FR2601178A1 - REACTOR BLOCK FOR A RAPID NUCLEAR REACTOR, PERMITTING NATURAL CIRCULATION EVACUATION OF THE RESIDUAL POWER OF THE HEART. - Google Patents

Abstract

THE REACTOR BLOCK FOR A RAPID REACTOR INCLUDES AUXILIARY SODIUM-SODIUM EXCHANGERS, THE SECONDARY CIRCUIT OF WHICH PROVIDES HEAT TO ROOM AIR USING SODIUM-AIR EXCHANGERS. THE AUXILIARY HEAT EXCHANGERS8 ARE IMMERSE IN THE COLD COLLECTOR1 AT THE LEVEL OF THE HOT COLLECTOR2 FROM WHICH THEY ARE SEPARATED BY AT LEAST ONE RING9, 10, AND THE EXTERIOR RING9 HAS CALIBERED HOLES 15. APPLICATION TO A RAPID REACTOR WITH NATURAL COOLING CIRCULATION THE SHUTDOWN OF THE FUEL ELEMENTS OF THE CORE, AS WELL AS USED FUEL ELEMENTS STORED AROUND THE CORE.

Description

2601178

  REACTOR BLOCK FOR A FAST NUCLEAR REACTOR, ALLOWING

  EVACUATION BY NATURAL CIRCULATION OF THE RESIDUAL POWER OF THE HEART

  It is known that it is necessary in fast neutron nuclear reactors to dissipate the residual power without causing considerable transient heating,

in case of shutdown of the reactor.

  For this purpose, a number of sodium-sodium exchangers are generally provided, the group of tubes of which is immersed in the hot collector, and the circulating sodium of which constitutes the primary fluid, the secondary fluid being, on the other hand, cooled by circulation. natural air in a

adequate chimney.

  In the case of natural circulation, this technical solution causes stratification of the sodium at a relatively low temperature in the hot collector and prevents the circulation of sodium from the core to the hot collector, thus preventing the correct implementation of the circulation. Primary sodium. It is difficult to guarantee that

  there will be no dangerous transient overheating.

  One of the objects of the present invention is to overcome these difficulties and this is achieved by immersing the tube bundle of the heat exchangers used to dissipate the residual power, no longer in the hot collector, but

in the cold collector.

  Thus, according to the invention, the reactor block for a fast type nuclear reactor comprising: a reservoir containing the primary sodium, a core of fuel elements, at least one hydraulic separation structure which divides

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  with the core the volume of the tank in a hot upper collector and a cold lower collector, intermediate exchangers that pass through the hydraulic separation structure as well as pumps for discharging the sodium contained in the cold collector towards the bottom of the core elements fuel, in which it is provided, to dissipate the residual power in the event of reactor shutdown, sodium-sodium auxiliary exchangers whose primary fluid is constituted by the sodium contained in the tank and the secondary fluid gives heat back to the ambient air by means of suitable sodium-air exchangers, is characterized by the fact that the auxiliary heat exchangers are immersed in the cold collector

  hot collector from which they are separated by at least one ring.

  According to another embodiment, the auxiliary exchangers pass through the hydraulic separation structure between the hot collector and the cold collector and are separated from the hot collector by at least one ring. The hydraulic separation structure has two elements which

  define an intermediate gap in which these elements both extend in the form of rings around the intermediate exchangers. According to the invention, these exchangers are thus surrounded by at least one ring which isolates them from the sodium of the hot collector.

  The outer ring surrounding each auxiliary exchanger has at its upper part a suitable number of calibrated holes. The reactor block according to the invention, equipped with intermediate exchangers having a cylindrical envelope surrounding each intermediate exchanger, said envelope having upper openings for the entry of the sodium contained in the hot collector and lower openings for the sodium outlet in direction of the cold collector, is characterized by the fact that the upper edges of these lower openings are located at a higher level than the center

thermal heart.

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  According to yet another embodiment, in case of reactor shutdown and cooling of the fuel by natural circulation, the sodium outlet means from the hot collector are located above the level of the thermal center of the core.

