GB1567119A

GB1567119A - Nuclear reactor installations

Info

Publication number: GB1567119A
Application number: GB9261/78A
Authority: GB
Original assignee: Kraftwerk Union AG
Current assignee: Kraftwerk Union AG
Priority date: 1977-03-09
Filing date: 1978-03-08
Publication date: 1980-05-08
Also published as: JPS53110793A; DE2710290A1; FR2383502A1

Description

(54) IMPROVEMENTS IN OR RELATING TO NUCLEAR REACTOR INSTALLATIONS (71) We, KRAFTWERK UNION ARTIENGESELLSCHAFT, a German company, of Mülheim (Ruhr), Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a nuclear reactor installation.

It is known to us to sink a nuclear reactor installation into the ground and cover it with a mound of earth. As well as acting as protection against extreme external influences, this construction has the advantage that the earth absorbs radioactive gases and vapour escaping from the nuclear reactor installation in the event of an accident leading to melting of the reactor core. In such a case the molten core cannot sensibly be contained in the reactor structure. However, the integrity of the reactor structure should largely be maintained during such an accident since collapse of the structure could result in uncontrolled movements of the earth mass covering the reactor installation and the release of radioactive material from within the reactor installation.

According to the present invention there is provided a nuclear reactor installation comprising a container for the nuclear reactor, the container including a base and being surrounded by granular material, a liquid-impermeable barrier extending downwardly from the base so as partially to define an enclosed space beneath the container, and a pressure relief device connecting the interior of the container to the surrounding granular material.

Conveniently the granular material comprises earth and the container is partially sunk below normal ground level and covered by a mound of earth.

Preferably the pressure relief device connects the interior of the container to the mound of earth. The pressure relief device limits the pressure in the reactor structure and thus prevents it from exploding and being destroyed, in which event the reactor coolant, for example, might uncontrollably escape from the reactor installation. The outlet cross-section of this device is preferably restricted, as the absorption ability of the mound decreases as the density increases. The barrier prevents the access to the reactor structure of the ground water or the water formed in the mound by condensation of vapour in the event of the melting reactor core burning through the base of the reactor structure.Without the barrier, the hole formed in the base of the container would permit virtually free entry of ground water or condensation water accumulating under a pressure of several bar at the reactor structure. The molten reactor core, however, remains in the space enclosed by the barrier which is dry or possibly has a small amount of water. Consequently vapour formation of such magnitude that the reactor structure would be imperilled by overpressure higher than the design pressure or by a pressure wave should in most cases be prevented.

The barrier may expediently be adapted to the ground conditions. Preferably the barrier extends downwards from the base into a liquidimpermeable layer of the ground beneath the reactor structure. If necessary, however, a complete seal can also be obtained by a barrier running largely horizontally, when the ground is water-permeable down to very great depths.

In a preferred embodiment, the water-impermeability need only be such that the amount of water suddenly coming into contact with the molten core remains below 500 t, preferably below 100 t. Thus the enclosure need not be completely water tight. The barrier therefore, may advantageously comprise sealing material which is introduced into loose earth in simple working procedures. Suitable sealing materials are clay, cement or bentonite, a natural mineral. The material may be introduced by injection, if necessary in liquid form, the barrier being produced during construction by freezing.

Advantageously, the horizontal cross-section of the space enclosed by the barrier is smaller than the area of the base, since the barrier is then protected by the reactor structure lying above it and a tight connection to the base of the reactor structure can be ensured. The base may have hollow spaces above the enclosed space which make it easier for the melting reactor core to burn through at a specific region, preferably in the centre of the base. By providing the base with a weakened region, the molten core and thus the heat source pass out from the reactor structure as quickly as possible and the molten core will almost certainly pass down into the space enclosed by the barrier. The hollow spaces should here be at least 10 m radially inwards from the barrier so that molten core penetrating through the hollow spaces cannot cause any damage to the barrier.Instead of just one barrier two or more barriers may be provided.

