Patent Number: 043953816
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The confinement enclosure with a suction means for leaks according to FIG. 1 comprises a wall consisting of a general frame 1 and an elevated structure 2, all made from reinforced prestressed concrete; the reinforcements are not shown, nor is the nuclear reactor which the enclosure contains. The thickness of the wall of the building is, for example, of the order of 1/20 to 1/10 of the radius. The drainage layer 2a shown by a broken line consists of juxtaposed zones or sub-groups such as 3, 3', 3", each ending in a collector 4, 4', 4"; filters such as 6a, 6b, 6c are provided inside the enclosure and are connected to the network 2a by ventilators such as 21a, 21b, 21c which ventilators place the drainage network under higher pressure compared with the pressure existing in the enclosure itself. The system is therefore a damming containment rather than a drainage system. If no accidents occur, the ventilators function normally at reduced power and, using devices (not shown), the preferential leakage zones can be detected so that they can be repaired. If an accident occurs, the power of the ventilators is increased in order to maintain the network 2a under overpressure relative to the inside and thus prevent any pollution from entering; thus, leaks towards the outside are from a gas which has been filtered previously. If the ventilators 21a, 21b, 21c break down, this filtering still occurs to a large extent, in that the filters 6a, 6b, 6c are much more permeable than the wall separating the interior of the enclosure from the drainage network 2a. The confinement enclosure with the water dam according to FIG. 2 is on the whole identical to that described above, but differs from it in that each drainage region is connected to a water reservoir such as 8 inside the enclosure, at a higher level than the said region. Thus, in the case of an accident with an internal build-up of pressure, the damming water pressure is automatically adjusted. For the upper region of the enclosure, the reservoir may be arranged in a turret 9 which cannot be guaranteed leaktight by the damming liquid itself but which may be specially resistant to prevent any local increase in the risks; the reservoir may also be mounted inside the enclosure and an automatic pump be used to complete the loading. The function of the damming water is to provide a liquid protective wall and to be the first substance to escape, instead of the polluted internal medium, if serious cracking occurs. It also brings about an important additional advantage in that it continuously moistens the concrete in its central part and thus prevents it from shrinking and cracking and keeps it leaktight. The water used is advantageously treated with basic additives to prevent deterioration of the concrete and corrosion of the reinforcements in the case of leakages circulating continuously, but every effort should, of course, be made to ensure that the walls are constructed so as to be perfectly leaktight and to repair them if there is any local deterioration, which is easy to detect by the patches of dampness. In some countries, it may also be advisable to incorporate anti-freeze additives in the water. In normal operation, monitoring the level of the reservoirs is a means of assessing the degree of leaktightness obtained; in the case of accidents, these reservoirs may be topped up with uncontaminated emergency cooling water from the reactor to prevent any radioactivity leak into the atmosphere. Periodic tests for leaktightness could be carried out by putting the networks under high pressure. In the example shown in FIG. 2, the drainage network 2a extends to the frame 1 of the enclosure 2 and communicates, via the duct 4"', with a corresponding water reservoir 8. Such protection of the frame 1 proves very useful in minimising the consequences of a serious accident such as the fusion of the core of a nuclear reactor, when the reinforced concrete frame is the chief obstacle in the path of the molten material coming from the core and tank. In fact, in an accident of this kind, the molten materials first of all progress downwards, spreading out laterally, decomposing the hydrates and liberating water, with reduction of this water by the metals, resulting in the production of hydrogen, and the carbonates with the release of carbon dioxide. A thermal front in the concrete precedes and promotes the chemical attack owing to the thermal stresses, which cause intensive cracking. When this front reaches the region of the channels, which are under water pressure, the cracking causes upward leaks and these cool the concrete, whilst the water evaporates and the vapour, having come into contact with the molten mass, escapes in the form of bubbles, thus causing the concrete to cool. The descent of the molten materials may thus stop slightly above the bed of channels. Naturally, the pitch and diameter of these channels must be made optimal, as must the water pressure, and several beds of channels of different diameters may be provided beneath the tank. FIG. 3 shows, in section, a vertical wall 20 equipped with a drainage system according to the invention. Vertical channels 10 are shown, which appear to be easier to construct, but the channels could also be horizontal or peripheral. They are produced during casting of the concrete, in known manner, by arranging in the formwork rubber tubes inflated with water and held in a straight line by internal steel rods, to resist any deviations caused by forces occurring during the positioning of the concrete; these rods and tubes can readily be removed after the concrete has set, at heights of more than 10 m. A set of vertical tubes is connected, by means of an upper collector 11 with a connecting tube 12, to the corresponding reservoir (not shown) in the case of the embodiment according to FIG. 2 and to the corresponding filter (not shown) in the case of the embodiment according to FIG. 1. The tight seal required round the tube 12 where it passes through the inner part of the wall 20 may be obtained using known devices such as guard discs 13 combined with an injection of cement mortar between the discs. Vertical prestressing reinforcements such as 14 and horizontal ones such as 15 and steel connecting means such as 16 and 17 which prevent in particular any widthways flaking of the wall are provided in the wall. The network of drainage or topping-up channels may be placed either in the ordinary concrete used to build the wall or in a special permeable layer 18, notably consisting of concrete with a large proportion of cavities. Permeable inclusions arranged in horizontal strips such as 19 which link the vertical channels and consist of permeable tubes filled with gravel may also be provided in the concrete during casting. The arrangement of all the channels and possible additional permeable devices should be such that, in the event of the wall 20 cracking due to an overload, there is no risk of anything passing directly through the wall without coming into contact with the drainage network. In a wall 2 m thick, there could be a network consisting of channels 0.04 m in diameter at a spacing of 0.5 m. Therefore, a confinement enclosure according to the invention when compared with known enclosures has the advantages of being very easy to produce and virtually leaktight in the case of any accidents which can be foreseen, whilst giving a resistance to extreme loads which is as great as the mass of materials used will permit. The features described for the wall with the drainage or topping-up network incorporated in it also extend to cover various other embodiments, especially in underground construction. The invention is not limited to the embodiments described and represented hereinbefore and various modifications can be made thereto without passing beyond the scope of the invention.