Arrangement to prevent the molten core of a nuclear reactor from

Details Arrangement to prevent the molten core of a nuclear reactor from Arrangement to prevent the molten core of a nuclear reactor from

1989-02-06

1990-12-18

Wasil, Daniel D.

Induced nuclear reactions: processes, systems, and elements

Reactor protection or damage prevention

Core catchers

G21C 1310

Patent

active

049784960

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD OF INVENTION

The invention concerns an arrangement to prevent the molten core of a nuclear reactor penetrating into the ground, particularly when the reactor has gone out of control and been mothballed.


STATE OF THE ART

To clarify the invention it will be recalled that a nuclear reactor operating by uranium fission generally comprises (FIG. 3) a core 1 containing the reactor proper, arranged inside a concrete vault 2. The vault is itself enclosed in another vault 3 known as the containment vault, which is also made of concrete and has a base 4 known as a "raft", which is 4 to 5 meters thick and generally supported on piles so as to form an anti-seismic assembly.
Recent occurrences at nuclear power stations have shown that risks of major accidents which may lead to fusion of the core are a possibility. In such cases, and particularly in power stations of the type installed in France, the system with a double shield and filters enables the radioactive emanations to be confined inside the vault. On the other hand it is virtually certain that the fused mass of the corium cannot be stopped, with all the risks that this implies to the strength of the safety and containment infrastructure and above all to the ground water table.
After any occurrence leading to the destruction of the core of a reactor one is left with a fused mass essentially made up of: chiefly comprising uranium oxide and zirconium oxide, other metals.
This mass, which is described as "corium", has a vast amount of energy and after a day or two will still have a residual output of approximately 10 to 15 MW, in the case of a reactor with an initial output of approximately 1000 MW, and a temperature estimated at about 2000.degree. C., due to the radioactivity of some elements and the exothermic reactions which take place as the concrete decomposes.
The fused mass attacks and decomposes the concrete of the raft, and it is thought that the attacking speed after a few hours would be a few centimeters per hour. After a few days the corium would pass inexorably through the concrete of the substructure of the raft and right into the substratum.
Furthermore, as soon as the corium has passed through the raft the containment vault is no longer sealed and radioactive gases may escape.


DESCRIPTION OF PROBLEM

Considering the dangers created by a reactor which has gone out of control, it is necessary to find a means for stopping the corium before it reaches the substratum, thus avoiding the major risks involved in pollution of the substratum and atmosphere due to the leakage.
An arrangement of this type must have the following dual function. a vis the above-mentioned constituents, to temperatures of approximately 2000.degree. to 2500.degree. C.; lower the temperature of the corium-refractory interface.
The inert refractory material must further be associated with cooling circuits designed and dimensioned to dissipate the thermal flux thus transferred.
At the present time no one material is known which can carry out both functions and which has high heat conductivity within the temperature range in question. The two functions must therefore be separated and each carried out by a suitable material.
French patent application Ser. No. FR 2336772 describes a protective coating for the safety pit of a nuclear reactor, comprising a layer of material with a high melting point, such as thorium dioxide or uranium dioxide, characterized in that it comprises, under a layer of this non-fusible material, a lower layer of a material with good heat conductivity such as graphite. However, the arrangement is directly exposed to the descent of the molten corium, in cases where the reactor has been moth balled, and the corium, which has still not spread out (as shown in FIG. 1) and is still at a temperature of over 2000.degree. C., is in danger of rapidly destroying the central part of the retaining basin and continuing its descent in an uncontrolled manner.


OBJECT OF INVENTION

In view of the above, the means for preventing the molten core of a nuclear r

REFERENCES:
patent: 3677892 (1972-07-01), Schabert
patent: 4036688 (1972-07-01), Golden et al.
patent: 4072561 (1978-02-01), Friedrich
patent: 4073682 (1978-02-01), Barleon et al.
patent: 4121970 (1978-10-01), Albrecht et al.
patent: 4146429 (1979-03-01), Slagley

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