WO2008090446A1

WO2008090446A1 - System for evacuating the residual heat from a nuclear reactor

Info

Publication number: WO2008090446A1
Application number: PCT/IB2008/000144
Authority: WO
Inventors: Luciano Cinotti
Original assignee: Del Nova Vis S.R.L.
Priority date: 2007-01-24
Filing date: 2008-01-23
Publication date: 2008-07-31
Also published as: ITTO20070052A1; WO2008090446A8; EP2122635A1

Abstract

The invention relates to a system (1) for evacuating the residual heat from a nuclear reactor (2), wherein the residual heat is evacuated by irradiation from the main tank (3) of the nuclear reactor towards a ring of substantially U-shaped tubes (15), inside which external air circulates in natural circulation and which are arranged circumferentially around the tank (3). The tubes (15) are grouped together in modules (11), each of which is made up of a set of U-shaped tubes (15) inserted inside one another with respective side-by-side branches (16) facing the tank (3) and arranged on opposite sides of a common central axis (X) and laterally separated from one another by empty spaces. A radiant heat reflecting wall (30), which also constitutes a structure (33) for containing an insulation filling (34) of the nuclear-reactor well (5), is set behind the tubes (15) with respect to the tank (3) to reflect back onto the tubes (15) the radiant heat passing through the empty spaces between one tube (15) and another. The reflecting wall (30) is shaped in such a way as to be partially inserted between the branches (16) of the tubes (15) to define a longitudinal channel (31) set substantially around each branch (16).

Description

"SYSTEM FOR EVACUATING THE RESIDUAL HEAT FROM A NUCLEAR REACTOR"

TECHNICAL FIELD

The present invention relates to a residual heat evacuation system of a nuclear reactor, in particular a liquid metal or molten salts cooled nuclear reactor.

BACKGROUND ART

It is known that in nuclear reactors there exists the need to evacuate the residual heat after the reactor is stopped. For safety reasons, the systems for evacuating the residual heat must be particularly reliable.

A reliable solution consists in dissipating the heat emitted by irradiation and marginally also by convection from the tank of the reactor towards the outside via heat-exchange tubes containing external air in natural circulation.

In particular, for reactors of the double-tank type, i.e., where the reactor tank is housed in a safety tank, resting for example on the reinforced-concrete supporting structures of the reactor, systems are known in which the heat-exchange tubes are housed in the annular space available between the safety tank and the reinforced-concrete supporting structures and are arranged in rings at a certain distance from the safety tank. The space between the reactor tank and the safety tank must on the other hand be reduced to the minimum in order not to complicate the system for supporting the reactor tank and the safety tank excessively.

The number of the heat-exchange tubes must not be excessive in order to reduce to the minimum their encumbrance in the high part, where they must run between the supports of the reactor tank, and to facilitate their connection to the necessary headers for respectively supplying cold air and for conveying heated air .

There are known, for example, systems in which the heat- exchange tubes are U-shaped and are arranged with respective cool branches (in which cooler air descends) constituting a radially outer ring and with respective hot branches (in which hotter air rises) constituting a radially inner ring. The tubes of each ring are arranged adjacent to and substantially in contact with one another. These solutions present the drawbacks of requiring a large number of tubes, occupying a large portion of the space of the reactor well, and using for the purposes of heat exchange approximately only a quarter of the surface of the tubes, i.e. , the part facing the reactor tank.

Other known systems envisage recourse to a single ring of tubes provided with an internal diaphragm that delimits within each tube a cold duct, facing the reactor well, in which cooler descending air circulates, and a hot duct, facing the reactor tank, in which hotter ascending air circulates. These solutions occupy less space than the preceding ones but still entail a large number of connections to be made with the external headers, use for heat exchange purposes only about half of the surface of the tubes, and create a thermal gradient between the cylindrical generatrices of the heated part of the tubes with respect to the part in the shade, with risks of deformation (warping) . Moreover, each tube requires a bottom closure weld, said weld having to undergo periodic checks because it is part of the confinement barrier of the reactor from external environment .

DISCLOSURE OF INVENTION

A purpose of the present invention is to provide a system for evacuating the residual heat from a nuclear reactor aimed at improving the -known solutions, by eliminating or reducing the drawbacks thereof. In particular, a purpose of the invention is to provide a system that requires a reduced number of heat- exchange tubes, that does not require welding of the tubes in their parts housed in the reactor well, that enables substantially total exploitation of the surface of the tubes for heat exchange purposes, and that causes limited circumferential thermal gradients.

