GB2030347A

GB2030347A - Nuclear Reactor Containment

Info

Publication number: GB2030347A
Application number: GB7931862A
Authority: GB
Original assignee: Kraftwerk Union AG
Current assignee: Kraftwerk Union AG
Priority date: 1978-09-14
Filing date: 1979-09-13
Publication date: 1980-04-02
Also published as: AR219182A1; ES484147A0; DE2840086A1; SE7907577L; ES8103437A1; DE2840086C2; BR7905616A; GB2030347B; JPS5555289A; FR2436474A1

Abstract

A nuclear reactor installation is housed inside a spherical safety casing (2). The reactor is disposed in a pressure vessel (1) and a discharge duct (3) connects the interior of the safety casing (2) to a chamber (4). In the event of an accident causing a core meltdown, the molten reactor core can pass through the discharge duct (3) into the chamber (4). The duct (3) is lined with an anhydrous fire-proof lining (23) of refractory elements, while the chamber (4) is lined with a water-cooled steel jacket (41). The molten core material, after passing through the duct (3), will flow over a distribution core 42 lying on a bed (43) of anhydrous material e.g., basalt or asbestos, which absorbs the molten material. The bed (43) is mounted on a support (44) provided with radial expansion slots through which the molten material can flow. A second collar-shaped, water-cooled steel jacket (45) is provided. <IMAGE>

Description

SPECIFICATION Nuclear Reactor Installation This invention relates to a nuclear reactor installation.

Frequently, in a nuclear reactor installation, the reactor core is disposed in a pressure vessel and comprises a plurality of fuel elements. The pressure vessel is housed inside a concrete chamber, and together with steam generators and other auxiliary apparatus is located inside a steelconcrete safety casing which is pressure resistant.

It is conveivable that the core of the nuclear reactor could melt if all the many cooling devices which are present for the reactor core fail. There follows a description of the possible sequence of events in the case of a core melt down in a reactor installation.

In the event of such a hypothetical assumption the reactor core would melt down in a matter of hours. The resultant molten mass would contain, in the first instance, the nuclear fuel and the material of the encasing tubes of the fuel rods, zirconium or steel for example. This molten mass of the core falls and is then caught firstly by the reactor pressure vessel. However, the barrier effect of the pressure vessel relative to the molten mass is limited as, in its typical position in the installation, the pressure vessel is virtually uncooled so that it heats up and its heat absorption capacity is used up relatively rapidly despite the high failure temperature of 1300 to 1 4000C, for example, of the material of the pressure vessel.

When the base of the reactor pressure vessel has been destroyed, the molten mass falls onto a concrete foundation in the reactor chamber. At this point in time, the molten mass comprises, for example, approximately 75 tonnes steel, 120 tonnes UO2 and 28 tonnes Zry. The molten mass is now substantially cooled for the first time by heat conduction in the concrete and by radiation from the surface of the pool of molten mass. As, however, the concrete also starts to melt at approximately 13000C, this mixes with the molten metal by convection. This is combined with a collapse of the concrete structures so that the possibility exists of the water in reactor sump coming into contact directly or indirectly with the molten mass, evaporating and causing an increased pressure build-up in the reactor safety vessel.As, however, the steel reactor safety vessel is also melted in the region of its base, this region is thereby weakened so that the resistance to pressure of safety casing is reduced.

According to the present invention there is provided a nuclear reactor installation comprising: a safety casing; a pressure vessel located within the safety casing and containing a nuclear reactor; a chamber disposed beneath the safety casing; and a discharge duct located in the safety casing and beneath the pressure vessel; so as to provide communication between the interior of the safety casing and the chamber whereby, in the event that the core of the nuclear reactor melts, the molten core can pass into the chamber.

Preferably the discharge duct is substantially vertical and comprises a plurality of anhydrous refractory elements.

The core of the nuclear reactor preferably comprises a plurality of fuel elements.

Preferably the safety casing comprises concrete and/or steel, and there is provided auxiliary equipment and a concrete housing within the safety casing, the pressure vessel and reactor core being disposed in the concrete housing.

Advantageously, the portion of the safety vessel defining an opening for the discharge duct is strengthened with a tie bar.

The base and at least part of the sides of the chamber can be defined by a first-water cooled jacket, for absorbing heat from a molten core, and a roof of the chamber can be defined by a second water-cooled steel jacket for preventing thermal radiation from within the chamber heating upon components of the installation immediately above the chamber.

