Patent Number: 
Section: claims

1. A boiling water reactor comprising:a reactor pressure vessel that includes a main body trunk having an upper open end and an openable upper lid covering the upper open end of the main body trunk from above;a reactor vessel vent line that penetrates lateral side of the main body trunk and has an opening section at a same level with or higher than the upper open end of the main body trunk in the reactor pressure vessel;a containment vessel that includes a dry well for containing the reactor pressure vessel;a sump arranged in the dry well and outside the reactor pressure vessel;a condensation tank arranged in the dry well and outside the reactor pressure vessel;a water level gauge arranged below the condensation tank in the dry well and outside the reactor pressure vessel;a nitrogen gas filling line branched from a gas phase piping;a water piping which extends downward from the condensation tank via the water level gauge and is connected to the main body at a position lower than the water level gauge, whereinthe reactor vessel vent line includes a gas phase piping extending substantially horizontally penetrating the main body trunk and connected to the condensation tank, and a sump connecting piping that is connected to the sump. 2. The boiling water reactor according to claim 1, wherein the containment vessel further includes a wet well having a suppression pool and communicating with the dry well. 3. The boiling water reactor according to claim 2, whereinthe reactor vessel vent line is connected to the suppression pool. 4. The boiling water reactor according to claim 1, whereinthe reactor vessel vent line is connected to a head spray line for supplying spray water for cooling the upper lid from inside into the reactor pressure vessel. 5. The boiling water reactor according to claim 4, whereinthe head spray line is so arranged as to directly spray water to the inner surface of the upper lid. 6. The boiling water reactor according to claim 4, whereinthe main body trunk and the upper lid are coupled together at a junction that has a flange structure such that a main body trunk flange and an upper lid flange are coupled together, andthe head spray line is so arranged as to directly spray water to inner surface of the main body trunk flange and inner surface of the upper lid flange. 7. The boiling water reactor according to claim 4, whereinthe head spray line is so arranged that the spray water is partly jetted out upwardly in the reactor pressure vessel.