of sodium likely to be contained in the feed circuit  61 . 
     The sodium storage tank  57  preferably has a storage capacity less than or equal to two days&#39; processing capacity of the facility. 
     Advantageously, the sodium storage tank  57  is at least partially located below ground level, which helps to reduce the overall height of the facility  55 . 
     The sodium storage tank  57  is a horizontal axis tank, typically a cylindrical horizontal axis tank. 
     For example, it has an axial length of around 2.5 m and a cross-section perpendicular to its axis of around 0.5 m 2 . 
     Placing such a tank with its axis horizontal is more advantageous than placing the same tank with its axis vertical. 
     Indeed, when a given volume of sodium is drawn from this tank, the difference in level is less if the tank is arranged with its axis horizontal than if it is arranged with its axis vertical. 
     In other words, the cross-sectional area of the tank, taken perpendicular to the vertical direction, is larger when the tank has its horizontal axis than when it has its vertical axis, as long as the volume of stored liquid sodium is greater than a minimum volume. 
     This helps to achieve a substantially constant pressure at the suction of the circulation member  63 . 
     The reaction vessel  59  comprises a sodium injection nozzle  102 , configured to eject sodium downwards. This nozzle is connected to the discharge line  71 . 
     The aqueous solution in the reaction vessel  59  is typically soda ash. 
     The reaction vessel  59  has an aqueous solution ejection nozzle  103 , positioned substantially below the sodium injection nozzle  102 . The aqueous solution ejection nozzle  103  ejects the aqueous solution upwards, so that the jet of aqueous solution meets the sodium injected by the sodium injection nozzle  102 . 
     The nozzle  103  is fed by a recirculation and cooling circuit  105 . The circuit  105  comprises a conduit  107 , an upstream end of which is stitched to a side wall of the reaction vessel  59 . The downstream end of the conduit  107  is connected to the nozzle  103 . The circuit  105  further comprises a recirculation pump  109  delivering the aqueous solution to the nozzle  103 . 
     The circuit  105  further comprises a heat exchanger  111 , one side of which is interposed on the conduit  107 . The other side is connected to a cooling unit  113 , thus allowing the aqueous solution circulating in the recirculation and cooling circuit  105  to be cooled. 
     A demineralised water supply line  115  is connected to the conduit  107 . It is connected to a demineralised water distribution network or to a demineralised water reserve. It allows the aqueous solution in the reaction vessel  59  to be diluted so as to maintain the concentration of the soda ash at a predetermined value. The demineralised 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 . 
     The facility  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 vessel  59  to the tank  119 . 
     The aqueous solution outlet  121  is typically by overflow. 
     In other words, the drain line  120  is connected to an overflow of the reaction vessel  59 , the aqueous solution 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 
     A drain  123  is interposed along the drain line  120 , immediately downstream of the outlet  121 . 
     Advantageously, the drain line  120  is configured to empty entirely by gravity into the drain tank  119 . 
     In other words, there is no lyre shape or siphon along the drain line  120  in which a plug of aqueous solution could build up. 
     The facility  55  further comprises a hydrogen processing circuit  125 , connected to the reaction vessel  59 . 
     The hydrogen processing circuit  125  comprises a hydrogen filter  127 , placed inside the reaction vessel  59 . 
     The hydrogen processing circuit  125  further comprises a gas scrubber  129 . A gas inlet of the gas scrubber  129  is fluidly connected to the hydrogen filter  127 . 
     The hydrogen processing circuit  125  further comprises a condenser  131 , located 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 chilled water circulates. 
     The hydrogen processing circuit  125  comprises a VHE (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 VHE filter  133 . 
     The scrubber  129  contains a volume of aqueous solution. The scrubber  129  comprises an overflow aqueous solution outlet connected to the drain line  120  by an outlet line  135  with a drain valve  137  interposed. 
     The condenser  131  has an outlet for the aqueous solution at the bottom, connected by an outlet line  139  to the drain line  120 . A drain valve is interposed on the outlet line  139 . 
