GB2189339A

GB2189339A - Reactor block of a fast reactor for evacuation by natural circulation of the residual power of the core

Info

Publication number: GB2189339A
Application number: GB08709415A
Authority: GB
Inventors: Luciano Cinotti
Original assignee: Ansaldo SpA
Current assignee: Ansaldo SpA
Priority date: 1986-04-21
Filing date: 1987-04-21
Publication date: 1987-10-21
Also published as: IT8612468A0; GB8709415D0; GB2189339B; FR2601178B1; IT1189748B; BE1000462A4; FR2601178A1; DE3713140A1; JPS6312992A

Description

GB 2 189 339 A SPECIFICATION cold heater, through the intermediate

exchangers 6, cooling down. Under normal conditions, circulation Reactor block of a fast reactor for evacuation by is ensured by pumps 70, immersed in the cold natural circulation of the residual power of the core header 1, and which, by means of suitable ducts 7, 70 feed the liquid sodium under the grid 71 supporting

Backgroundof the invention the fuel elements forming the core 5.

It is well-known that in fast reactors there is a need When a shutdown situation occurs, dissipation of to dissipatethe residual powerwithout any the residual power remains entrusted to the auxiliary considerable heattransients, in the event of a sodium-sodium exchangers 8, known appropriately shutdown of the reactor. 75 as the residual power evacuation exchangers. The Forthis purpose a certain number of secondary sodium of these auxiliary exchangers in sodium-sodium exchangers are provided, the tube turn gives up the heat absorbed to an air current in a nest of which is immersed in the hot header, the suitable chimney.

circulating sodium of which constitutes the primary This second heat exchanger of the residual power fluid; the secondaryfluid, on the other hand, is 80 dissipation system is normally called the sodium-air cooled by natural circulation of air in a suitable exchanger.

chimney. The pumps 70 are generally all working; according In the case of natural circulation, this design to a first known solution, it is arranged that even in engineering solution creates in the hot header plantshutdown conditions, one or more of them stratifications of sodium at a relatively low 85 remain in operation.

tempeaturewhich blockthe circulation of the In this case, in reactorshutdown conditions,the sodium from the coreto the hot header, preventing sodium of the hot header 2 is cooled bythe auxiliary proper implementation of the circulation of the exchangers 8,then, through the intermediate primarysodium. exchangers 6 it passes intothe cold header 1,from It istherefore difficuitto ensurethe absence of 90 where it isforced bythe pumps 70 and the conduits 7 dangerous heattransients. to passthrough the core 5, where it absorbs heat,to return once moreto the hot header.

Briefsummaryof theinvention If, on the other hand, in reactorshutdown

The aim of this invention isto overcomethese conditions, it is envisaged that powerto all the problems, and this is achieved by immersing the 95 pumps may be cut off, the sodium must continueto tube bundle of the heat exchangers used for circulate naturally. However, the usual arrangement dissipation of the residual power no longer in the hot of the auxiliary exchangers 8 as illustrated in Figure 1 header, but in the cold header. does not guarantee efficient circulation of the According to the invention, these exchangers can sodium between the hot header and the warm be surrounded by one or more rings which separate 100 headerwhen all the pumps 70 are at a standstill.

them f rom the sodium of the hot header. Indeed, the arrangement shown in Figure 1 allows cooling of the sodium in the hot header but hinders Briefsummary of the drawings circulation of the sodium in the core: in actual fact,

Figure 1 shows schematically the cross-section of directly above the core the sodium cooled by the a traditional faste nuclear reactor. In this Figure the 105 auxiliary exchangers 8 stratifies, and this relatively parts referring to the residual power evacuation cool sodium tends to drop towards the bottom, that system stand out. isto saytowardsthe core 5.

Figure2 is similarto Figure 1, but referstothe Consequently, the flow rate of the sodium through installation of the heat exchanger implemented the core 5 is unstable and difficuitto assess, and may according to this invention: 110 lead to high temperatures in the corefuel elements.

Figures 31A and 318 show schematicallythe Figure 2 illustrates the solution according to the auxiliary exchangers of the known type and invention; forthe sake of simplicity,the according to the invention respectively, in a stand-by corresponding parts in the Figure have been condition: indicated bythe same numbers.

Figures41A and 4/Bare similarto Figures 3/Aand 115 In this case, the sodium-sodium auxiliary 3/B, but referto the situation existing in the auxiliary exchangers 8 of the residual power dissipation exchangers according to the state-of-the art and circuit are installed in the cold header 1; the auxiliary according to the invention respectively, sometime exchangers 8, however, are installed atthe height of after shutdown of the reactor. the cold header 2.

