Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Emergency core coolant systems
Reexamination Certificate
1999-09-15
2001-09-04
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Emergency core coolant systems
C376S280000, C376S283000
Reexamination Certificate
active
06285727
ABSTRACT:
BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention relates to a nuclear reactor plant with a light water reactor, comprising a containment having an upper space and a lower space, which is separated from the upper space by a separating member and which is arranged to house a cooling medium, and a reactor vessel housing a reactor core and provided in the upper space.
Such nuclear reactor plants are known and these have proved to function in a satisfactory manner. However, if the reactor core for any reason would reach such a temperature that the fuel starts to melt and the geometry of the core is changed, it might happen that the core may fall down from its original position and penetrate the bottom of the reactor vessel, i.e., a so-called core melt. In this case, the core will fall down onto the bottom surface of the containment. As long as the containment is intact and the core may be maintained within the containment, there is no real risk that radioactivity in any greater amount will leak to the environment. However, if the core would melt through the containment, the risk for such a leakage is imminent.
Different measures have been-proposed in order to avoid this risk. One such measure is to sprinkle cooling liquid over the core located on the bottom surface. However, such a cooling from above may prove to be insufficient, during a longer period of time, to prevent in a secure manner any part of the core from penetrating the containment. Another measure proposed is to let the core fall down into a water pool provided beneath the reactor vessel. A further measure, which has been proposed, is to let the core fall down into a container having double walls between which a cooling medium circulates to cool down the core.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a way to take charge of a core which has fallen down through the bottom of the reactor vessel in such a manner that the risk of radioactive emissions to the environment may be further reduced.
This object is achieved by the reactor plant initially defined and characterized in that the separating member comprises a portion which is arranged to be located at such a position that the surface of the portion facing the lower space is in contact with the cooling medium, and that the reactor vessel is provided above the portion. By such a design of the reactor plant, which enables the feature that the portion is submerged in a cooling medium, the core at a possible core melt will fall down onto the portion. Thereby, the following essential advantages are obtained: on the one hand, the core in a passive manner will be cooled from beneath by the fact that the lower surface of the portion is in contact with the cooling medium in the lower space and, on the other hand, a further barrier for the core to be penetrated before it may reach the floor surface of the containment has been provided. Furthermore, if the core anyway would penetrate the portion, the complete floor surface of the containment is available for the core, i.e., the core may be distributed over a large area covered by the cooling medium. Advantageously, the portion may comprise a surface arranged in such a manner that the cooling medium may flow along the surface and remove heat from the portion. In such a manner, one may ensure efficient cooling from beneath of a core which has fallen down at a possible core melt, since the cooling medium flows along the surface and cools it down by natural recirculation.
According to a further embodiment of the invention, the separating member comprises a wall portion extending upwardly from and surrounding the portion in such a manner that the portion and the wall portion form a cavity of the separating member. In such a manner, the core will, in case of a possible core melt, be located in a delimited space and it is also possible to let the wall portion be cooled down from outside by the cooling medium in the lower space. In addition, it is possible to fill up the cavity by water before the melt reaches the portion, enabling the cooling to be further improved.
According to a further embodiment of the invention, an openable connection is provided to extend between the cooling medium in the lower space and the cavity and to enable the supply of the cooling medium to the cavity. In such a manner it is possible to cool down the core not only from beneath through the portion but also by direct supply of cooling medium onto the core. Thereby, the openable connection may comprise a conduit extending through the wall portion. In such a manner, the core may, in addition, be cooled down from above by, for instance, cooling medium flowing down over the core. Furthermore, the openable connection may comprise a fuse portion arranged to fuse at a predetermined temperature and thereby the openable connection. In such a manner, the additional cooling will take effect as soon as a sufficiently high temperature has been reached by the fuse portion. Thereby, the fuse portion may be comprised by the portion, which thus involves the flowing of the cooling medium into the cavity from beneath.
According to a further embodiment of the invention, the lower space comprises a bottom surface formed by the lower limiting wall of the containment, the portion being provided at a distance from the bottom surface, wherein at least one essentially vertical support plate extends between the portion and the bottom surface.
According to a further embodiment of the invention, at least one channel extends through the separating member and connects the upper and lower spaces. The channel has an orifice in the lower space, which is arranged to be located in the cooling medium. By such a channel, possible discharge of steam into the upper space, which may lead to a pressure increase therein, will be conveyed down into the lower space and will be condensed in the cooling medium. According to an advantageous embodiment, the openable connection has an orifice in the cavity, which is provided at a lower level than the orifice of the channel. In such a manner, it is ensured at the pressure of the cooling medium at the openable connection will be sufficiently high to permit the cooling medium to flow through the openable connection and into the cavity, and in such a manner that the core may be cooled down at the same time as the steam from the water above the core may be cooled down by the colder cooling medium in the lower space. In order to ensure, over a longer period of time, an efficient cooling of the core, means may be arranged to cool down the cooling medium in the lower space.
Further features and advantages of the present invention will appear from the following description of different embodiments.
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Bredolt Ulf
Engstrom Johan
Ivung Bengt
ABB Atom AB
Connolly Bove Lodge & Hutz
Jordan Charles T.
Richardson J. A.
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