     The facility  55  further comprises a gas treatment unit  141 , configured to dilute the gases and release the diluted gases to the atmosphere. One outlet of the VHE filter is connected to the gas treatment unit  141 . 
     The gas treatment unit  141  comprises a duct (not shown) in which the gases from the hydrogen processing circuit  125  are diluted so that the hydrogen content in the diluted gas is below a predetermined level that excludes any risk of explosion. 
     In practice, the gases from the hydrogen processing circuit  125  contain, during the operation of the facility, approximately 100% hydrogen. In the gas treatment unit  141 , these gases are diluted to a hydrogen content of approximately 1%. 
     Due to the absence of a lyre shape in the drain line  120 , it is possible for hydrogen to flow out of the reaction vessel  59  through the aqueous solution outlet  121  and into the drain tank  119 . 
     Advantageously, the drain tank  119  has a gas outlet  143  fluidly connected to the gas treatment unit  141  by a line  155 . 
     The gas outlet  143  is provided in the upper part of the tank  119  and opens into the headspace of the tank  119 . 
     Furthermore, the inert gas supply unit  75  is configured to supply inert gas to the drain tank  119 . 
     The facility comprises a control member  152  configured to maintain an inert gas pressure in the drain tank  119  above a predetermined minimum. 
     The control member  152  is a spillway located on the line  155  to keep the headspace of the tank  119  at an inert gas pressure. 
     The spillway  152  is a fitting that regulates the pressure of the fluid upstream of the spillway. In other words, the spillway is a restriction controlled by the upstream pressure level. 
     The spillway  152  is calibrated to maintain a pressure in the tank  119  above a minimum. This minimum is between 10 and 100 mbar rms, preferably between 20 and 60 mbar rms and is for example 40 mbar rms. 
     Furthermore, the controller  79  is configured to drive the inert gas supply unit  75  to provide a flow of inert gas into the tank  119 , with an inert gas flow rate within a predetermined range. 
     For this purpose, the drain tank  119  has an inert gas inlet  145  at the top of the tank. This inlet  145  is connected to the source of inert gas  81  by a line  147  on which is interposed a regulator  149 . 
     The regulator  149  controls the flow of inert gas delivered to the drain tank  119  through the line  147 . It is driven by the controller  79 . 
     The control member  149  is for example an adjustable valve. 
     In addition, the drain tank  119  is equipped with a gas pressure gauge  151 . The gauge  151  provides the measured pressure values to the controller  79 . 
     Furthermore, the liquid effluent treatment unit  117  is preferably equipped with a flow meter  153  configured to measure the flow of gas circulating in the headspace of the tank  119 . 
     The controller  79  drives the control unit  149  according to the flow values measured by the flow meter  153 , so as to maintain the gas flow within a predetermined range. 
     The predetermined gas flow range is for example [0.1; 10] Nm3/h, preferably [0.2; 5] Nm3/h, even more preferably [0.5; 1] Nm3/h. 
     The controller  79  is preferably programmed to match the gas flow to the soda ash transfer rate to the soda ash 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 caused by the soda ash transfer. 
     In the variant shown in  FIG.  2   , the flow meter is located on line  147 . In this case, the controller  79  is programmed to drive the control member  149  not only using the measurement provided by the flow meter  153  but also using the pressure gauge  151 . 
     For example, the degree of openness 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 
     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 into the tank  119 , P mes  is the value provided by the gas pressure gauge  151 , Q mes  is the value provided by the flow meter  153 . 
     Alternatively, the flow meter  153  is arranged to measure the gas flow in the line  155  connecting the gas outlet  143  of the tank to the gas treatment unit  141 . 
     In this configuration, the controller  79  drives the control unit  149  using the measurement provided by the value provided by the flow meter  153 , without using the measurement from the pressure gauge  151 . 
     Thus, the head of the draining tank is filled with inert gas. 
     Hydrogen from the reaction vessel  59  flowing into the drain tank  119  via the drain line  120  in an incidental situation cannot cause an explosion. 