120 Forthis purpose, the auxiliary exhangers 8 are Detailed description of thepreferred embodiment surrounded by rings, in this specific case bytwo

As shown in Figure 1, in the known manner,the cylindrical rings 9 and 10, connected atthe bottom to reactor block is divided up into a lower area, known the hydraulic separation structures 3 and 4 between as the cold header 1, and an upper and central area 2, the hot and cold headers.

known as the hot header. 125 This solution has no contraindications when the These two areas are separated from one another plant is operating in normal conditions: in this case by one or more hydraulic separation structures 3,4. the sodium in the cold header has a temperature of The sodium normally circulates from the cold about 400'C while the sodium in the hot header has a headerto the hot header, through the core 5 of the temperature of about 550'C.

reactor, heating up, and from the hot header to the 130 The auxiliary exchangers 8 remove a small fraction 2 GB 2 189 339 A 2 of powerfrom the sodium of the cold header, and to the hot header 2.

give it up to the atmosphere through the above From here it entersthe intermediate exchangers 6 mentioned sodium-air exchangers. through the upperwindows 61 and leavesthrough This loss of heat does however have a positive the lowerwindows 62, in the cold header 1.

effect in that it serves to lowerthe temperature of the 70 Through the pumps 70 and pipes 7 itthen returns layer of sodium contained between the structures of into the core 5, to retrace the above path.

9 and 10 and therefore between the structures 3 and According to the invention, the hot sodium coming 4, of which structures 9 and 10 are the extension out of the lower windows 62 of the intermediate surrounding the exchangers 8. exchangers 6 stratifies in the upper part of the cold In this manner, the sodium contained inside the 75 header 1 (in addition of course to mixing with the - innermost ring 10 is maintained at a temperature cold sodium in it), and therefore penetrates inside fairly close to 400'C, in spite of the heat contribution the ring 10 which surrounds the auxiliary exchanger by conduction due to the sodium of the surrounding 8.

hotheader. Thus it penetrates through the upperwindows 81 In this way, furthermore, the residual power 80 of the auxiliary exchanger 8 and having cooled down evacuation circuits are kept in normal working inside it, still in natural circulation, it leavesthrough conditions, in a stand-by state at a temperature of the lowerwindows 82: from here it is drawn back about 400'C, while according to the known solution through the pumps 70 and the ducts 7 towards the illustrated in Figure 1, in stand-by conditionsthey core 5.

remain at a temperature of about 5500C. 85 It should be noted that in orderto ensure natural This situation is illustrated in Figures 3/A and 3/13, circulation of the primary sodium inside the core 5, it both referred to the situation which is present in a is necessary forthe upper edge 63 of the lower stand-by state in the auxiliary heaters 8 and in the windows 62 of the intermediate exchangers 6 to be sodium-air exchangers 80. higherthan the heat centre (q) of the core 5.

As already mentioned, Figure 3/A refers to the 90 The solution putforward by the invention, when known solution of Figure 1 while Figure 3/B refersto the separation between the cold header and the hot the solution according to the invention, as per Figure header is made with at leasttwo structures 9 and 10, 2. After shutdown of the reactor, due to the inertia of as shown in Figure 2, allows cooling by natural the pumps which continue to turn for sometime,the circulation of exhausted fuel elements in internal temperature between the hot and cold headers tends 95 storage around the core 5.

to become uniform at an intermediate value, and this With the plant solution claimed bythis invention, situation is shown schematically in Figures 4/A and this cooling takes place in any operating condition of 4113. the reactor.

Figure 4/A refersto the known solution while Forthis purpose,the upper part of the external ring Figure 4/B refersto the solution according tothe 100 9which surroundsthe auxiliary e)changers 8 is invention. It can be seen clearly in Figure 4/Athatthe equipped with gauged holes 15.

secondary sodium present in the auxiliary Through these gauged holes the sodium of the hot exchanger 8 has a temperature lowerthan that of the header 2 penetrates into the interspace existing secondary sodium in the sodium-air exchanger 80 at between the two rings 9 and 10 and, by conduction, a greater height: therefore, according to the known 105 gives up heatto the sodium of the cold header solution, until the temperature inside the hot header contained between the internal ring 10 and the outer starts to rise again, the onset of natural circulation of wall of the auxiliary exchanger 8.

the secondary sodium between the auxiliary This heat is in turn given up to the secondary exchanger 8 and the sodium-air exchanger 80 above sodium of the auxiliary exchanger 8 and, at least it is delayed. 110 partly, bythis to the environment by means of the On the contrary, according to the system sodium-air exchanger.

