LWR type reactor

A reactor pressure vessel is submerged in a pool water of a reactor container, and a water-tight vessel is disposed at the periphery of the pressure vessel. A buffer header is formed around the water-tight vessel. A blowing tube of a safety valve connected to the inside of the water-tight vessel is connected to the buffer header. A large number of small boreholes in communication with a pool water which is disposed at the outside are formed on the buffer header. High temperature steams discharged from the safety valve are introduced and temporality stored in the buffer header. Then, streams converted to small bubbles by passing through the small boreholes perforated to the buffer header are jetted out into pool water uniformly, brought into contact with pool water and condensated. This can promote lowering of stream pressure and condensation of high temperature steams, thereby enabling to prevent abrupt elevation of the pressure in the pressure vessel. (I.N.)