Abstract: A heat sink according to the invention comprises a multilayered body prepared by laying one upon the other a plurality of heat sink fin elements having pin-fin sections formed by cutting a number of slits through thin plates of a thermally conductive material with spacers, each being inserted between two adjacent heat sink fin elements to separate them by a given distance.

Abstract: A nuclear reactor internal pump has a pump shaft connected to a motor and rotatably passing through the bottom of a nuclear reactor pressure vessel, an impeller fitted to a front end portion of the pump shaft, a diffuser disposed on the downstream side of the impeller and having an inner cylinder fixed to the bottom of the reactor pressure vessel to surround the pump shaft, a purge water passage defined between the pump shaft and the inner cylinder to communicate with the inside of the reactor pressure vessel and a purge water supply device for supplying purge water to reactor pressure vessel through the passage. Further, the nuclear reactor internal pump comprises a clad dropping preventive device for trapping clad coming from the reactor pressure vessel in the passage to prevent the clad from dropping.