  Other purposes, advantages and features will become apparent upon reading the description of an embodiment,

  BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the trans-versal section of a conventional fast type nuclear reactor. In this figure, the parts relating to the system of evacuation of the residual power appear clearly; - Figure 2 is similar to Figure 1 but relates to the installation of heat exchangers embodying the present invention; - Figures 3 / A and 3 / B schematically show the auxiliary exchangers, respectively of known type and according to the invention, in standby situation; FIGS. 4 / A and 4 / B are similar to FIGS. 3 / A and 3 / B but relate to the situation existing in the auxiliary exchangers,

  respectively according to the state of the art and according to the invention, shortly after stopping the reactor.

  As shown in FIG. 1, in known manner, the reactor block is divided into a lower zone called the cold collector 1, and an upper and central zone 2.

called the hot collector.

  These two zones are separated from one another by one or

  several hydraulic separation structures 3, 4.

  Sodium circulates normally from the cold collector

  in the direction of the hot collector, via the core 5 of the reactor, by heating up and from the hot collector to the cold collector, via the intermediate exchangers 6 in cooling

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  dissant. Under normal conditions, the circulation is ensured by the pumps 70 immersed in the cold collector 1 and which, by means of suitable pipes 7, discharge the liquid sodium under the grid 71 supporting the fuel elements forming the core 5. When a shutdown situation occurs, the dissipation of the residual power is carried out by the sodium-sodium 8 auxiliary exchangers, which are usually referred to as the residual heat dissipation exchangers. The secondary sodium of these auxiliary exchangers gives back the heat it has absorbed to air flowing in a suitable chimney. This second heat exchanger of the residual power dissipation system is usually referred to as

the sodium-air exchanger.

  The pumps 70 generally work properly.

  According to a first known solution, it is arranged that at least one of the pumps remains in service. In this case and at the reactor shutdown, the sodium of the hot collector 2 is cooled by the auxiliary exchangers 8 and then passes through the intermediate exchangers 6 into the collector 1, where it is discharged by the pumps 70. and the pipes 7 to cross the heart 5 where it absorbs heat and then returns

  once again at the hot collector.

  If, on the other hand, under shutdown conditions of the reactor, it should be considered that the supply of all the pumps is interrupted, the sodium must continue to circulate naturally. However, the usual arrangement of the auxiliary heat exchangers 8, as illustrated in FIG. 1, does not ensure the efficient circulation of sodium between the hot collector and the heated collector when all the pumps are in operation.

are stopped.

  Indeed, the arrangement shown in Figure 1 allows the cooling of the sodium in the hot collector but prevents the circulation of sodium in the heart: in fact, the sodium which is located directly above the core and which is cooled by the Auxiliary exchangers 8 is stratified and this relatively cold sodium tries to descend to the bottom, that is to say towards the heart 5. It follows that the flow

2601178

  of sodium through the heart 5 is unstable and difficult to insure, which can lead to high temperatures in

  the combustible elements of the heart.

  Figure 2 illustrates the solution according to the invention; for the sake of simplification, the corresponding parts of this figure have been assigned the same reference numerals. In this case, the sodium-sodium auxiliary heat exchangers 8 of the residual heat dissipation circuit are installed in the cold collector 1. The auxiliary heat exchangers 8 are however installed at the cold collector 2. For this purpose, the auxiliary exchangers 8 are surrounded by rings which, in this specific case, are cylindrical rings 9, two in number and which are connected at their base to the

  hydraulic separation structures 3 and 4 between the hot and cold collectors.

  This solution does not have a contraindication when the installation is operating under normal conditions: in this case, the sodium of the cold collector

  temperature of about 400 ° C., whereas the sodium of the hot collector has a temperature of about 550 ° C.

  Auxiliary exchangers remove a small portion of the sodium power in the cold collector and give it to the atmosphere via the sodium-air exchangers mentioned previously. This dissipation of power, however, has a positive effect because it serves to lower the temperature of the sodium layer contained between the structures 9 and 10, and therefore between the structures 3 and 4, whose structures 9 and

  constitute the extension surrounding the interchanges 8.