illustrated in Figure 4113,the auxiliary exchanger 8 A second fraction of this heat is given up to the and 10 and the sodium-air exchanger 80 are found sodium of the cold header in the area underlying the one at a temperature slightly higherthan that of the auxiliary exchangers 8.

otherso the onset of natural circulation of the 115 The sodium which descends into the interspace secondarysodium isfavoured. existing between the rings 9 and 10 continues in its Therefore, according to the invention, in shutdown path in the interspace existing between the two conditions the intervention of the auxiliary structures 3 and 4for hydraulic separation between exchangers and of the air-sodium exchangers is the cold and hot headers.

quicker. 120 In this waythe sodium circulating in the interspace Rwill be seen laterthat according to the invention, 16 already cooled on the basis of the mechanism even after natural circulation has started inside the described above, does not create excessive thermal auxiliary exchangers, dissipation of the residual gradients in the thickness of the structure 3.

power is carried out more efficiently. The flow of sodium circulating in the interspace 16 Evacuation of the residual power by means of the 125 ends up by cooling the fuel elements 14 stored circulation of the primary sodium is due to the around the core 5.

motive power generated in thefuel elements. According to a different embodiment, the auxiliary During the phase of dissipation of the residual exchanges 8 may be located externallyto the power by means of natural circulation of the primary hydraulic separation elements 3 and 4.

sodium, the primary sodium passes from the core 5 130 In this case the auxiliary exchangers 8 maybe 3 GB 2 189 339 A 3 inserted into the cold header without the confirming surrounded by a cylindrical shell which has upper structures 9 and 10 provided in the solution already openingsforthe entry of the primary coolantfrom illustrated. the hot header and lower openingsforthe exit of the In this casethe structures 3 and 4 must provide primary coolanttowards the cold header, the upper circumferential loops in orderto leave the space 70 edges of the lower openings being at a level higher necessary for installation of the sodium-sodium than the heat centre of the core.

Claims

exchangers. This structural solution makes it 8. The reactor blockof any

of Claims 1 to 6, possible to eliminate the structure 90 confining the wherein the outlet of the sodium from the hot header sodium coming from the cold header (Figure 1). via the auxiliary exchanger is abovethe height of the This confining structure 90 has thefunction of 75 heat centre of the core.

conveying a small percentage of the total flow of cold 9. The reactor block of any of the preceding primary sodium so as to lap the main tank to cool it. Claims, wherein the primary coolant is sodium.

In effect, the presence of the sodium-sodium 10. The reactor block of Claim 9, wherein the exchangers 8 outside the separation structure 3 auxiliary exchanger is a sodium-sodium exchanger, makes it possible to absorb the heat transmitted to 80 the primary fluid being sodium in said tank and the the sodium in the upper part of the cold header by secondaryfluid giving up heat in turn to the ambient conduction through the above structures 3 and 4. air by means of a sodium- air exchanger.

Although for descriptive reasons this invention is 11. A reactor block, substantially as herein based on the above descriptions and illustrations, described with reference to, and as shown in, many alterations and variations may be made in the 85 Figures 2,313 and 413 of the accompanying drawings.

embodiment of the invention, said alterations and 12. Afast reactor containing the reactor block of variations must be considered, however, as being any of the preceding Claims.

based on the following claims.

CLAIMS Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd,8187, D8991685.

Published byThe Patent office, 25 Southampton Buildings, London, WC2A l AY, 1. A reactor block of a fast reactor, arranged for from which copies maybe obtained.

the dissipation of the residual power by natural circulation in shut-down conditions, and comprising:

a tank containing primary coolant; a core of fuel elements; at least one separation structure which, together with the core, divides the tank into an upper, hot header and a lower, cold header; at least one intermediate heat exchangerwhich interconnects the hot and cold headers; at least one pump forfeeding the primary coolant towards the bottom of the core; and at least one auxiliary heat exchanger which puts the primary coolant in heat exchange relationship with the ambient air, the auxiliary exchanger being at the level of the hot header but immersed in the cold header.
2. The reactor block of Claim 1, wherein the auxiliary exchanger is within said tank and is separated from the hot header by at least one casing.
3. The reactor block of Claim 2, wherein the casing is of circular cylindrical form.
4. The reactor block of Claim 2 or 3, wherein the auxiliary exchanger passes through said separation structure.
5. The reactor block of any of Claims 2 to4, wherein said separation structure comprisestwo elements which define an intermediate interspace, which elements are connected to respective inner and outer said casings around the auxiliary heat exchanger.
6. The reactorblockof anyof Claims 2to5, wherein there is an inner and an outer said casing surrounding the auxiliary exchanger, the outer casing having holes in its upper part and communicating with the hot header.
7. The reactor block of any of the preceding Claims, in which the auxiliary heat exchanger is