In accordance with said purposes, the present invention relates to a system for evacuating the residual heat from a nuclear reactor, in particular a nuclear reactor cooled with liquid metal, molten salts, or gas, as defined essentially in the annexed Claim 1 and, as regards its preferred characteristics, in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of example in the following non-limiting embodiment, with reference to the accompanying drawings, in which:

Figure 1 is a partial schematic longitudinal section view, with a detail enlarged, of a nuclear reactor provided with a system for evacuating the residual heat in accordance with the invention;

Figure 2 is a partial schematic cross-sectional view of the reactor of Figure 1; and

Figure 3 is a schematic perspective view, with parts removed for reasons of clarity, of the system for evacuating the residual heat of Figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION

In the figures, designated as a whole by 1 is a system for evacuating the residual heat of a nuclear reactor 2 of a substantially known type, in particular a nuclear reactor which uses liquid metal or molten salts as primary fluid.

Reactor 2 comprises a main tank 3, which extends substantially along an axis A and in which the various components of the reactor 2 are housed (not represented in so far as they are known and do not concern the present invention) .

The tank 3 is closed at the top by a roof 4 and is housed in a reactor well 5, which in the example shown is delimited by a safety tank 6 resting on a reactor supporting structure 7, for example made of concrete. The tank 3 is contained within the safety tank 6 and is positioned at a pre-set distance from the safety tank 6 in such a way that between the tank 3 and the safety tank 6 there is defined a substantially annular gap 8. The tank 3 is supported by a plurality of supports 9 that are arranged along a top perimetral edge 10 of the structure 7 and are circumferentially spaced apart from one another.

The system 1 comprises a plurality of cooling modules 11, substantially the same as one another and arranged circumferentially around the tank 3 and hence around the axis A. Each module 11 comprises a cold header 12 and a hot header 13, arranged circumferentially around the roof 4 on the outside of the reactor well 5, and a plurality of substantially U-shaped heat-exchange tubes 15, which connect the cold header 12 to the hot header 13 and are arranged within the gap 8 in the proximity of the safety tank 6 and facing the tank 3.

Each tube 15 is substantially fork-shaped and has: a pair of substantially vertical side-by-side branches 16, arranged on opposite sides of a central axis X and gradually converging towards each other; a U-shaped curved portion 17, which joins the two branches 16 at the bottom in the proximity of a bottom area of the tank 3; and two substantially horizontal portions 18, which connect at the top the branches 16 to the cold header 12 and to the hot header 13 respectively. In particular, each tube 15 has a descending branch 16a, connected via a portion 18a to the cold header 12, and an ascending branch 16b, connected via a portion 18b to the hot header 13 .

The tubes 15 are arranged in a ring around the tank 3, and all the branches 16 of the tubes 15 face the tank 3, i.e. the tubes 15 are arranged to form a single ring in which all the branches 16 are substantially arranged at the same distance from the tank 3. The branches 16 are arranged basically along an internal surface 19 of the safety tank 6 facing the tank 3 and follow the profile (possibly curved towards the axis A) of the safety tank 6.

The tubes 15 are mechanically supported by the safety tank 6. In particular, the tubes 15 are fixed, with the possibility of vertical sliding to enable adaptation to any thermal expansion, in a certain number of points on the internal surface 19 of the safety tank 6, for example via collars 20 set around the branches 16 and anchored to the internal surface 19.

Preferably, each tube 15 consists, at least for the part contained within the reactor well 5, of a monolithic tubular body 21 bent to form a U and having no welds. In other words, the branches 16 and the U-shaped curved portion 17, and optionally also the portions 18, of each tube 15 constitute a monolithic tubular body 21 without any welds.

In each module 11, the descending branches 16a and the ascending branches 16b of the tubes 15 are arranged symmetrically on opposite sides of a common central axis X. All the branches 16 are set at a distance from one another and separated laterally by empty spaces. The branches 16 are set at a greater distance from one another at respective top ends (joined to the portions 18), and are closer to one another at respective bottom ends (joined to the portions 17) , in the bottom area of the tank 3. Advantageously, the tubes 15 of each module 11 are arranged on opposite sides of a support 9 positioned substantially along an axis X. Further supports 9 are possibly arranged also between one module 11 and another.

The cold headers 12 of the various modules 11 converge in one or more supply ducts 22 (just one is represented schematically in Figure 3 for reasons of simplicity) , which terminate with an external air intake 23, set on the outside of a building 24 that houses the reactor 2. The hot headers 13 converge in turn in one or more outlet ducts 25 (just one is represented schematically in Figure 3 for reasons of simplicity) , which terminate with a chimney 26 set on the outside of the building 24.

The system 1 further comprises a radiant heat reflecting wall 30, set radially outer and behind the tubes 15 with respect to the tank 3 to reflect back onto the tubes 15 the radiant heat passing through the empty spaces between one tube 15 and another .