There can be provided within the chamber; a heat-resistant distribution member located beneath a lower end of the discharge duct for distributing the molten core around the chamber; a bed of anhydrous material for absorbing heat from the molten core, which bed is disposed beneath the distribution member and a refractory support member disposed beneath the bed of anhydrous material, the refractory support member including a plurality of radial slots, guides or ducts for distributing the molten core uniformly over the floor of the chamber.

The bed of anhydrous material can be supported by a dish-shaped or trough-shaped member and can comprise refractory material, basalt and/or asbestos.

The tie bar for the safety vessel can be annular and the distribution member can be conical.

The provision of water-cooled steel jackets around the chamber and the anhydrous material for absorbing heat enables the molten mass of the core to be stabilised and cooled. If the anhydrous material melts, its heat of fusion, absorbed from the molten mass, aids the cooling of the molten mass.

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 crosssection through a nuclear reactor installation according to the present invention.

The installation shown in the Figure is å light water cooled reactor, although the present invention is not limited to light water cooled reactors but can be used in any nuclear reactor installation.

The actual nuclear reactor comprises a reactor core 1 The reactor core 11 is enclosed by a pressure vessel 1 and built into the so-called reactor chamber 12. The pressure vessel 1 is disposed in a concrete housing which is part of concrete components 13 which serve on the one hand as a radiation shield and on the other hand as supporting structures for the rest of the components of the installation, such as steam generators and other auxiliary apparatus which for the sake of clarity are not represented here in more detail. Only the reactor sump 14 is shown which forms a collecting chamber for water discharging from primary or secondary lines which can be supplied from the sump 14 back to the emergency cooling installation of the power plant.

The afore-mentioned parts of the installation are located inside a spherical steel safety casing 2 which is also enclosed and spaced apart from a concrete cover 21.

In contrast to known structures the reactor chamber 12 is not sealed below but communicates with a discharge duct 3 which leads into a chamber or well 4 for the molten mass of the molten core. The chamber 4 is arranged underneath the safety casing 2. The opening required for the discharge duct in the safety casing 2 is provided with an annular tie bar 22 so that the strength of the safety casing 2 component is not affected by the opening.

The discharge duct 3 is lined with an anhydrous fireproof lining 23 made of refractory elements, so that when the molten mass of the reactor core passes through no water is released and therefore no increased steam pressure is produced in the safety casing 2. As already mentioned, the discharge duct 3 opens into the chamber 4 for the molten mass, which is lined with a water-cooled steel wall or jacket. If the core melts, it will eventually pass through the discharge duct 3 into the chamber 4. The molten core will then flow over a distribution cone 42, also consisting of anhyrous, and heat resistant or fire-proof material. The distribution cone 42 lies on a trough-shaped bed 43, for absorbing the molten mass, filled with anhydrous materials, for example refractory material basalt, asbestos etc.

The bed 43 has an outer wall of steel. The molten mass absorbing bed 43 is mounted on a refractory support member or foundation 44 which rests directly on the base of a trough 41 and which is provided with substantially radial expansion slits through which the molten core can flow. The trough 41 provides a first watercooled steel jacket on the base and part of the sides of the chamber 4. A second collar-shaped, water-cooled steel jacket 45 is provided. This second water-cooled steel jacket 45 defines the roof of the chamber 4 and provides a radiation shield, which prevents radiant heat from affecting the concrete components 1 3.

As already mentioned, in the event of a highly improbable, hypothetical accident causing a core melt down, the resultant molten mass would flow out through the discharge duct 3 away from the range of influence of water in the sump 14 into the chamber 4 for the molten mass. To avoid the molten core coming into contact with normally hydrated concrete, the discharge structures, i.e.

more particularly the discharge duct 3 are lines with fireproof refractory material which can be manufactured on a large scale, for example with fire concrete asbestos etc. Below this discharge duct 3 the molten core reaches a distribution cone 42, also constructed from anhydrous refractory material, which distributes the discharging molten mass onto a similarly anhydrous molten mass absorbing bed 43. In addition to its distribution function, the cone 42 also acts as a radiation shield to reduce the transfer of energy from the chamber 4 for the molten mass to the safety casing 2.

