GB2342770A

GB2342770A - Water nuclear reactor with in-built receptacle

Info

Publication number: GB2342770A
Application number: GB9922968A
Authority: GB
Inventors: Jacques Marseille
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1998-10-14
Filing date: 1999-09-28
Publication date: 2000-04-19
Anticipated expiration: 2019-09-28
Also published as: KR100677735B1; GB9922968D0; DE19949585A1; JP2000121772A; FR2784784A1; DE19949585B4; FR2784784B1; KR20000029086A; JP4786007B2; GB2342770B

Abstract

In a water nuclear reactor, a receptacle (20) filled with porous mineral material (30) is placed underneath the core (12), inside vessel (10). In the event of serious accident causing fusion of the core (12), the corium (36) then formed is collected in receptacle (20). The presence of material (30), free of water, avoids the occurrence of a vapour explosion. Also, material (30) cools the corium (36) to a temperature below the melting point of the refractory bricks (24) of receptacle (30).

Description

WATER NUCLEAR REACTOR WITH IN-BUILT RECEPTACLE 2 WATER NUCLEAR REACTOR

WITH IN-BUILT RECEPTACLE

DESCRIPTION

Field of the invention

The invention relates to a pressurized water or boiling water nuclear reactor, whose vessel contains a basin-shaped receptacle intended to collect the "corium", that is to say the solid or liquid debris derived from the core of the reactor in the event of the latter's accidental fusion.

State of the art In nuclear reactors, a large number of systems have been considered and developed over the last few years for the purpose of limiting the consequences of a serious accident causing partial or full fusion of the reactor core.

In particular, as illustrated by document FR-A-2 341 181, it has already been put forward that, in the bottom of the nuclear reactor vessel, a withholding device can be placed intended to avoid perforation of the vessel by the corium. formed during accidental fusion of the reactor core. This withholding device comprises several spaced out horizontal plates, fixed to the wall of the vessel and crossed by openings which are staggered from plate to plate and whose edges project upwards. In the event of accident, the corium which flows through these openings subsequently moves up into a bell-shaped distributor positioned in the B 13122.3 GP 3 centre of the horizontal plates and resting on the bottom of the vessel.

Also, as illustrated by document US-A-3 964 9.66, the idea has been put forward of placing a corium receptacle under the core of the nuclear reactor cooled by a liquid metal, inside the reactor vessel. This receptacle, made in steel, is directly suspended from the lower horizontal plate supporting the core. Heat exchange piping crosses the base of the receptacle and projects upwards inside the receptacle. These pipes are closed at their top end and communicate with the inside of the receptacle via holes provided for this purpose.

Irrespective of the geometry of the receptacle device it is considered incorporating inside the vessel of a water reactor, there is a risk that accidental fusion of the core is followed by an explosion. For the corium. fragments into small particles while it flows towards the bottom of the vessel. The water contained in the reactor vessel vaporises on contact with the small corium particles. A vapour film is formed around each particle, creating favourable conditions for an explosion causing a high energy shock wave. Given the origin of this type of explosion it is usually called explosion".

At the present time, the receptacle devices described to collect and maintain the corium in the bottom of a water reactor vessel are not designed to prevent a vapour explosion. Also, these devices are not protected from the effects of such an explosion should such occur. Despite the low probability of interaction between the vaporised water and the corium, the possibility of this type of explosion cannot be fully B 13122.3 GP 4 dismissed. The extremely powerful pressure peak generated by the explosion could damage the intactness of the receptacle device and therefore its effectiveness.

Disclosure of the invention

The subject matter of the invention is precisely a pressurized water or boiling water nuclear reactor fitted with a corium collection device incorporated into the vessel, whose original design can prevent the 10, occurrence of a vapour explosion and, consequently, an preserve the intactness of the receiving device in the event of accident.

According to the invention, this result is obtained by means of a water nuclear reactor, comprising a vessel and a reactor core housed in the vessel, said reactor being characterized in that it also comprises a basin-shaped receptacle housed in the vessel underneath the core and made, at least in part, of refractory material, said receptacle being able to collect the corium formed during any accidental fusion of the core and being fully filled with a porous mineral material able to mix with the corium. and lower its temperature to an equilibrium temperature lower than the melting point of the refractory material.

In a nuclear reactor built in this way, the corium formed during accidental fusion of the core flows out and fragments within the porous mineral material which does not contain water. The conditions propitious to a vapour explosion, which are the presence of a vapour film around each particle of corium, are therefore avoided. Consequently such B 13122.3 GP explosion cannot occur and the intactness of the receptacle is preserved.

This effect is heightened by the gradual cooling of the corium during its downward movement into the porous mineral material contained in the receptacle. In this way, damage to the receptacle due to melting of the refractory material is also avoided.

In one preferred embodiment of the invention, means for spreading and dispersing the corium, such as a grid or perforated horizontal plate, are placed ab6ve the receptacle and the porous mineral material it contains.

