Patent Number: 
Section: description

FIG. 1 discloses schematically a nuclear reactor plant according to the present invention. The plant comprises a casing 1 enclosing a primary space 2 and a secondary space 3, which are separated from each other by means of a separating member 4 in the form of an intermediate wall. The separating member 4 forms a peripheral, essentially plane floor member 5 and a cavity 6 centrally provided and defined by a bottom portion 7 and a wall portion 8 extending around the bottom portion 7 and connecting the bottom portion 7 to the floor member 5. A reactor vessel 9 is provided in the primary space 2 and in such a manner that the reactor vessel 9 at least partly extends downwardly into the cavity 6. The reactor vessel 9 houses a reactor core 10 schematically disclosed. The nuclear reactor plant disclosed is of a so-called boiling water type and includes a steam conduit 11 extending out of the casing 1 to a turbine plant (not disclosed) for generating electrical energy. From the turbine plant, a feed water conduit 12 for the condensate extends through the casing back into the reactor vessel 9. Furthermore, a number of driving members 13, schematically disclosed, are provided at the bottom portion of the reactor vessel 9 and arranged to displace control rods (not disclosed) into and out of the reactor core 10 for controlling the nuclear reaction in the reactor core 10. Moreover, two pump devices 14 are disclosed, which are arranged to recirculate liquid in the interior of the reactor vessel 9. The lower part of the cavity 6 is accessible via a transport passage 15 extending between the lowest part of the cavity 6 and the part of the primary space 2 located above the floor member 5. The transport passage 15 comprises means for transporting staff and material, for instance a schematically indicated elevator device 15a and a staircase (not disclosed). The primary space 2 is accessible from outside via a passage 16 through the casing 1. The passage 16 comprises doors 17 permitting the opening and closing of the passage. It is to be noted that the casing 1 in accordance with the present invention comprises no openings or passages from the cavity 6, which are provided at a lower level than the one disclosed, i.e. all such openings are to be located above the floor member 5 and more closely above the most highly located part of the reactor core 10. Furthermore, according to the present invention, there is no opening or passage through the wall 8 and the floor 7 into the secondary space 3. When work is to be performed at the lower parts of the reactor vessel 9, staff and material thus are to be transported via the passage 16 and the transport passage 15 down to the lowest part of the cavity 6. The casing 1 and the primary space 2 are upwardly limited by an upper limiting wall 18. Above this limiting wall 18, and thus above the casing 1, there is an upper space 19 housing a volume of a liquid. In the example disclosed, this upper 35 space 19 is connected to a pool 20 intended for temporary storing of fuel rods and located outside the casing 1. The interior of the reactor vessel .9, i.e. the reactor core ID, is accessible from the upper space 19 via an openable cover arrangement comprising a first dome-shaped cover 21 forming a part of the casing 1 and the upper limiting wall 1B, and a second dome-shaped cover 22 forming a part of the reactor vessel 9. The secondary space 3 is arranged to house a cooling medium in the form of a liquid and is connected to the primary space 2 via a number of channels 23, one of which is disclosed in FIG. 1. The channel 23 extends through the floor member 5 and has an orifice in the secondary space 3, which is arranged to be located in the liquid present in the secondary space 3. In FIG. 2, the first cover 21 and the second cover 22 are in order to, for instance, enable maintenance or revision of the nuclear reactor plant. One wall portion 24 extends around the upper part of the reactor vessel 9 and connects the reactor vessel 9 to the upper limiting wall 18 in such a manner that the primary space 2 is separated from the upper space 19. Furthermore, in FIG. 2 is disclosed that one of the pump devices 14 has been dismounted by mistake in such a manner that the reactor vessel 9 has an opening or a hole at its bottom portion. The liquid present in the reactor vessel 9 will flow therethrough out of the reactor vessel 9 and down into the cavity 6. Since the reactor vessel 9 is open upwardly against the upper space 19, the liquid present in the upper space 19 will flow downwardly through the reactor vessel 9 an fill it to the same degree as the liquid flows out through the opening at the bottom portion. Thus, the liquid level in the cavity 6 will rise until it reaches the final liquid level 25 disclosed in FIG. 2. As appears from FIG. 2, the final water level 25 is located above the most highly located part of the reactor core 10, i.e. above the upper edge of the reactor core 10. This means that the reactor core 10 still is completely submerged in water. Thereby, in spite of an opening at the bottom portion of the reactor vessel 9, one may ensure cooling of the reactor core 10. It is to be noted that the arrangement according to the invention relies on completely passive measures, i.e. there is no precondition that pumps or other actively driven members operate in order to supply liquid to the interior of the reactor vessel 9 in case of a leakage at the bottom portion of the reactor vessel 9. The present invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the following claims. For instance, it is to be noted that more than one transport passage may be provided in order to ensure sufficient evacuation capacity for the staff located in the cavity 6 in case of the scenario described above. Although the embodiment disclosed refers to a boiling water reactor, it is to be noted that the principles according to the invention also are applicable to other types of nuclear reactors, in particular light water reactors, such as a pressure water reactor. According to the embodiment disclosed, the inventive casing 1 forms the reactor containment of the plant. However, it is to be noted that the casing 1, within the scope of the invention, may form a separate casing which is separated from the reactor containment provided according to conventional technique and which may be provided either inside or outside the reactor containment. For instance, the casing 1 may be formed by a reactor building.