A SYSTEM FOR THE DISSIPATION OF HEAT FROM THE INTERIOR OF A CONTAINMENT STRUCTURE OF A NUCLEAR REACTOR

Abstract: A system (10) for the passive dissipation of the heat from the interior (8) of a containment structure (3, 9) of a nuclear reactor (2), which would develop particularly in the case of accidents, comprises a first metal wall (11) lining the side and/or top walls (5, 6) within the said containment structure (3), a second metal wall (16) intermediate between the said first metal wall (11) and the said side and/or containment walls (5, 6), a sealed interspace (18) confined by the said first and second metal walls (11, 16) and containing a heat collection fluid (22) and means (24) for the passive dissipation of heat from the said interspace (18).

Description: A system for the dissipation of heat from the interior of a containment structure of a nuclear reactor The present invention relates to a system for dissipating heat from the interior of a containment structure of a nuclear reactor, particularly for the dissipation of heat which would be generated in the case of an unexpected and accidental malfunctioning of the normal cooling systems. As is known, at present nuclear reactors, with their associated primary cooling circuit, are housed within a primary containment structure of steel or concrete usually made of several layers or walls. This primary containment structure is in turn housed within a building the side walls and the roof of which communicate with the external environment. In the case of an accident the heat generated within the primary containment structure by the nuclear reactor must necessarily be dissipated away from this structure and from the building without releasing into the atmosphere any fluid or material previously contained within the primary structure. It is important to note that a nuclear reactor, even if shut down as a consequence of an accident, continues to generate heat by the decay of the nuclear fuel. This generation of heat is initially significant and then decreases overtime without, however, reaching zero. The dissipation of heat therefore is necessary even if the reactor is shut down. In order to comply with current security requirements the manner in which the heat is dissipated must be of passive type, that is to say it must not depend upon the operation of manual or automatic control systems, the activation of pumps or the like, the opening of valves, or the availability of sources of energy of any type, but must be intrinsically activated by natural physical phenomena connected with permanent structural characteristics. In particular, in the case of buildings and containment structures made predominantly of concrete or pre-compressed reinforced cement, the side walls and the top, being insulating materials, constitute an obstacle to the dissipation of heat. Heat dissipation systems have been proposed comprising hydraulic circuits which have external heat exchangers and heat exchangers within the containment structure through which a heat transfer fluid flows in natural circulation. The external heat exchangers must be of very large dimensions since they must have a particularly extensive heat exchange surface, and therefore have a high cost. Moreover, in normal operating conditions of the nuclear reactor and with winter temperatures outside, the heat exchange fluid contained in the external heat exchanger can freeze rendering the heat dissipation system ineffective. Furthermore, such hydraulic circuits, if subjected to rupture, constitute a preferential escape route from the containment structure for radioactive material. The object of the present invention is that of making available a system for the dissipation of heat which makes it possible for the above- mentioned disadvantages of the prior art to be overcome. This object is achieved by a system for the dissipation of heat, of the type specified, as defined in the characterising part of the attached Claim 1. The characteristics and advantages of the system for the dissipation of heat according to the invention will become clearer from the description of two embodiments thereof made hereinafter with reference to the attached drawings, provided by way of non-limitative indication. In the drawings: - figure 1 is a sectional view of a nuclear reactor containment building, of the type having two concrete walls, which incorporates a heat dissipation system according to the invention; - figure 2 is a perspective view, partially in section, of a detail of the system of figure 1 ; - figure 3 is a partial sectioned view, of a containment building, of the single concrete wall type, of a nuclear reactor which incorporates a different embodiment of the system according to the invention; - figure 4 is a schematic partial perspective view, in section, of a detail of the building and system of figure 3; and - figure 5 is a partially sectioned perspective view of a detail of the system of figure 3. In figures 1 and 2 a nuclear reactor containment building is generally indicated with the reference numeral 1. It houses the nuclear reactor itself, indicated 2. The building 1 comprises an inner concrete containment structure 3, of substantially cylindrical form, which has a base 4, side walls 5 and a top wall 6. The side and top walls 5, 6 have a common internal surface 7 which generally defines and encloses an internal environment 8 of the inner containment structure 3. The inner containment structure 3 of the reactor is enclosed in an outer concrete containment structure 9, also of substantially cylindrical form, which also rests on the base 4 and has side walls 5' and a top wall 6'. Between the inner containment structure 3 and the outer containment structure 9 there is a space 30 of cylindrical shape delimited by the facing side walls and the top walls 