REACTOR PRESSURE VESSEL

PROBLEM TO BE SOLVED: To maintain intact environment in a housing by providing holes in the vicinity of welding part in a neutron flux monitor housing. SOLUTION: When primary cooling water 19 injected of hydrogen is passed in reactor bottom, the cooling water 10 flows in a housing 8 and ICM guide tube 6 through holes 8A provided on an ICM housing, based on the pressure difference between the upper part and the lower part of a core support plate 5. The cooling water 10 having flown in them flows into a neutron meter 15 through a lower hole 15A provided in the meter 15, flows up in the meter 15 and flows out to the core through an upper hole 15B of the meter 15. As flow of cooling water 10 like this is generated in the housing 8, the corrosion environment of welding heat effect part 8B in the housing 8 is greatly improved.