In this preferred embodiment of the invention, the porous mineral material is a ceramic foam preferably containing approximately 99 % silica. The porosity of the mineral material contained in the receptacle means that it is possible to have available inside the receptacle a volume that is sufficient to take a large mass of corium. This porosity can in particular lie between approximately 63 and approximately 80 %.

Advantageously, the receptacle is separated from the bottom of the vessel by a space which leads upwards and ensures the circulation of water around the receptacle. This water circulation aids in cooling the receptacle and therefore also contributes towards maintaining its intactness.

The receptacle may, as required, be either substantially hemispherical, or formed of a substantially hemispherical part over which is mounted a substantially cylindrical part.

B 13122.3 GP 6 In the preferred embodiment of the invention, the refractory material is in the form of bricks which are placed inside a steel casing which is also part of the receptacle.

Brief description of the drawing

As a non-restrictive example a preferred embodiment of the invention will be described referring to the appended drawing in which the single figure is a vertical cross section, giving a diagram of the lower part of a water nuclear reactor whose vesel incorporates a receptacle according to the invention in the event of an accident leading to fusion of the reactor core.

Detailed description of a preferred embodiment of the invention The embodiment shown in the figure concerns a pressurized water nuclear reactor. However, as has already been pointed out, the invention is not limited to this type of reactor and generally concerns all water reactors. It therefore also applies to boiling water reactors.

In the single figure, reference 10 denotes the reactor vessel. More precisely, only the lower part of the vessel is shown. In its central part, vessel 10 contains reactor core 12, and its associated inner equipment.

Core 12 is usually formed of a large number of assemblies of nuclear fuel, arranged vertically and juxtaposed. The assemblies rest on a lower horizontal plate 14. This plate 14 is perforated next to each nuclear fuel assembly to allow circulation of the B 13122.3 GP 7 cooling water contained in vessel 10, inside these assemblies. A perforated flow-distribution plate 16, preferably having a convex lower part, is generally fixed to and below plate 14.

Core 12 of the reactor, and its associated inner equipment, such as horizontal lower plate 14 and perforated plate 16, rest on the vertical cylindrical wall of vessel 10 via supporting and guiding devices 18.

It is to be noted that core 12 and its associated inner equipment may be of any shape, different to those shown in the figure, while remaining within the scope of the invention.

In accordance with the invention and as shown in the single figure, a basin-shaped receptacle 20 is placed inside vessel 10 below core 12 of the reactor. More precisely, receptacle 20 is placed in between perforated plate 16 and the convex, generally hemispherical, base of vessel 10.

Receptacle 20 is designed to collect the corium in the event of accidental fusion of reactor core 12.

It is recalled that the term "corium" denotes the fusion mass that is produced should such an accident occur and generally contains the nuclear fuel, its cladding materials and the cladding materials of the reactor control rods and the internal structures associated with core 12. Receptacle 20 is arranged and dimensioned such as to collect the entirety of the corium. in the event that a serious accident should lead to complete fusion of reactor core 12.

In the preferred embodiment of the invention shown in the single figure, receptacle 20 is separated B 13122.3 GP 8 from the base of vessel 10 by a space leading upwards between the peripheral upper edge of the feceptacle and vessel 10. This space 22 allows circulation of the water contained in reactor vessel 10 as shown by the arrows in the f igur e. Should an accident occur, water circulation is set up in this space 22 by natural convection. The effect produced by this water circulation is to cool receptacle 20.

Receptacle 20 comprises at least one layer of 10. bricks 24 in refractory material. This material -is chosen such as to have as high a melting temperature as possible, and good chemical compatibility with the corium. It may in particular be a zirconium-based ceramic material which also has the advantage of being widely available on the industrial market.

As illustrated in the figure, bricks 24 are juxtaposed and preferably have complementing adjacent edges, for example in U or dovetail shape which interlock into one another.

Receptacle 20 also comprises a metal casing 26 inside which bricks 24 are placed. This metal casing 26 may in particular be made of stainless steel. It comprises an inner and an outer skin which fully cover bricks 24. Casing 26 also comprises an upper flange connecting together the upper edges of the inner and outer skins of receptacle 20.

The upper flange of metal casing 26 may be used to suspend receptacle 20 in vessel 10. In this case it rests on supports 28 provided inside vessel 10 as illustrated in the diagram of the sole figure.

As a variant or complement, radiating reinforcements (not shown) having a radial orientation B 13122.3 GP 9 in relation to the vertical axis of vessel 10 may be interposed in space 22 separating the latter from receptacle 20. In this case the radial reinforcements are provided with holes to contribute towards the 5 circulation of water in space 22.

In the embodiment shown, receptacle 20 has been given a substantially hemispherical shape, concentric with the shape of the base of vessel 10.

In a variant of embodiment not shown, this substantially hemispherical shape may be extenbed upwards by a substantially cylindrical part centred on the vertical axis of vessel 10. This arrangement may be chosen in particular if the total volume of the corium likely to be formed during a serious accident exceeds the available volume inside receptacle 20 if the latter is solely formed of a hemispherical part. Evaluation of the volume available inside receptacle 20 is made by taking into consideration the presence of a porous mineral material 30 inside the receptacle as will be described more in detail below.

