Patent Number: 051924943
Section: summary

The invention relates to an arrangement for protecting a cylindrical surface in a nuclear reactor containment against the effects of a core meltdown, the surface being located below the reactor vessel in a pool intended to be filled with water in order to granulate and cool molten core falling into it. In many types of reactors, if the core melts in a reactor accident and penetrates through the bottom of the reactor vessel, the molten core material will run down into a pool in the reactor containment. This is the case, for instance, in the type of reactor represented by the boiler reactors Oskarshamn 3(O3) and Forsmark 3(F3) in Sweden. In these reactors the central space below the reactor vessel is dry during normal operation, but in the event of an accident this space will be filled with water from a surrounding pool before the molten core penetrates the bottom of the reactor vessel. It is known from many industrial processes and from laboratory experiments that when a melt runs down into water it is fragmented into particles which solidify on their way through the water, thus forming a bed of particles at the bottom of the pool. The particles vary in size. Normally, approximately 98-99% of the bed mass will consist of particles having equivalent diameters between 1 and 12 mm. However, in certain cases steam explosions may occur and particles are then obtained which may be smaller than those obtained at normal fragmentation. The molten core material will form a particle bed of up to 25-30 m.sup.3 on the concrete floor of the reactor containment if the whole reactor core melts. Due to the high radioactivity of the core granulate enormous quantities of heat are generated in the bed, which must therefore be cooled if melting is to be avoided. Melting occurs at about 2000.degree. C. and a meltdown at this temperature would seriously damage the concrete and rapidly destroy the penetrations of steel or the like located in the floor and possibly also in the walls of the water pool. This would cause severe and unacceptable radioactive emission. The particle bed is cooled by water from the pool penetrating the bed from above. The water boils in the bed, forming steam which flows up through the bed and condenses in the water mass above the bed. If the bed containing small particles, is too high, or generates too much energy, the upwardly flowing steam will prevent the water from penetrating down through the bed. This phenomenon is known as dryout and means that the particle bed dries out and is heated to melting point which, as mentioned, results in concrete attack and destruction of penetrations. Reactor containment integrity is lost, with severe radioactive emission as a result. The object of the invention is to offer an arrangement by means of which the integrity of the reactor containment can be maintained in the event of an accident of the type mentioned. A specific object of the invention is to provide an arrangement which protects the penetrations, but the invention is also aimed at offering an arrangement which will protect the bottom and walls of the pool against attack from the bed. For this purpose the invention offers an arrangement for protecting a tubular surface (e.g. a penetration or the side wall of the pool) in a nuclear reactor containment against the effects of a core meltdown, the surface being located below the reactor vessel in a pool intended to be filled with water in order to granulate and cool molten core material falling into it. The arrangement is substantially characterised by a pipe or a tubular element located on the side of the tubular surface to be protected, thereby forming a gap between the surface and the pipe, the pipe having an inlet located below the surface of the water in the pool and above the highest level in the pool which granulate could be expected to reach, an outlet in the pipe communicating with the lower part of the pool, so that water is permitted to pass via the inlet, the gap and the outlet to the lower part of the granulate bed, and by screen devices preventing granulate from entering the gap. The tubular surface to be protected may thus be a penetration or a pipe through the bottom or side wall of the pool. Alternatively the tubular surface may constitute the side wall of the pool itself. According to a further development of the invention, the bottom of the pool may be lined with a perforated bottom wall to form a bottom gap between the bottom of the pool and the bottom wall, the perforations in the bottom wall then communicating with the gap below. The protective devices may comprise a screen over the inlet to the gap, said screen preventing granulate from falling directly into the gap. The protective device may even comprise a grating or the like covering the outlet from the gap or the pipe to the granulate bed to prevent granulate particles from entering and blocking the gap. Alternatively the protective device at the outlet may consist of slits in the pipe or of a labyrinth or the like. The invention is defined in the appended claims. It has been discovered that the dryout effect for particle beds can be increased by a factor of up to 3 if counter flow is avoided between steam and water through the particle bed. This can be achieved by placing vertical pipes in the pool, the pipes having an inlet located above the surface of the bed and below the surface of the water, and an outlet (e.g. in the form of axial slits in the lower end of the pipe) at the bottom of the pool. Water will thus run down through the pipe, permeate into the bed and form steam which will pass up through the particle bed without impeding the downward flow of water through the pipe. However, these simple pipes are not sufficient if the granulate bed is formed of layers with relatively small particles on top of a layer of relatively large particles; such bed structures cannot be entirely excluded since they may be formed by particles from small steam explosions settling on top of particles formed by normal hydrodynamic fragmentation. If the bed were to be cooled only by pipes extending down through the particle bed, the integrity of the penetrations of steel in the concrete floor of the pool might be endangered. However, this problem is solved according to the invention by arranging the pipes around the penetrations, thus preventing the penetrations from coming into direct contact with the core granulate and also ensuring that they are cooled by the water flowing down through the gap. Of course, an equivalent arrangement is possible for the side wall of the pool. In both cases the bottom of the pool can be protected by a bottom wall such as that described above. In the case of reactors with relatively small pool area, the granulate bed will be relatively deep and therefore difficult to cool. In such situations it is particularly suitable to allow the pipe to communicate with a bottom gap between the bottom of the pool and the bottom wall, as well as when the penetrations extend through the side wall of the pool, spaced from the bottom of the pool.