  In this way, the sodium contained within the innermost ring 10 is maintained at a temperature relatively close to 400 ° C, despite the heat input achieved by conduction due to the hot collector 'surrounded. In this way, the residual power discharge circuits are maintained under normal working conditions in the active waiting state, at a temperature of about 400 ° C., whereas in the known solutions represented in FIG. and in the active waiting state they

  remain at a temperature of about 550 C.

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  This situation is illustrated in FIGS. 3 / A and 3 / B, both of which relate to the situation which exists in the active waiting state in the auxiliary collectors 8 and

in sodium-air heat exchangers 80.

  As already mentioned, FIG. 3 / A relates to the known solution of FIG. 1 while FIG. 3 / B relates to the solution according to the invention shown in FIG. reactor, as a result of the inertia of the pumps that continue to rotate for some time, the temperature between the hot and cold collectors is trying to

  to be uniform to an intermediate value and this situation is shown schematically in Figures 4 / A and 4 / B.

  Figure 4 / A refers to the known solution while solution 4 / B refers to the solution according to the invention. It can clearly be seen in FIG. 4 / A that the secondary sodium present in the auxiliary exchanger 8 has a temperature lower than that of the secondary sodium in the sodium-air exchanger 80 located at a higher level: thus, in the solution known, the start of the natural secondary sodium circulation between the auxiliary exchanger 8 and the sodium exchanger 80 located above it is delayed until the temperature inside the hot collector begins to increase again. On the contrary, in the system illustrated in FIG. 4 / B, the auxiliary heat exchangers 8 and 10 and the sodium-air heat exchanger 80 are at a temperature slightly higher than that of the other, so that the start-up of the natural circulation of secondary sodium

is accelerated.

  Thus, according to the invention, in a shutdown situation, the intervention of auxiliary exchangers and exchangers

air-sodium is faster.

  We will now see that, according to the invention, even after

  the departure of the natural circulation inside the auxiliary exchangers, the dissipation of the residual power 35 is implemented more effectively.

  The evacuation of the residual power as a result of the circulation of the primary sodium is due to the motor effect caused by the fuel elements. During the phase

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  dissipation of the residual power using the natural circulation of the primary sodium, it flows from the core 5 towards the hot collector 2. From there, it enters the intermediate heat exchangers 6, via the upper openings 61 , and is evacuated by the lower openings 62 to the cold manifold 1. It then returns via the pumps 70 and the pipes 7 in the core 5 to make

  again the journey just mentioned.

  According to the invention, the hot sodium from the lower windows 62 of the intermediate heat exchangers 6 is laminated in the upper part of the cold collector 1 (in addition to the cold sodium mixing stroke inside this part) and penetrates thus inside the ring 10 which surrounds the auxiliary exchanger 8. It thus enters via the upper openings 81 of the auxiliary exchanger 8 and, after having cooled inside this exchanger, again in natural circulation, it goes out through the lower openings 82. It is then sucked back from

  these openings by the pumps 70 and the pipes 7 towards the core 5.

  It will be noted that in order to ensure the natural circulation of the primary sodium within the heart 5, it is necessary that the upper edge 63 of the lower openings 62 of the

  intermediate exchangers 6 is at a level higher than that 25 of the thermal center (q) of the core 5.

  This solution made with the aid of the invention, when the separation between the cold collector and the hot collector is carried out using at least two structures 9 and 10, as shown in the figures, allows the cooling by 30 natural circulation of spent fuel elements disposed in the internal storage located around the heart 5. With the aid of the installation according to the invention, this cooling is

  realized in any operating state of the reactor.

  For this purpose, the upper part of the outer ring 9 which surrounds the auxiliary exchangers 8 is equipped with calibrated holes 15. The sodium of the hot collector 2 penetrates through these calibrated holes in the gap existing between the two rings 9 and 10 and, by conduction, yields heat to

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  Sodium of the cold collector contained between the inner ring 10

  and the outer wall of the auxiliary exchanger 8.