     It is continuously discharged to the gas treatment unit  141 . This is achieved by the drain tank  119  being connected to the inert gas supply unit  75 . This unit maintains a continuous overpressure of inert gas in the drain tank  119  and provides a continuous flow of inert gas into the tank head. 
     Keeping the drain tank  119  pressurised helps to reduce the risk of air entering the tank. 
     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 vessel  59 , plus a nominal volume of aqueous solution contained in the gas scrubber  129 ; and   a maximum volume equal to the production of soda ash over a period of 6 hours.       

     In other words, the drain tank  119  is sized to receive the volume of aqueous solution contained in the reaction vessel  59 , and the volume of aqueous solution contained in the recirculation and cooling circuit  105 , in the drain line  120  and in the hydrogen processing circuit  125 . However, it is not designed to store the volume of aqueous solution resulting from the operation of the facility  55  for a significant period of time, for example one day. 
     For this purpose, the facility  55  comprises an aqueous solution storage tank  157 , with a storage capacity greater than one day&#39;s production of the facility at nominal treatment capacity. 
     For example, the aqueous solution storage tank  157  has a storage capacity of one week&#39;s production at nominal treatment capacity. 
     If necessary, the facility comprises several storage tanks for aqueous solution. 
     For example, the drain tank  119  has a storage capacity of the order of 1 m 3  of aqueous solution. The facility  55  also has three storage tanks for aqueous solution  157 , each with a volume of 30 m 3  . 
     The storage tank  157  has an air inlet  158 , through which the ceiling 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 roof 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 hydrogen-containing process gases are directed. 
     The drain tank  119  is at least partially below ground level. The or each aqueous solution storage tank  157  is instead located above ground level. 
     The facility  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 . 
     The use of 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 facility. 
     It is then possible to arrange the facility  55  on one level. 
     In the facility shown in  FIG.  1   , the equipment is arranged on two levels. The sodium storage tank  3  and the aqueous solution drain and storage tank  47  are located at ground level, with the reaction vessel  19  located at a higher level. 
     As a result, the cost of the civil engineering for the sodium destruction facility  55  in  FIG.  2    is reduced. 
     The inert gas supply unit  75  has a line  162  connected to the head of the reaction vessel  59 . This line is designed to inert the reaction vessel and the hydrogen processing circuit before the facility starts up. 
     The facility  55  further comprises a circuit  163  supplying the coil  132  with chilled water. The circuit  163  is connected to a chiller  165 . 
     A variant of the facility  55  will now be described with reference to  FIG.  3   . Only the points in which the facility in  FIG.  3    differs from the facility in  FIG.  2    will be detailed below. 
     Elements that are identical or perform the same function will be designated by the same references. 
     In the embodiment shown in  FIG.  3   , the aqueous solution outlet  121  of the reaction vessel  59  is not an overflow outlet. The aqueous solution outlet  121  is flooded and opens below a nominal level of aqueous solution in the reaction vessel  59 . 
     Typically, the aqueous solution outlet  121  is a spigot, provided in a side wall  173  of the reaction vessel  59 , below the nominal level of aqueous solution. 
     In this case, the reaction vessel  59  advantageously comprises an aqueous solution level gauge  175 . This level gauge is of any suitable type. It measures the level of aqueous solution in the reaction vessel  59 . It transmits the measured values to the controller  79 . 
     The drain line  120  further comprises a regulating shut-off member  177 . 
     The shut-off member  177  is interposed on the drain line  120 , and allows the passage cross-section offered to the aqueous solution flowing in the drain line  120  to be modulated. 
     The shut-off member  177  is for example an adjustable valve. 
     The controller  79  controls the valve  177  using the level values measured by the level gauge  175 . The controller  79  controls the valve  177  to maintain the level of aqueous solution in the reaction vessel  59  within a predetermined range. 
     When the aqueous solution level is within the predetermined range, the aqueous solution outlet  121  is flooded. 
     Therefore, 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 . 