The molten mass absorbing bed 43 is filled with for example granular basalt, fire concrete, asbestos. The molten core, which has a viscosity similar to that of water, penetrates into the pockets of this fill and is cooled in the first instance by using the heat absorbing capacities of the filler bed. The filler bed itself will then melt and the fusion heat needed for this is again further cools the molten core.

At the same time the steel trough of the molten mass absorbing bed 43 is destroyed and the molten mass reaches the trough 41 of the chamber 4 for the molten mass via the expansion slits incorporated in a refractory supporting foundation 44. In the same way as the side walls this base is provided with water cooling, so that in view of the temperatures of the molten core, whiich have in the mean-time been lowered, no further termperature increase of the molten core may occur and the molten core finally solidifies by means of constant cooling. This action is assisted by the afore-mentioned collar-shaped jacket 45 which is also water cooled.

In summary, it may be said that, by means of the construction of the collecting arrangement for the molten core according to the above described embodiment of the present invention, the molten core is safely taken away from the region of the reactor sump 14. Furthermore, by using anhydrous refractory materials oxidation and the formation of hydrogen inside the molten core are practically exciuded. The heat entering the safety vessel 2 from the molten core by heat conduction is relatively low. The heat generating capacity of the molten core, which is located in the chamber 4, may be eliminated without special cooling of the reactor safety casing 2, and the reactive effect of the molten mass on this vessel is slight. The afore-mentioned molten mass absorbing bed 43 can be designed in this connection so that it is capable of retaining the molten core for several hours enabling the cooling of the trough 41 to be begun with safety. In this connection the cooling lines may be supplied from the outside via transportable pumps with river water or surface water from the vicinity; in some circumstances a cooling chain may also be set up here which may absorb the heat generating capacity of the molten core via natural circulation in the cooling tower of the reactor. The use of heat pipes is also possible.

Thus, by means of the collecting arranged described herein, the environment of a nuclear reactor installation can remain unpolluted even after such a serious reactor accident resulting in a core melt down.

Claims

1. A nuclear reactor installation comprising: a safety casing; a pressure vessel located within the safety casing and containing a nuclear reactor; a chamber disposed beneath the safety casing; and a discharge duct located in the safety casing and beneath the pressure vessel so as to provide communication between the interior of the safety casing and the chamber whereby, in the event that the core of the nuclear reactor melts, the molten core can pass into the chamber.

2. A nuclear reactor installation as claimed in claim 1, wherein the discharge duct comprises a plurality of anhydrous refractory elements.

3. A nuclear reactor installation as claimed in claim 1 or 2, wherein the core of the nuclear reactor comprises a plurality of fuel elements.

4. A nuclear reactor installation as claimed in claim 1,2 or 3, wherein the safety casing comprises concrete and/or steel, and wherein there is provided auxiliary equipment and a concrete housing within the safety casing, the pressure vessel and reactor core being disposed in.the concrete housing.

.

5. A nuclear reactor installation as claimed in claim 1, 2, 3 or 4, wherein the discharge duct is substantially vertical.

6. A nuclear reactor installation as claimed in any preceding claim, wherein the portion of the safety vessel defining an opening for the discharge duct is strengthened with a tie bar.

7. A nuclear reactor installation as claimed in any preceding claim, wherein the base and at least part of the sides of the chamber are defined by a first, water-cooled steel jacket, for absorbing heat from a molten core and a roof of the chamber is defined by a second, water-cooled steel jacket for preventing thermal radiation from within the chamber heating up components of the installation immediately above the chamber.

8. A nuclear reactor installation as claimed in any preceding claim, wherein there is provided, within the chamber, a heat-resistant distribution member located beneath a lower end of the discharge duct for distributing the molten core around the chamber; a bed of anhydrous material for absorbing heat from the molten core, which bed is disposed beneath the distribution member; and a refractory support member disposed beneath the bed of anhydrous material, the refractory support member including a plurality of radial slots, guides or ducts for distributing the molten core uniformly over the floor of the chamber.

9. A nuclear reactor installation as claimed in claim 8 when appendant to claim 6, wherein the tie bar is annular and the distribution member is conical.

10. A nuclear reactor installation as claimed in claim 8 or 9 wherein the bed of anhydrous material is supported by a dish-snaped or trough shaped member and comprises refractory material, basalt and/or asbestos.

11. A nuclear reactor installation substantially as hereinbefore descibed with reference to, and as shown in, the accompanying drawing.