  This heat is returned to secondary sodium

  of the auxiliary heat exchanger 8 and at least partially via it to the atmosphere using the sodium-air heat exchanger.

  A second fraction of this heat is transferred to the sodium of the cold collector in the zone below the auxiliary exchangers 8. The sodium which falls in the gap existing between the rings 9 and 10 continues its course in the range 16 existing between the two structures 3 and 4, serving as hydraulic separation between the collector cold and hot. In this way, the sodium circulating in the gap 16 already cooled by the mechanism just described, does not create excessive thermal gradients in the mass of. structure 3. The flow of circulating sodium in the gap 16 ends by cooling the fuel elements 14 stored around the core 5. According to an alternative embodiment, the auxiliary exchangers 8 may be located outside by relative to the hydraulic separation elements 3 and 4. In this case, the auxiliary exchangers can be inserted inside the cold collector without the containment structures 9 and 10 provided in

the solution mentioned above.

  In this case, the structures 3 and 4 must provide circumferential loops in order to leave the necessary space

  for the installation of sodium-sodium exchangers. This structural solution eliminates the sodium confinement structure 90 from the cold collector (FIG. 1).

  This containment structure 90 has the function of passing a small percentage of the total flow of cold primary sodium so that it comes to lick the main tank to cool. Indeed, the presence of the sodium-sodium exchangers 8 outside the separation structure 3 makes it possible to absorb the heat transmitted to the sodium at the upper part of the cold collector by conduction via the

  structures 3 and 4 cited above.

  Of course, the present invention is not limited to the embodiments described and shown and is capable of numerous variants accessible to the man of

  art without departing from the spirit of the invention.

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Claims (6)

  1.- Reactor block for a fast nuclear reactor, allowing the evacuation by natural circulation of the residual power at standstill, of the type comprising: - a reservoir containing the primary sodium, - a core of fuel elements, - to minus a hydraulic separation structure which divides, with said core, the volume of the reservoir into a hot upper manifold and a cold lower manifold; - intermediate exchangers which pass through the hydraulic separation structure, as well as - pumps to circulate the sodium contained in the cold collector towards the bottom of the core of the 15 fuel elements, in which it is expected, to dissipate the residual power in case of reactor shutdown, sodiumsodium auxiliary exchangers whose primary fluid is constituted by the sodium contained in the reservoir and whose secondary fluid yields heat back to the ambient air using sodium-air heat exchangers. characterized by the fact that the auxiliary exchangers (8) are immersed in the cold collector (1) at the hot collector (2) from which they are separated by minus one ring (9, 10).   2. Block reactor according to claim 1, characterized in that the auxiliary exchangers (8) pass through the hydraulic separation structure (3, 4) between the collector   hot (2) and the cold collector (1) and are separated from the hot collector by at least one ring (9, 10).   3. Reactor block according to one of claims 1 or 2,   characterized by the fact that the hydraulic separation structure comprises two elements (3, 4) which define an intermediate gap (16) in which these elements extend   both in the form of rings (9, 10) around the intermediate exchangers (8).   4. reactor block according to any one of the preceding claims, characterized in that the outer ring (9) surrounding each auxiliary exchanger (8) is provided, 2601178   at its upper part, a suitable number of calibrated holes (15).   5. Reactor block according to any one of the preceding claims, equipped with intermediate exchangers which   have a cylindrical envelope surrounding each intermediate exchanger, said envelope having upper openings for the sodium inlet of the hot collector and lower openings for the sodium outlet towards the cold collector, characterized in that the upper edges (63) lower openings (62) are located at   a level higher than the thermal center (q) of the heart (5).   6. Reactor block according to any one of the preceding claims, characterized in that in case of stopping   of the reactor and cooling the fuel by natural circulation, the outlet means (62) of sodium from the hot collector (2) are located above the level   the thermal center (q) of the heart (5).