PURPOSE: To cool melting debris in a short time, and prevent leakage of the melting debris by burying a heat absorbing part of a cooling device in an inner wall of a dry well, and radiating heat absorbed by this heat absorbing part by a heat radiating part arranged outside.

CONSTITUTION: A heat absorbing part 10 is buried in a horizontal inner wall 6 of a dry well 3 under a nuclear reactor, and a cooling device 16 is constituted so that heat is radiated from a heat radiating part 12 through a heat insulating part 14 passing penetratingly airtightly through a nuclear reactor housing container. If melting debris 5 is accumulated in a lower part of the housing container since melting of a reactor core is caused, the cooling device 16 is operated, and the melting debris 5 is cooled, and is prevented from leaking out while destroying a part with which the melting debris 5 comes into contact. This cooling device 16 is a heat pipe, and the heat absorbing part 10 corresponds to an evaporation part, and the heat radiating part 12 corresponds to a condensation part. For example, though it is preferable to use sodium or lithium having a high operation temperature as operating fluid, when it is buried in the concrete inner wall part 6 like this example, water may be also allowed to be used.