5, 5', 6, 6'. The containment building 1 is associated with a heat dissipation system generally indicated 10. This comprises a first metal wall 11 backing onto the side walls 5 and the top wall 6 within the said internal containment structure 3 in such a way as almost completely to cover the inner surface 7 but maintained from it by a substantially constant distance. The said first metal wall 11 is formed by a plurality of contiguous internal sheets 12 disposed in superimposed circumferential rows 13 starting from the base 4 of the inner containment structure 3 and entirely covering the top wall 6. Each successive row 13 is rested on horizontal beams 14, preferably of the double-T type, rigidly fixed to the inner surface 7 of the side and top walls 5, 6, which in succession form a ring 15 for each circumferential row 13 of inner plates 12 which, as well as being supported on the underlying horizontal double-T beams 14, is connected to the overlying horizontal double-T beams 14 in such a way as to be put in mechanical engagement between counter posed horizontal double-T beams 14. Between the said first metal wall 11 and the side and top walls 5, 6 of the inner containment structure 3 the system 10 includes a second metal wall 16 formed by a plurality of contiguous intermediate plates 17 corresponding to the inner plates 12, which rest on the horizontal double-T beams 14 and are rigidly fixed to the said internal surface 7 of the inner containment structure 3. Overall, then, the second metal wall 16 is adherent to the internal surface 7 and is rigidly fixed to it. In this way the second metal wall 16 is also fixed in its own plane with respect to the side and top walls 5, 6. The installation 10 further includes an interspace 18 confined by the said first and second walls 1 1 , 16 and subdivided into a plurality of sectors 19 which are delimited by the horizontal double-T beams 14 and by vertical beams 20, preferably of the double-T type, disposed between the first and second walls 1 1 , 16 in engagement with the horizontal double-T beams 14 and corresponding to the contiguous edges of the internal and intermediate plates 12, 17. Between the internal and intermediate plates 12, 17 and the vertical and horizontal double-T beams 14, 20 are hydraulic sealing members 21 which in this example are bellows. Due to the presence of these bellows the interspace 18 of each sector 19 is entirely sealed. The interspace 18 contains a heat collection fluid 20, in this embodiment water, which does not completely fill each sector 19 but leaves an air and steam header chamber 33. In normal operating conditions of the reactor 2 the absolute pressure within each sector 19 and therefore, generally, within the interspace 18 is at a predetermined value generally not greater than atmospheric pressure and advantageously less than 10,000 Pa. The air content is also limited to facilitate the heat exchange phenomenon by evaporation and condensation. Within each sector 19 is contained a vertical corrugated metal sheet 40 rigidly connected to the said inner and intermediate sheets 12, 17. The system 10 further includes means 24 for the passive extraction of the heat from the said interspace 18. In this embodiment the said means 24 for the passive extraction of heat comprise a plurality of inner condensers 25 contained in the space 30 between the containment structures 3 and 9. Each inner condenser 25 corresponds to a respective sector 19 of the interspace 18 located on the side walls 5 of the inner containment structure 3 and is in hydraulic communication with it through a concentric tube duct 26 which opens into the sector 19 in the header chamber 23 and is connected to a manifold 27 of the inner condenser 25. The manifold 27, and therefore also the condenser 25, is located in an elevated position with respect to the corresponding sector 19 and from it extends a U-shape bundle 28 of finned tubes 29. The means 24 further include a plurality of apertures 31 formed in the outer containment structure 9 between the outside and the space 30 and disposed close to the top walls 6, 6'. The said means 24 include a descending channel 33 within the space 30, starting from the said apertures 31 , and a corresponding ascending channel 34. The said channels 33, 34 are defined by a third metal wall 32 disposed vertically within the space 30 in an intermediate position between the side walls 5, 5' of the inner and outer containment structures 3, 9 and connected to the top wall 6' of the outer containment structure 9. The third metal wall 32 does not extend down to the base 4 such that it divides the space 30 into the descending channel 33, communicating with the outside through the apertures 31 , and the ascending channel 34, in which the internal condensers 25 are located, the said channels 33 and 34 therefore being concentric and communicating at their lower ends. The means 24 for the passive extraction of heat further includes a chimney 35 located centrally on the top wall 6' of the outer containment structure 9, which puts the ascending channel 34 into communication with the outside. From the apertures 31 , through the descending and ascending channels 33, 34 to the chimney 35 defines a permanently open natural air circulation route which contains the inner condensers 25. For the passive dissipation of heat from the fluid contained in the sectors 19 of the interspaces 18 located in correspondence with the top wall 6 of the inner containment structure 3, the means 24 comprise a plurality of outer condensers 25' disposed outside the building 1 on the top wall 6' of the outer containment structure 9. These outer condensers 25' are in hydraulic communication with the said sectors 19 by means of tubes 36 and are preferably immersed in corresponding water-filled baths 37 formed on the top