Reduction of the quantity of water suddenly coming into contact with the molten core can further be promoted by water-bearing spaces in the reactor structure having a temperaturedependently openable outlet of small crosssection. In this way fairly large quantities of water in the reactor structure itself can be prevented from coming suddenly into contact with the molten core and the structure parts heated by it in the event of thermal destruction of the concrete. Instead a temperature-de pendently openable outlet with its small crosssection ensures slow emptying of a water store of this type before the strength of the concrete is made to fail due to excess temperatures and the water store suddenly escapes.In this context small cross-section means a few tens of cm2 (about 100 cm at the most) so that the through-flow quantities do not exceed a few t/h (at most about 100 t/h).

The outlets may have fusible plugs for tem- perature-depencsent opening, made for example from metal alloys with melting points between 200 and 3000C. In the case of fuel element storage tanks with storage supports which with a volume of 1200 m3 and more can be particularly dangerous as sources of water, the outlets should lie above the top edges of the storage supports. In this case, there is still a residual quantity of about 1/3 the normal water level in tanks of this type.In the case where the concrete does not fail, this quantity can still be enough to remove the heat of the stored fuel elements, whilst, in the case of concrete failure, the generation of vapour due to the presence of a quantity of water which is 1/3 the quantity normally present should normally be controllable so as to load the discharged pressure-less concrete vessel suddenly but only within its design pressure of, for example, 5 bar.

For a better understanding of the present invention and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows diagrammatically a vertical section through a nuclear reactor installation according to the invention.

The drawing shows a nuclear reactor installation, with a pressurized water reactor, which generates, for example, 1300 MWe. The pressurized water reactor is disposed in a spherical safety envelope 1 formed from steel which encloses the concrete internal fittings 2, the primary circuit components, and reactor pressure vessel 3. The concrete fittings 2 in dude, amongst other things, a tank 5 for fuel elements which may be housed in a storage support 6. The safety envelope 1 is enclosed by a called secondary shield 7. This is an inverted dish-shaped concrete structure having circular cross-section which, together with the concrete fittings mentioned and the safety envelope 1, is hereinafter called the reactor structure 8. The shield 7 forms a container of the reactor structure 8.

The reactor structure 8 has a fiat base plate 9 which, in this embodiment, is sunk 30 m deep into the ground 10. For this purpose there is a pit 13 provided beneath the earth's surface 11 within a water-bearing gravel layer 12. The pit 13 is closed off by water tight barriers 14 and 15. The double enclosure is a particularly sure safeguard against radioactivity being washed out with the ground water.

The part of the reactor structure 8 projecting above the earth's surface 11 has a mound of earth 16 piled on top of it, the top of which mound is sealed with a dense clay layer 17 and a mechanically rigid layer 18.

The mound is subdivided by partitions 19 so as to ensure optimum enclosure of vapour which under certain circumstances may be radioactive, as described, for example, in British Patent Applications Nos. 29926/77 (Serial No. 1,549,471) and 32056/77 (Serial No. 1,549,475).

Provided beneath the base plate 9 is a further barrier 20, which encloses a zone of circular cross-section 23 having a diameter about 20 m smaller than the diameter of the base plate. Like the barrier 14 and 15, the barrier 20 extends into a water-impermeable zone 21 of the ground 10, and may in fact extend a few metres into this zone. The barrier 20 is formed of bentonite, with a thickness of 2 to 5 m.

Within the portion of the base plate 9 corresponding to the cross-section 23 there are provided hollow spaces 22. The height of the hollow spaces 22 is approximately half the thickness of the base plate 9 which is several metres.

They ensure that within the zone enclosed by the barrier 20, namely in the central area, a defined weakened region is provide in the base plate 9. A reactor core which melts as a result of a conceivable breakdown would penetrate through the base plate 9 at this weakened region. The resultant opening in the reactor structure 8 which is intended to enclose the reactivity opens into the zone enclosed by the barrier 20, which is closed off from the surrounding ground water. For this reason the quantity of vapour produced by the melting core cannot be increased by ground water flowing into this zone. This prevents the reactor structure 8 from being brought to the point of explosion and collapse by a sudden mcrease in vapour pressure.

