PURPOSE: To reduce the exposer ray quantity of radioactive ray of workmen, by means of setting up a water supplying device at the well and the bit of storage equipments of a reactor, and of subjeting water to be supplied from an outside water source when the reactor is in inspection.

CONSTITUTION: A supper jar 40 for filling with water is set up at the upper part of a reactor vessel 2, and is connected with an outside water source by a water supplying line 41. The fuel exchaging work etc., are effected by removing a pool gate 43 between a well 15 of the reator and a storage pool 6 by supplying the water to the well 15 of the reactor, or to a bit 8 of storage equipment, by hanging a separator 12 by a crane, and by transfering it to the next seat. The safety working can be performed without the contaminated water in the reactor vessel 2 being overflowed in the well 15 of the reactor and the bit 8 of the storage equipment.