wall 6' of the outer containment structure 9. The outer condensers 25' are also located in elevated positions with respect to the corresponding sectors 19 of the interspace 18. With reference to Figures 1 and 2 the operation of the system 10 according to the above-described embodiment will now be described. When the system 10 is in "waiting" conditions, that is in the absence of accident situations which involve the generation of heat in the interior environment 8, the condensers 25, 25' are only filled with air and steam from the sector 19 to which they are connected and at the same pressure thereas. The temperature of the water 22 contained in the interspace 18 is largely influenced by the temperature of the environment 8 rather than the temperature outside the building 1 so that the said water 22 does not run the risk of freezing. Upon occurrence of an accident situation the air within the containment structure 3 of the reactor 2 is subject to a strong heating. In particular, if the nuclear reactor 2 is of the water-cooled type it is very probable that a large quantity of steam will also be liberated. At this point the temperature difference existing between the internal environment 8 of the containment structure 3 and the water 22 in the interspace 18 rises and therefore starts a transfer of heat between the environment 8 and of the water 22 through the first metal wall 11 which, due to the heating effect, is free to expand even though retained between horizontal double-T beams 14. The expansion takes place radially, that is to say each inner sheet 12 curves towards the interior of the sector 19 compressing the corrugated sheet 40. The second metal wall 16 will also be subjected to heating and, resting on the inner surface 7 of the inner containment structure 3 will compress transferring the force to which it is subjected directly to the structure 3. Upon achievement of the saturation temperature the water 22 in the interspace 18 begins to boil releasing saturated steam into each condenser 25, 25' which, consequently, will be subject to heating. The heating of the internal condensers 25 initiates natural circulation of the outside air through the apertures 31 , the descending and ascending channels 33, 34 and the chimney 35. The temperature of the outside air is generally less than the saturation temperature of the water 22 contained in the interspace 18 so that, within the tube bundles 28 the steam condenses returning by gravity to the interior of the respective sector 19. Similarly, the outer condensers 25' heat the water in the baths 37 which evaporates condensing the steam contained in each outer condenser 25'. Upon exhaustion of the water in the baths 37 the outer condensers 25' can nevertheless function by exchanging heat with the outside air. The overall operation of the means 24 for the passive extraction of heat from the water 22 therefore proceeds on the basis of naturally initiated heat transfer phenomena which can continue for an indefinite time. With reference to Figures 3, 4 and 5 there will now be given a description of a further embodiment of the system 10, in which components which perform the same function as those previously described will be indicated with the same reference numerals. In Figure 3 a containment building of a nuclear reactor, indicated 1 , is of the single concrete containment structure type, indicated 3, which has a base 4, side walls 5 and a top wall 6. The heat dissipation system 10 according to the invention comprises a first metal wall 11 positioned as in the preceding embodiment, formed by a plurality of contiguous inner sheets 12 rigidly fixed together in such a way that the said first wall 11 is self supporting and rests on the base 4. Rings 15 of horizontal double-T beams 14 are suspended to the first metal wall 11 by means of a plurality of hooks 41. The system 10 further includes a second metal wall 16 positioned as in the previous example but spaced from the inner surface 7 in such a way as to leave free a space 30' between itself and the side and top walls 5, 6. In this embodiment of the invention the second metal wall 16 is also self supporting and rests on the base 4. The first and second metal wall 11 , 16 are therefore free to move within their own planes with respect to the side and top walls 5, 6. Moreover, the first and second movable walls 11 , 16 are each free to move within their own planes with respect to the other. Between the horizontal double-T beams 14 and the metal walls 11 , 16 are hydraulic sealing members 21 which, in this embodiment, are bellows. In the sealed interspace 18 are vertical double-T beams 20 also suspended from the said first wall 1 1 by means of the said hooks 41 , disposed in such a way as to create a preferential path for the natural circulation, due to convection, of the water 22 which is contained in the interspace 18 filling it completely. Also between the vertical double-T beams 20 and the metal walls 1 1 , 16 are hydraulic sealing members 21 which, in this embodiment are bellows. These vertical beams 20 delimit, together with the horizontal beams 14, sealed sectors 19 within which the absolute pressure is advantageously, in normal operating conditions of the reactor 2, less than 10,000 Pa. Within the space 30' the system 10 includes a cage 42 of metal beams 43 located between the second metal wall and the side and top walls 5, 6 of the containment structure 3. Between the cage 43 and the second metal wall 16 there is, however, located an expansion space 44. The system 10 for the passive dissipation of heat further includes means 24 for the extraction of the heat from the said interspace 18, which in this embodiment of the invention includes the said second metal wall 16. The means 24 further include a plurality of apertures 31 formed in the containment