Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Core catchers
Reexamination Certificate
1996-05-22
2001-02-20
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Core catchers
Reexamination Certificate
active
06192097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a retaining device with a multi-layer protective cladding for protecting the bearing and containment structure of a spreading chamber, for the controlled spreading and cooling of a hot melt, in particular a core melt, which after emerging from the nuclear reactor pressure vessel of a nuclear reactor installation designed on the spreading concept can be carried into a spreading chamber and cooled therein.
2. Description of the Related Art
It has been known heretofore, for retaining core melt emerging from a reactor pressure vessel of a nuclear reactor installation, to dispose a crucible-like catching container directly below the reactor pressure vessel, in the reactor cavern carrying the reactor pressure vessel. The catching container can hold the core melt within a compact volume and can be cooled both on its outside and in its interior. British Patent 2 236 210 A describes one such retaining device with a catching container, also called a core catcher. The core catcher is embodied by a supporting structure that is spaced apart from the wall and bottom of the reactor cavern and is supported by support elements disposed on the bottom. In the interior, the core catcher is lined with a core retention layer of zirconium oxide blocks. The core retention layer is lined with a sacrificial layer, preferably of a steel, for its protection during normal power plant operation. The core retention layer is thus sandwiched between the bearing structure and the sacrificial layer. For mechanical stability of the sacrificial layer, the zirconium oxide blocks are joined to one another and to the bearing structure and to the sacrificial layer by a zirconium oxide cement. To enable the core catcher to hold all the emerging core melt, the container is drawn upward in crucible fashion to beyond the bottom of the reactor pressure vessel. As a result the core melt can be caught in a kind of smelting crucible with a small directly-coolable surface area.
A retaining device for receiving a spreading core melt is described in the conference offprint “Emerging Nuclear Energy Systems 1989, ICENES '89, Karlsruhe July 3-6, Proceedings of the Fifth International Conference on Emerging Nuclear Systems, pp. 19-24; see particularly
FIG. 1
on page 23 in conjunction with the text on page 21. In that known retaining device, a cooled catch basin (core catcher) is likewise disposed inside the reactor containment directly below the reactor pressure vessel; in this basin, the melt can spread over a large area and can be cooled with water in direct contact with an enlarged surface area. The steam generated by the decay heat of the melt condenses in the upper part of the steel shell of the reactor containment and from there flows back to the retaining device. Specifically, the bottom of the catch basin is supported by a support structure made up of double-T beams, and is formed by a perforated steel plate. Core melt can be cooled directly with water both through the steel plate and via openings in the wall of the catch basin. What is known as a “sacrificial concrete layer” is located on the steel plate. That sacrificial layer is reinforced in the surface region of the shielding pit, or in other words below the reactor pressure vessel. Cooling water in the lower part of the reactor containment permanently surrounds the catch basin. The cooling water can also spread along the top of the sacrificial layer, or in other words the surface of the sacrificial layer toward the reactor pressure vessel. However, as much as possible, an immediate contact of a core melt emerging from the rounded bottom portion of a reactor pressure vessel with water should be avoided. Moreover, in the known retaining device the water layer below the catch basin is the only protective layer for the structural concrete located beneath it.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device for retaining a hot melt, in particular a core melt-through inside the spreading chamber of a nuclear reactor installation, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for improved protection for the bearing and containment structure of a catch space. Moreover, the retaining device should be constructed such that immediate direct contact of descending core melt masses with the cooling water cannot occur, so as to enable delayed and metered water cooling.
In general terms, the above-noted objects further pertain to a retaining device for a hot melt. The more specific object of the invention, however, can be defined as follows: In a retaining device for a core melt that operates on the spreading concept and in which, after spreading of the core melt, the core melt is cooled on its outer surface, in particular with water, the following problems are to be solved:
preventing contact of the core melt with the bearing and containment structure in general and with the structural concrete of the building or structural concrete in particular, and preventing erosion of the concrete;
stabilizing the core melt in the region of the protective cladding, especially above the protective and insulating layer;
varying properties and consistency of the core melt, such as viscosity, melting point, fragmentation;
reducing the thermal burden from the core melt on the bearing and containment structure of the spreading chamber.
With the foregoing and other objects in view there is provided, in accordance with the invention, a retaining device for core melt in a spreading chamber of a water-cooled nuclear reactor installation, wherein the spreading chamber has a bearing and containment structure and is configured for controlled large-area spreading and cooling of a core melt in the case of a core meltdown. The retaining device comprises:
a multi-layer protective cladding disposed on and protecting the bearing and containment structure of the spreading chamber;
the protective cladding including an outer sacrificial layer acting as a thermal shock barrier and as melting substance, and a protective and insulating layer for the bearing and containment structure downwardly adjacent the sacrificial layer;
the protective and insulating layer including a first partial layer of fireproof concrete adjacent the bearing and containment structure, and a second partial layer of temperature-proof ceramic blocks adjacent the sacrificial layer.
In other words, the retaining device of the invention has a protective liner, i.e. a protective cladding formed with at least two layers including the and which is composed of the following:
an outer sacrificial layer as a thermal shock barrier and as melting substance, and
a protective and insulating layer, adjacent the inside of the sacrificial layer, for the bearing and containment structure located beneath, which as thermal protection and as a retention layer for the hot melt includes:
a first partial layer of fireproof concrete adjoining the bearing and containment structure, and
a second partial layer of temperature-proof ceramic blocks adjoining the sacrificial layer.
The advantages attainable with the invention are considered above all to be that the novel retaining device, because of its specific layer construction, is capable of resisting the chemical strain, the pressure strain, and the temperature strain of a spreading core melt, without involving the bearing and containment structure of the spreading chamber in the process, even if the core melt is not cooled on its underside. Cooling of the core melt at its top is advantageous for the sake of rapid dissipation of the further fission or decay heat and for shielding reasons and is within the scope of the preferred embodiment of the invention in retaining and cooling a core melt. In the case of a hot melt that is not a core melt, water cooling of the surface may be dispensed with under some circumstances.
In accordance with an added feature of the invention, the ceramic blocks of the second partia
Fischer Manfred
Hau Gerhard
Hollmann Josef
Wistuba Lothar
Behrend Harvey E.
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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