Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Core catchers
Reexamination Certificate
1999-06-07
2002-02-12
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Core catchers
Reexamination Certificate
active
06347129
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention lies in the field of nuclear power generation technology. More specifically, the invention relates to a container for collecting and spreading core melt in a nuclear power plant with a wall part and a bottom. The invention further pertains to a nuclear power plant with a reactor pressure vessel which is disposed in a reactor cavity and contains a reactor core, and with a container for the collection and spreading of core melt.
The design of nuclear power plants, especially light-water nuclear reactors, must provide safety provisions for the control of extremely unlikely accidents in which the reactor core melts down. This is accomplished, for example, by providing a space immediately underneath or below and laterally adjacent of the reactor pressure vessel in which core melt formed by a melting reactor core can be collected and cooled. It is important, thereby, to avoid pollution of the surroundings. The effects of an unlikely accident of this kind thus remain confined to the nuclear power plant. In the case of a lateral disposition, the collecting space is of large-area design, as described, for example, in the international PCT publication WO 94/29876. That core catcher allows the core melt to spread out thinly over a large area, thereby forming a large surface area which can be cooled in an effective manner. A passage leads underneath the reactor pressure vessel from the reactor cavity in which the reactor pressure vessel is accommodated to the spreading space. The passage is closed during normal operation of the nuclear power plant. The spreading space and the reactor cavity are free from water during normal operation. The possibility of a steam explosion is thus reliably avoided in the event of the emergence of core melt. Only when the core melt enters the spreading space is water introduced passively into the spreading space. Steam which forms during this process can be discharged from the spreading space over a large area. In all cases, it is ensured that uncontrolled steam formation in the reactor cavity does not occur.
German published patent application DE 43 22 107 A1 has disclosed a structure for collecting and cooling core melt, in which a coolant flows through cooling passages which are arranged in the bottom of a core melt spreading chamber. As a result, only the bottom is cooled.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a collecting container for core melt in a nuclear power plant, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensures even more effective and passive cooling of the core melt with at most little and controlled formation of steam. It is a further object of the invention to provide a nuclear power plant with a collecting container for core melt in which the formation of steam and hydrogen in the event of the emergence of core melt is largely prevented.
With the foregoing and other objects in view there is provided, in accordance with the invention, a container for collecting and spreading core melt in a nuclear power plant, comprising:
a structured bottom
formed of a material of good thermal conductivity; and
formed with a plurality of geodetically lowest points
and a plurality of geodetically highest points;
an outer wall extending with an upward slope between a respective one of the geodetically lowest points and a respectively adjacent one of the geodetically highest points; and
a steam conduit extending through an interior of the container and at each of the geodetically highest points.
In accordance with an added feature of the invention, the bottom has an inner wall extending with an upward slope between a respective one of the geodetically lowest points and a respectively adjacent one of the geodetically highest points.
The term “geodetically highest points” as used herein is to be understood as describing the local maxima of the bottom, or floor. They can lie in one-plane or at different geodesic heights. The geodetically lowest points are local minima of the bottom and can lie in one plane or at geodetically different levels. An upward slope of the outer wall of the bottom between a geodetically highest point and an adjacent geodetically lowest point means that a horizontal profile between these two points, i.e. a profile at a constant geodesic level, is largely excluded. This has the advantage that cooling liquid in contact with the outside of the rising or falling bottom rises along the bottom due to heating, and steam bubbles which form in it can be discharged via the steam conduit. These are designed such that a two-phase mixture (steam/cooling liquid) can flow through in a sufficient quantity even in a transient flow phase. They are composed, for example, of a thermally insulating material or are surrounded by such a material. The upward slope is an effective means of preventing the formation of a steam-bubble region on the outside of the bottom, leading to impairment of heat transfer out of the container to the cooling liquid. It is particularly advantageous that the core melt flowing into the container does not come into direct contact with a cooling liquid, thus reliably preventing the uncontrolled formation of steam and a steam explosion. All that occurs is heat transfer between the core melt and a cooling liquid surrounding the bottom or a cooling device connected to the container. By avoiding direct contact with metal which may flow into the container after the core melt, a hydrogen-forming reaction between a cooling liquid, especially water, and this metal is additionally avoided. The bottom preferably extends between a geodetically lowest point and an adjacent geodetically highest point with a strictly monotonic slope, it being possible for the slope between a geodetically lowest point and a geodetically highest point to be constant. This makes the bottom particularly simple to produce from the point of view of manufacturing technology. The bottom preferably has a wall thickness of 1 cm to 10 cm, in particular 5 cm, this wall thickness being used for preference in particular in the case of a bottom where the thermally conductive material is a steel. Since the bottom comes into direct contact with the core melt, the thermal conductivity of the material for the bottom should be chosen in such a way that a crust forms as rapidly as possible in the region of the bottom. The thermal conductivity of a steel is so high that solidification of the core melt in the immediate vicinity of the bottom can be achieved within a few minutes. The container is preferably closed by a top made of a material of good thermal conductivity. Although the top does not come into direct contact with the core melt when core melt enters the container, a high thermal load is imposed on the top due to heat radiation. The top is therefore preferably to be designed in such a way that the heat given off by heat radiation can be rapidly dissipated. The top can be connected to a separate cooling system or, in particular, can be connected by way of the steam conduits to a cooling liquid surrounding the bottom.
In accordance with an added feature of the invention, a plurality of conduits pass through the top and a differential-pressure-dependent shut-off element closes the conduits for a fluid flow. This avoids the formation of excess pressure in the container. The shut-off element can, for example, be a one-way bursting disc. The bursting disc is preferably triggered only at a differential pressure at which the pressure within the container is higher than outside the container. For this purpose, the bursting disc is, for example, supported in the direction of the container by a metallic undernet. The conduits which pass through the top are taken to a geodesic level such that they project beyond the level of a cooling liquid serving to cool the top.
In accordance with an additional feature of the invention, the bottom is formed in a dome shape or a pyramid shape around a respective ge
Carone Michael J.
Richardson John
Siemens Aktiengesellschaft
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