structure 3 between the outside and the space 30' and disposed in proximity to the top wall 6, a descending channel 33 and an ascending channel 34 defined by the relative position of the metal beams 43 of the cage 42. The descending channel 33 communicates with the outside through the apertures 31 and the ascending channel 34 is delimited on one side by the said second metal wall 16. The means 24 for the passive extraction of heat further includes a chimney 35 located centrally on the top wall 6 of the containment structure 3, which puts the space 30' into communication with the outside. As in the preceding embodiment, from the aperture 31 to the chimney 35, through the descending and ascending channels 33, 34, there is therefore always open a natural circulation path for the air which flows in contact with the second metal wall 16. Moreover, the thermal communication between the interspace 18 and the outside is ensured through the second metal wall 16 which presents a wide surface contacted by the air in natural circulation. With reference to Figures 3, 4 and 5, the operation of the system 10 according to the second embodiment of the invention described above will now be described. When the system 10 is in waiting condition, in the absence of accident situations which involve generation of heat in the internal environment 8, the temperature of the water 22 contained in the interspace 18 is greatly influenced by the temperature of the environment 8 rather than the temperature outside the building 1 , so that the said water 22 does not run the risk of freezing. Upon occurrence of an accident situation the temperature difference existing between the internal environment 8 of the containment structure 3 and the water 22 in the interspace 18 rises and a transfer of heat between the environment 8 and the water 22 starts to take place through the first metal wall 11 which, by the effect of the heating, is free to expand even though it is held rigid by the double-T beams 14, 20. Expansion of the first metal wall 11 continues until it contacts against the second metal wall 16. The second metal wall 16 will also be subject to heating but this too is free to expand, like the first metal wall 11 , contacting against the cage 42 which contains it by making use of the expansion space 44. Contact of the second metal wall 16 against the cage 42 contributes to reinforcing the entire installation 10 and to increase the capacity of the interspace 18 to resist high pressures. Heating of the second metal wall 16 initiates natural circulation of outside air through the aperture 31 , the descending and ascending channels 33, 34 and the chimney 35 which cools the second metal wall 16 and therefore also the water 22 in the interspace 18. The increase of pressure in the interspace 18 is supported by the double-T beams 14, 20 and by the second metal wall 16 which, however, is held rigid being engaged with the beams 43 of the cage 42. In this case too, the overall operation of the means 24 for passive extraction of heat from the water 22, therefore proceeds on the basis of naturally occurring thermal transfer phenomena which can continue for an indefinite time. The principal advantage associated with the heat dissipation system according to the present invention lies in the fact that the efficiency of the means for passive extraction of heat, increased by the effect of the mechanisms which increase the thermal exchange coefficients with the outside air, allows a reduction in the heat exchange surface. Moreover, the freezing of the fluid which accumulates the heat is prevented by keeping it entirely within the reactor containment building. Further, the metal walls provide a supplementary containment for preventing the escape of radioactive agents. By the effect of the heat exchange mechanisms used, the temperature within the reactor containment structure can be reduced progressively, after the occurrence of the accident situation, down to values below 100°C. Consequently, the pressure within the building can be held under control for an indefinite period. It is important to note that, in place of water as the heat collection fluid, it is possible to use different substances such as, for example, sodium in metal form. The subdivision of the interspace into sectors, due to the presence of the reinforcing structural elements such as the double-T beams, reduces and further distributes the hydraulic pressure which the water contained in the interspace exerts even in normal operating conditions of the nuclear reactor. Moreover, the presence of the double-T beams, supported either by the containment structure or by the second metal wall, increases the ability of the whole system to resist seismic events. The system according to the invention, having an entirely passive function, lends itself to be integrated with any devices having an active function in such a way as to improve its performance if it is possible for them to be utilised. For example, conventional cold water spray systems on the second metal wall can be envisaged to increase the extraction of heat from the interspace. It will also be clear that the embodiments of the system described with reference to a containment building having a single containment structure can conceptually be adapted also to a double containment structure building. This system can be applied also to preexisting containment buildings in substitution for, or in addition to, active intervention systems. It is to be understood that numerous variations to the heat dissipation system of the invention and the manner in which it is made operative, can be introduced by a man skilled in the art, to satisfy details and contingent requirements, all however lying within the ambit of protection of the invention as defined in the following claims.