The wall 30 is positioned close to the internal surface 19 of the safety tank 6 and is between the internal surface 19 and the tubes 15, at a relatively small pre-set distance from the tubes 15, i.e. in strict proximity of the tubes 15. The wall 30 is shaped in such a way as to be partially inserted between the branches 16 of the tubes 15 and to surround respective rear portions of the branches 16, substantially defining a longitudinal channel 31 positioned substantially around each branch 16.

The wall 30, for example made of sheet metal, has a reflecting surface 32, facing the tubes 15, with high reflecting power, for example with a mirror finish.

The wall 30 defines a structure 33 containing an insulation filling 34 with high capacity of thermal insulation, which is positioned between the wall 30 and the internal surface 19 of the safety tank 6.

A layer 35 of refractory material is inserted between the safety tank 6 and the reactor supporting structure 7; moreover, a plurality of supplementary tubes 36 are embedded in the material (concrete) of the structure 7, in the proximity of the layer 35 of refractory material, and in which cooling water circulates and which are arranged circumferentially at a distance apart from one another around the safety tank 6. The layer 35 of refractory material is thus positioned between the supplementary tubes 36 and the safety tank 6.

The system 1 further comprises air-suction pipes 40, substantially vertically housed in the reactor well 5 and specifically in the gap 8, for cooling the air contained in the reactor well 5 and for creating, if need be, a current of cold air entering the reactor well 5. The suction pipes 40 are arranged substantially along the internal surface 19 of the safety tank 6 and follow the profile thereof. The suction pipes 40 extend up to the bottom area of the tank 3 and are open at the bottom. Advantageously, the suction pipes 40 are arranged on the supports 9 and hence substantially along the axes X of the modules 11 (i.e. within the innermost tube 15 of each module 11) and/or between one module 11 and another.

The suction pipes 40 are connected to an air circulation and cooling auxiliary system 41, comprising one or more ducts 42 in which the suction pipes 40 merge, at least one heat exchanger 43, for example an air-water heat exchanger, and at least one air circulation blower 44.

The gap 8 is closed at the top by removable closing structures 45. In the modules 11, a cooling fluid circulates which is designed to evacuate to the external atmosphere the residual heat or power of decay of the core of the reactor 2 that is irradiated from the tank 3.

Preferably, the cooling fluid circulating in the system 1 is air. In the system 1 and specifically within the tubes 15, external cooling air, taken from outside the reactor 2, hence circulates .

Air circulation in the system 1 occurs preferably in natural circulation, thanks, for example, to the chimney 26, connected to which are the outlet ducts 25 coming from the hot headers 13, which is able to ensure a suitable draught. Clearly, there may also be provided an air forced circulation in the system 1 by means of one or more blowers .

The air enters from one or more air intakes 23, located outside the building 24 that houses the reactor 2 and branches off into the supply ducts 22 of the cold headers 12 of the modules 11. The air descends in the descending branches 16a of the tubes 15 and is heated, rises in the ascending branches 16b, heating up further, and circulates in the hot headers 13 and in the outlet ducts up to the chimney 26, to be emitted into the atmosphere .

Evacuation of the heat to the external atmosphere hence occurs by irradiation from the tank 3 to the outer surface of the tubes 15, by thermal conduction through the walls of the tubes 15, and by transfer of heat to the external air that circulates within the tubes 15.

A first part 47 of radiant energy emitted by the tank 3 reaches the tubes 15 directly, whilst a second part 48, which passes through the empty spaces between the tubes 15, is reflected towards the tubes 15 by the wall 30, which performs at the same time the function of containing the insulation filling 34. The energy reflected is sent back towards the part of tube 15 in the shade from direct radiation, distributing more uniformly the radiating power on the outer surface of the tubes 15 and reducing the circumferential thermal gradients and the risk of warping of the tubes .

By causing both the descending branches 16a of the tubes 15 (in which the cooling air enters) and the branches 16b (from which the heated air exits) to face the tank 3 directly and by keeping the tubes 15 at a certain distance from one another, it is possible to reduce the number of necessary tubes significantly with respect to the known solutions and improve the mechanical behaviour thereof .

Preferably, the outer surface of the tank 3 is made in such a way as to increase the thermal emissivity, since it is, for example, painted with high-emissivity paint (at least on the unwelded surfaces) . Also the outer surface of the tubes 15 is painted with high-emissivity paint.

The closing structures 45 prevent any dispersion of heat by convective motion of air between the reactor well 5 and the atmosphere of the building 24.

