Patent Number: 
Section: claims

1. A nuclear reactor plant comprising: a casing enclosing a primary space;  a reactor vessel, arranged in the primary space;  a reactor core provided in the reactor vessel;  at least one reactor component sealingly and releasably provided in a through-going opening at a bottom portion of the reactor vessel,  wherein said reactor component comprises at least one of a pump device arranged to recirculate liquid within an interior of the reactor vessel, and a driving member arranged to displace a control rod into and out of the reactor core for controlling a nuclear reaction in the reactor core;  an upper space provided above the primary space; and  a wall portion separating said primary space from said upper space, the reactor core being separated from the upper space by means of an openable cover arrangement, wherein the primary space is limited downwardly by a floor member and a cavity which is open upwardly and extends downwardly from the floor member, and wherein the reactor vessel is at least partly provided within the cavity and said reactor component is provided within the cavity,  wherein the casing is designed in such a manner that the primary space is completely closed against the environment at least to a level corresponding to the most highly located part of the reactor core, that the upper space is arranged to house a volume of a liquid, which volume is sufficiently large to permit the filling of the primary space with said liquid to a level reaching at least the most highly located part of the reactor core, and that at least one transport passage extends between the lower part of the cavity and the part of the primary space located above the floor member, said transport passage comprising means for transporting people and material. 2. A nuclear reactor plant according to  claim 1 , characterized in that the reactor vessel is provided in the primary space in such a manner that it extends downwardly into the cavity to such a level that the most highly located part of the reactor core is located in the cavity below the floor member. claim 1 3. A nuclear reactor plant according to  claim 1 , characterized in that the cavity is defined by a bottom portion and a wall portion which delimit completely the cavity from the surrounding parts of the casing up to the level at which the floor member is located. claim 1 4. A nuclear reactor plant according to  claim 1 , characterized in that the casing also encloses a secondary space separated from the primary space and provided below the floor member and arranged to house a cooling medium. claim 1 5. A nuclear reactor plant according to  claim 4 , characterized by at least one channel extending through the floor member and connecting the primary space to the secondary space and that said channel has an orifice provided in the secondary space and arranged to be located in said cooling medium. claim 4 6. A nuclear reactor plant according to  claim 1 , characterized in that the primary space is accessible from outside via an openable passage, extending through the casing at a level located above the floor member. claim 1 7. A nuclear reactor plant according to  claim 1 , characterized in that the cover arrangement comprises a first cover arranged to close an upper limiting wall of the primary space and second cover arranged to close the reactor vessel. claim 1 8. A nuclear reactor plant according to  claim 1 , characterized in that the casing is arranged to be permanently closed against the environment up to said level during the operation of the plant and when the plant is shut down. claim 1