Claims: CLAIMS 1. A system (10) for passive dissipation of heat from the interior (8) of a containment structure (3, 9) of a nuclear reactor (2) which would develop particularly in the case of accidents, in which the said containment structure (3, 9) comprises side walls (5, 5') and at least one top wall (6) which have a common inner surface (7) characterised in that it comprises: - a first metal wall (11 ) backing onto the said side walls (5) and top walls (6) within the said containment structure (3); - a second metal wall (16) intermediate between the said first metal wall (11) and the said side walls (5) and/or containment wall (6); - a sealed interspace (18) confined by the said first and second metal walls (11 , 16) and containing a heat collection fluid (22), and; - means (24) for the passive dissipation of heat from the said interspace (18). 2. A system (10) according to Claim 1 , characterised in that the interspace (18) is subdivided into a plurality of sealed sectors (19) delimited by horizontal and vertical beams (14, 20). 3. A system (10) according to Claim 2, characterised in that the absolute pressure within the said sectors (19), in normal operating conditions of the nuclear reactor 2, is less than 10,000 Pa. 4. A system (10) according to Claim 2, characterised in that the said horizontal and vertical beams (14, 20) are double-T beams. 5. A system (10) according to Claim 1 , characterised in that the said second metal wall (16) is adherent to the inner surface (7) and is rigidly fixed to it. 6. A system (10) according to Claim 2, characterised in that the said means (24) for the passive extraction of heat from the interspace (18) comprises a plurality of condensers (25, 25') each corresponding to a respective sector (19) of the interspace (18) and hydraulically communicating with it. 7. A system (10) according to Claim 6, characterised in that the condensers (25, 25') are located in elevated positions with respect to the corresponding respective sector (19) of the interspace (18). 8. A system (10) according to Claim 7, characterised in that the said means (24) for the passive extraction of heat comprise a plurality of apertures in the said side walls (5, 5') close to the said at least one top wall (6, 6'), a descending channel (33) from the said plurality of apertures (31), an ascending channel (34) and a chimney (35) located centrally on the said top wall (6, 6') which puts the ascending channel (34) into communication with the outside, the said descending channel (33) and the said ascending channel (34) being in communication at their lower parts, the said ascending channel (34) containing the said condensers (25). 9. A system (10) according to Claim 1 , characterised in that the first metal wall (11) is free to move within its own plane with respect to the said side and/or top walls (5, 6). 10. A system (10) according to Claim 1 , characterised in that the said second metal wall (16) is free to move within its own plane with respect to the said side and/or top walls (5, 6). 11. A system (10) according to Claim 1 , characterised in that the said first and second metal walls (11 , 16) are each free to move within their own plane with respect to the other. 12. A system (10) according to Claim 1 , characterised in that the said means (24) for the passive extraction of heat include the said second metal wall (16). 13. A system (10) according to Claim 12, characterised in that the said means (24) for the passive extraction of heat include a plurality of apertures in the said side walls (5) close to the said at least one top wall (6), a descending channel (33) from the said plurality of apertures (31), an ascending channel (34) and a chimney (34) located centrally on the said top wall (6) which puts the ascending channel (34) into communication with the outside, the said descending channel (33) and the said ascending channel (34) being in communication at their lower parts, the said ascending channel (34) being delimited on one side by the said second metal wall (16). 14. A system (10) according to Claim 12, characterised in that the said first metal wall (11) is self supporting. 15. A system (10) according to Claim 12, characterised in that the said second metal wall (16) is self supporting. 16. A system (10) according to Claim 12, characterised in that the said second metal wall (16) is spaced from the inner surface (7) in such a way as to leave free a space (30') between the said second metal wall (16) and the side and top walls (5, 6). 17. A system (10) according to Claim 16, characterised in that it includes, in the said space (30') a cage (42) of metal beams (43) located between the said second metal wall (16) and the side and top walls (5, 6) of the said containment structure (3), the said descending channel (33) and the said ascending channel (34) being defined by the relative position of the said metal beams (43) of the cage (42). 18. A system (10) according to Claim 17, characterised in that there is an expansion space (44) between the said cage (42) and the said second wall (16). 19. A system (10) according to Claim 2, characterised in that the said horizontal and vertical beams (14, 20) are supported by the first metal wall (1 1 ) by means of a plurality of hooks (41 ).

Inventors: RIZZO FRANCO LUIGI, RIZZO, FRANCO, LUIGI
Applicants: FINMECCANICA SPA, RIZZO FRANCO LUIGI, FINMECCANICA S.P.A. AZIENDA ANSALDO, RIZZO, FRANCO, LUIGI
Application year: 1996
Main classification: G21C15
IPC codes: G21C15