Abstract: In a plant equipment comprising apparatus of a plurality of systems and control means for switching the systems and changing capacities of the apparatus, an apparatus of each system bas a rated capacity which is 1/N of a capacity of the whole equipment, and the plant equipment is constituted of (N+1) systems. In conditions of normal operation, all the (N+1) systems are operated at capacities less than the rated capacity through the control means. During normal operation, when one system is inspected, or when an abnormality occurs in one system, this system is stopped by the control means. Capacities of apparatus in N of the systems which are operated are increased to the rated 1/N capacity through the control means, and operation is continued with a structure of 1/N capacity.times.N systems. When the maintenance of apparatus in the stopped system is finished, the stopped system is restarted, so as to operate all of the (N+1) systems at capacities less than the rated capacity again.

Abstract: In a boiling water nuclear reactor, a reactor primary cooling water line is filled with water at the start-up time, and the inside of a pressure vessel is pressurized by a pressurized tank and at the same time control rods are withdrawn to thereby heat cooling water in a state of single-phase flow to high temperature. Succeedingly, the pressurization is released and the pressure of the pressure vessel is gradually approximated to the saturation pressure corresponding to the cooling water temperature to thereby make the cooling water transit into a state of two-phase flow, and thereafter the cooling water is heated by nuclear reaction in the state of two-phase flow to thereby obtain predetermined reactor running temperature and pressure.