The fuel element storage tank 5 is provided with an outlet line 25, closed by a fusible plug 26. The fusible plug is, for example, a metal alloy based on tin with a melting point of 2500C. Because of this the outlet 25 is opened if, in the event of a breakdown, a temperature greater than 2500C occurs in the concrete of the reactor structure 8, with the result that the level of water present in the fuel element stor age tank 5 is reduced to the height of the fuel element storage support 6. The normal water volume of 1300 m3, for example, is thereby reduced to 1/3 so that there can be no sudden formation of water vapour.

Provided in the space 30 between the safety envelope 1 and the secondary shield 7 is a socalled flood vessel 31, provided for flooding of the reactor chamber in the event of a loss of coolant accident. This flood vessel with a water volume of 400 m3 is similarly provided with an outlet 32 normally closed with a fusible plug 33.

As the drawing shows, in the nuclear reactor installation illustrated, there are several discharge possibilities provided within the reactor structure which convey the media under pressure out of the reactor suucture into the loose layer of the mound 16 piled on top of the structure 8. In the interior of the steel sphere 1 there is an outlet provided in the form of a line 35 which runs from the upper zone of the steel sphere and has a nominal width of, for example, 400 mm. The line runs via an excess pressure valve 36 with a response pressure of 6 bar to an outlet 37 which pre ferably lies in a zone of coarse gravel in the mound 16, which enables the gases and vapour to spread into the rest of the mound 16.

Arranged parallel to the excess pressure valve 36 is a remotely controllable valve 38. With this valve a reduction of pressure in the safety envelope 1 so as to virtually equalize the pressures in the mound 16 and the safety envelope 1 can be achieved. This is important when, in the event of a breakdown, a sudden generation of vapour occurs which could ex ceed the pressure of 6 bar, determined by the excess pressure valve 36, and which could overload the steel sphere 1.

Running from the space 30 between the sphere 1 and secondary shield 7 to the same outlet 37 there is an excess pressure valve 40 with a response pressure of 2 bar, for ex ample. There is additionally provided a fur ther excess pressure valve 41 which has a response pressure of, for example, 5 bar. With this valve a further outlet with a cross-section of, for example, 1500 cm2 is opened, with the result that the strength of the reactor structure, which corresponds to an internal pressure of 7 bar for example, is not put at risk as long as the intermittent generation of pressure due to vaporisation of water at heated parts of the structure is not greater than 500 t of water.

The outlet openings mentioned earlier are quite sufficient to allow this quantity, continuously vaporised by afterheat of the reactor core, to flow away.

If it is assumed that the reactor is deprived of any cooling during a breakdown of a type which under normal conditions is totally excluded, for example due to military action, there is then produced even with the control rods fully inserted, so much heat that the core may melt. As an example, in the first 46 days after the occurrence of such a breakdown, about 4 x 10' MWs of heat are released.

Theoretically this quantity of heat would be capable of vaporising 16,000 t of water. However, only a quantity of heat equivalent to that needed to vaporise 320 t of water can be stored in the steel sphere 1. Consequently, with further heat generation resulting in vaporisation, heat is removed from the safety envelope 1 in the form of vapour.

If the steel sphere 1 should fail, the vapour can spread into the interior of the secondary shield 7. The energy content storable therein, however, is similarly very limited. For a short time it is equivalent to about 500 t of vapour, excluding any condensation occuring at the walls. In total, however, there is normally about 500 t water present in the reactor structure. This quantity of water results in vapour being released from the reactor structure 8 into the mound via the outlet 37 and outlet 42. There the vapour finally condenses. The condensate flows downwards and, with a mean pore volume of 15 ó of the gravel layer enclosing the reactor structure 7 and the available cross-sectional surfaces, it produces a water level of 2.4 m height.