For inspections on the tank 3 to be carried out with the reactor 2 not running but with the reactor primary fluid at a temperature conveniently higher than its melting point, it is necessary to remove at least partially the closing structures 45. In this case, to keep the temperature of the air in the gap 8 as low as possible and prevent violent exit of hot air from the gap 8 , the hot air contained in the gap 8 can be aspirated via the suction pipes 40 and cooled before being released into the building 24. An equal current of cold air will circulate from the building 24 to the gap 8, protecting both any staff that were to be in the proximity of the supports 9 and the equipment used for inspection.

The supplementary tubes 36 have the function of keeping the temperature of the material of the structure 7 substantially uniform, by removing the modest thermal power transmitted through the insulation filling 34, the safety tank 6, and the layer 35 of refractory material.

In the event of failure of the tank 3 with flooding of the reactor well 5 with the primary fluid, the system 1 preserves its own function of removal of the residual heat with the tubes 15 that exchange directly with the primary fluid.

In the case where the primary fluid is lead, the insulation filling 34 immersed in the lead degrades, causing an increase in the temperature of the safety tank 6 and increasing the thermal leaks, which, however, can still be limited by the layer 35 of refractory material and evacuated by the supplementary tubes 36.

Finally, it is understood that numerous modifications and variations can be made to the system described and illustrated herein, without thereby departing from the sphere of protection of the annexed claims.

It is also understood that, even though generic reference has been made, by way of example, to liquid-metal or molten-salt reactors, the system according to the invention finds application also in different reactors, for example gas reactors, obviously once the necessary modifications in the conformation and arrangement of the heat-exchange tubes have been made so as to adapt to the different reactor scheme.

Claims

C L A I M S

1. A system (1) for evacuating the residual heat from a nuclear reactor (2) , in particular a nuclear reactor cooled with liquid metal or molten salts, the system comprising a plurality of cooling modules (11) , arranged circumferentially around a main tank (3) of the reactor in a reactor well (5) and having heat-exchange tubes (15) in which a cooling fluid circulates; the system being characterized in that each module

(11) comprises a plurality of substantially U-shaped heat- exchange tubes (15) , having respective pairs of side-by-side branches (16) facing the tank (3) and arranged on opposite sides of a common central axis (X) and separated laterally by empty spaces; and by comprising a radiant heat reflecting wall

(30) , set behind the tubes (15) with respect to the tank (3) to reflect back onto the tubes (15) the radiant heat passing through the empty spaces between one tube (15) and another.

2. A system according to Claim 1, wherein the reflecting wall (30) is shaped in such a way as to be partially inserted between the branches (16) of the tubes (15) to define a longitudinal channel (31) positioned substantially around each branch (16) .

3. A system according to Claim 1 or Claim 2 , wherein the reflecting wall (30) is provided with a reflecting surface (32) , for example with a mirror finish, facing the tubes (15) .

4. A system according to any one of the preceding Claims, wherein each module (11) comprises a cold header (12) and a hot header (13), which are arranged outside the reactor well

(5) and in which respective branches (16) of the tubes (15) merge .

5. A system according to any one of the preceding Claims, wherein the branches (16) of each tube (15) are arranged at a greater distance from one another at respective top ends , and are closer to one another at respective bottom ends.

6. A system according to any one of the preceding Claims, wherein the heat-exchange tubes (15) are painted externally with high-emissivity paint.

7. A system according to any one of the preceding Claims, wherein cooling air circulates in the heat-exchange tubes

(15) .

8. A system according to Claim 7, wherein the cooling air circulates within the tubes (15) by natural draft promoted, for example, by means of a chimney (26) .

9. A system according to any one of the preceding Claims, wherein the reflecting wall (30) defines a structure (33) containing an insulation filling (34) , the insulation filling (34) and the heat-exchange tubes (15) being arranged on opposite sides of the reflecting wall (30) .

10. A system according to any one of the preceding Claims, wherein each tube (15) consists, at least for the part contained within the reactor well (5) , of a monolithic U- shaped tubular body (21) having no welds.

11. A system according to any one of the preceding Claims, wherein the heat-exchange tubes (15) are substantially arranged along an internal surface (19) of a safety tank (6) of the reactor (2), set around the main tank (3), and are mechanically supported by said internal surface.

12. A system according to any one of the preceding Claims , comprising air-suction tubes (40), substantially vertically housed in the reactor well (5) and open at the bottom, for cooling the air contained in the reactor well (5) and for creating a current of cold air entering the reactor well (5) .

13. A system according to Claim 12, wherein the suction pipes (40) are connected to an air circulation and cooling auxiliary system (41) .

14. A system according to Claim 12 or Claim 13, wherein the suction pipes (40) are positioned between one module (11) and another and/or along the central axes (X) of the modules (11) .