     Indeed, because the outlet  121  is constantly flooded, there is no hydrogen flowing from the reaction vessel  59  to the drain tank  119  through the drain line  120 . 
     In contrast, the discharge tank  119  has a gas inlet  179  that puts the headspace of the storage tank  119  in fluid communication with the atmosphere  181  of the room in which the tank  119  is located. 
     The drain tank  119  also has a gas outlet  183  which connects the tank  119  to the building ventilation  160 . 
     According to a second aspect, the present disclosure relates to a method for the destruction of radioactive sodium, typically from a fast neutron reactor. 
     This method is especially suitable for the facility  55  described above. 
     Conversely, the facility  55  is specially designed to implement the method that will be described. 
     The method comprises the following steps:
         Transferring the liquid metallic sodium to be treated to a liquid metallic sodium storage tank  57  located at a first level above ground.   Feeding sodium to a reaction vessel  59  containing an aqueous solution, via a sodium feed circuit  61 , the sodium feed circuit  61  comprising a sodium circulation member  63  located at a second level above ground level higher than the first level, the circulation member  63  having a suction  65  in fluid communication with the sodium storage tank  57  and a discharge  67  in fluid communication with the reaction vessel  59 ;   Supplying the sodium storage tank  57  with an inert gas;   Maintaining a pressure at the suction  65  of the sodium circulation member  63  within a predetermined range by controlling a gas pressure in the sodium storage tank  57 .       

     The sodium storage tank  57  is as described above. 
     The reaction vessel  59  is as described above. 
     The sodium feed circuit  61  is as described above. 
     The sodium storage tank  57  is supplied with inert gas through a supply unit  75  as described above. 
     The suction pressure of the sodium circulation member  63  is controlled within a range of 0 to 100 mbar effective. 
     The method, according to a further aspect independent of the first, comprises the following steps:
         Supplying liquid metallic sodium to a reaction vessel  59  containing an aqueous solution, the reaction vessel  59  having a flooded aqueous solution outlet  121  opening below a nominal level of aqueous solution in the reaction vessel  59 ;   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 , the drain line comprising a regulating shut-off member  177 ;   Controlling the shut-off member  177  to maintain the level of aqueous solution in the reaction vessel  59  within a predetermined range.       

     The reaction vessel  59  is as described above. 
     The aqueous solution outlet  121  is as described above with reference to  FIG.  3   . 
     The reaction vessel  59  is supplied with liquid metallic sodium from a storage tank  57  via a supply circuit  61 . The tank  57  and the supply circuit  61  are advantageously as described above with reference to  FIG.  2   . 
     The drain tank  119  is as described above with reference to  FIG.  3   . The drain line  120  is as described above with reference to  FIG.  3   . 
     The regulating shut-off member  177  is as described above with reference to  FIG.  3   . 
     It is driven by the controller  79  using measurements taken by an aqueous solution level gauge  175  in the reaction vessel  59 . 
     The predetermined range is chosen so that when the level of aqueous solution remains within the range, the aqueous solution outlet  121  is constantly flooded. 
     The method of sodium destruction, according to a third aspect independent of the first and second aspects, comprises 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 inert gas to the drain tank  119 ;   Diluting the gases from the discharge tank  119  and releasing the diluted gases into the atmosphere.       

     The reaction vessel  59  is as described above with reference to  FIG.  2   . 
     The aqueous solution outlet  121  is by overflow, or in other words by spillage. This outlet has been described above with reference to  FIG.  2   . 
     The drain tank  119  is as described above with reference to  FIG.  2   . The drain line  120  is as described above with reference to  FIG.  2   . 
     The drain tank  119  is supplied with inert gas as described above. 
     The gases from the discharge tank  119  are directed to a gas treatment unit  141 , of the type described above. 
     The gases from the drain tank  119  are likely to contain hydrogen, as the drain tank  119  is connected to an overflow liquid outlet from the reaction vessel  59 . In the gas treatment unit  141 , the gases from the discharge tank  119  are diluted with a dilution ratio that ensures that the hydrogen content after dilution excludes any risk of explosion.