Heat conduction in the gravel layer is almost negligibly small and the water-saturated layer cannot remove a very great quantity of heat either, so that, without further supply of water, the concrete structure with its internal fittings is considerably heated. Temperatures of more than 2000C can be expected. It is therefore important that, even when the molten core burns through the shield 7, which occurs in a controlled manner by means of the hollow spaces 22 in the base plate 9, there is no substantial inflow of water which would lead to sudden vaporisation at the hot parts of the structure.This is achieved by the barrier 20 which ensures that the volume of water temporarily coming into contact with the molten core and the heated parts of the structure is not greater than the quantity of vapour which can be removed in the same time vaa outlet lines 37 and 42. Explosion of the reactor structure 8, which could lead to collapse of the mound 16 and thus a release of radioactivity, is thereby avoided.

Further shown in this embodiment is an outlet 45 with an excess pressure valve 46 with a response pressure of 1.5 bar. The outlet 45 extends from the loose material of the mound into the open air. This outlet should prevent excess pressure due to the presence of noncondensable air from arising initially during condensation of the vapour in the mound, as this would burst open the layer of clay 17 and the mechanically rigid layer 18. The barrier 20 ensures that, when the water store in the reactor structure has been used up, no further vapour can pass into the mound, although vapour may heat the mound locally to the point where uncondensed vapour escapes through the outlet 45 into the open air.

WHAT WE CLAIM IS:- 1. A nuclear reactor installation comprising a container for the nuclear reactor, the container including a base and being surrounded by granular material, a liquid-impermeable barrier extending downwardly from the base so as partially to define an enclosed space beneath the container, and a pressure relief device connecting the interior of the container to the surrounding granular material.

2. A nuclear reactor installation as claimed in claim 1, wherein the granular material comprises earth, and wherein the container is partially sunk below normal ground level and covered by a mound of earth.

3. A nuclear reactor installation as claimed in claim 2, wherein the pressure relief device connects the interior of the container to the mound of earth.

4. A nuclear reactor installation as claimed in any one of the preceding claims, wherein the barrier extends downwards from the base into a liquid-impermeable layer of the ground.

5. A nuclear reactor installation as claimed in any one of the preceding claims, wherein the or each layer is formed from a mixture of clay, bentonite or cement, and loose earth.

6. A nuclear reactor installation as claimed in any one of the preceding claims, wherein the area of the cross-section of the enclosed space, in a horizontal plane, is smaller than the area of the base.

7. A nuclear reactor installation as claimed in any one of the preceding claims, wherein the portion of the base above the enclosed space includes a plurality of small hollow chambers.

8. A nuclear reactor installation as claimed in claim 7, wherein the small chambers are at least 10 metres away from the barrier.

9. A nuclear reactor installation as claimed in any one of the preceding claims, wherein the or each water containing vessel within the container is provided with an outlet, the opening of which is temperature dependent.

10. A nuclear reactor installation as claimed in claim 9, wherein the or each outlet is sealed by a fusible plug.

11. A nuclear reactor installation as claimed in claim 9 or 10, wherein one vessel is a fuel element storage tank, said outlet of which opens into the interior of the tank above the level of a storage support within the tank.

12. A nuclear reactor installation as claimed in any one of the preceding claims, wherein there is a plurality of barriers extending downwardly from the base of the container.

13. A nuclear reactor installation substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Further shown in this embodiment is an outlet 45 with an excess pressure valve 46 with a response pressure of 1.5 bar. The outlet 45 extends from the loose material of the mound into the open air. This outlet should prevent excess pressure due to the presence of noncondensable air from arising initially during condensation of the vapour in the mound, as this would burst open the layer of clay 17 and the mechanically rigid layer 18. The barrier 20 ensures that, when the water store in the reactor structure has been used up, no further vapour can pass into the mound, although vapour may heat the mound locally to the point where uncondensed vapour escapes through the outlet 45 into the open air. WHAT WE CLAIM IS:-

1. A nuclear reactor installation comprising a container for the nuclear reactor, the container including a base and being surrounded by granular material, a liquid-impermeable barrier extending downwardly from the base so as partially to define an enclosed space beneath the container, and a pressure relief device connecting the interior of the container to the surrounding granular material.