So that the entirety of the corium produced by core 12 during a serious accident may be effectively collected in receptacle 20, it is possible to place a ring collector 32 over its upper edge as illustrated in the single figure. This collector 32 may in particular rest on the upper flange of receptacle 20 via a crossbeam structure 33.

The top surface of collector 32 has approximately the shape of a hopper curved inwards and extending upwards as far as the proximity of the wall of vessel 10. More precisely, an allowance limited to a few centimetres is provided between collector 32 and the B 13122.3 GP wall of vessel 10, such as to prevent debris and flow from the core in fusion from penetrating inside space 22 provided between receptacle 20 and the base of vessel 10.

S In accordance with one essential characteristic of the invention, as above mentioned, receptacle 20 is fully filled with a porous mineral material 30. This porous mineral material is chosen such that it can mix with the corium formed during accidental fusion of core 12 and can lower corium temperature to an equilibrium temperature that is lower than the melting point of the refractory material forming bricks 24. By mixing with the corium, material 30 free of water opposes the occurrence of pre-conditions for a vapour explosion.

1S This is because material 30, being free of water, prevents the formation of a vapour film around each corium particle formed by the fragmentation of the corium when it moves down inside receptacle 20. The risk of damage to the receptacle due to a vapour explosion is therefore practically ruled out.

The porous mineral material 30 which fills receptacle 20 is advantageously a ceramic foam containing approximately 99 % silica. This type of material allows gradual cooling of the corium during its downward movement into receptacle 20. The temperature can therefore be stabilised below the melting point of the refractory material forming bricks 24. In this way it is possible to avoid damage to receptacle 20 through melting of the refractory bricks.

The porosity of material 30 is determined so as to provide a sufficient volume inside receptacle 20 to collect the entirety of the corium formed by fusion of B 13122.3 GP core 12. A material with a porosity of approximately 63 % to approximately 80 % is considered as satisfactory from this viewpoint, while ensuring efficient slowing and cooling of the corium during its downward movement.

Preferably, as illustrated in the single figure, means for spreading and dispersing the corium are placed above receptacle 20 and the porous mineral material 30 it contains. These means may in particular be formed of a horizontal perforated plate 34 or a horizontal grid.

The pathway 36 of the corium. produced by the fusion of core 12 is shown in the event of a serious accident. When it falls on perforated plate 34, the corium. spreads over the entire width of receptacle 20 which it enters via the holes of the plate. The corium then f ragments within the porous mineral material 30 free of water. As already mentioned, this material eliminates any risk of vapour explosion. The downward movement of the corium slowed down by material 30 is accompanied by its cooling to a temperature below the melting point of refractory bricks 24. The latter are also cooled, like all receptacle 20, by the circulation of water which is set up by natural convection within space 22. The intactness of receptacle 20 can therefore be preserved under the best conditions.

The invention is evidently not limited to the above-described embodiment given by way of example. As already pointed out, it can indifferently be applied to any type of water nuclear reactor, irrespective of the arrangement of the core and associated inner structures. Also, the shape and structure of the receptacle may be different from those described.

B 13122.3 GP 12 Furthermore, even if space 22, the corium spreading means and ring collector 32 are desirable, their presence may, in some cases, be omitted. Finally, the nature of the porous mineral material may, in some cases, differ from the described silica-based ceramic foam even if this material is preferred.

B 13122.3 GP 13

Claims

CIAIMS

1. Water nuclear reactor comprising a vessel'-and a reactor core hous6d in the vessel, said reactor also comprising a basin-shaped receptacle housed in the vessel below the core and made at least in part of refractory material, said receptacle being able to collect corium formed during accidental fusion of the core and being entirely filled with a porous mineral material able to mix with the corium and to lower its temperature to an equilibrium temperature below the melting point of the refractory material.

2. Reactor according to claim 1, in which corium spreading and dispersion means are placed above the receptacle and the porous mineral material it contains.

3. Reactor according to claim 2, in which said corium. spreading and dispersion means comprise a grid or a horizontal perforated plate.

4. Reactor according to any preced4ng claim, in which the porous mineral material is a ceramic foam.

5. Reactor according to claim 4, in which the ceramic foam contains approximate.i.y 99 % silica.

6. Reactor according to any preceding claim, in which the porous mineral material has a porosity of approximately 63 % to approximately 80 %.

7. Reactor ac cording to any preceding claim, in which the receptacle is separated from the bottom of the vessel by an upward- leading space for the circulation of water.

B 13122.3 GP 14

8. Reactor according to any preceding claim, in which the receptacle is substantially hemispherical.

9. Reactor according to any of Claims 1 - 7, in which the receptacle comprises a substantially hemispherical part over which is mounted a substantially cylindrical part.

10. Reactor according to any preceding claim, in which the receptacle comprises a metal casing, in which bricks of said refractory material are placed.

11. Reactor substantially as hereinbefore described with reference to the accompanying drawing.