PURPOSE: To prevent fluid intrusion from a low temperature plenum and to intend prevention of thermal fatigue by providing, at an outlet part, a piping equipped with a contraction in its inside and by providing a hole on a pipe arrangement surface located at a position lower then the contraction.

CONSTITUTION: A contraction 14 is attached to an outlet 13 piping and a hole 15 is provided on a pipe arrangement surface located at a position lower than the contraction 14. When a reactor is shut down and circulation flow rate decreases to lower fluid temperature at the outlet 13 lower than fluid temperature of a low temperature plenum 5, the contraction 14 accelerates flowing-out fluid and the accelerated fluid flows out to the plenum 5 via a center part of the outlet 13 piping. Here, fluid of the plenum 5 of which temperature is higher than the flowing-out fluid, rises up in the piping along an outer circumference of the outlet 13 piping. The rising fluid is retarded its rise by the contraction 14 provided on the pipe inside surface and the retarded fluid flows out outside the piping through a hole 15, and therewith fluid intrusion to a part higher than the contraction 14, is prevented. By these procedures, the liquid instruction from the lower temperature plenum 5 is prevented in a very beginning stage of scram and thermal fatigue is well prevented, too.