Abstract: A new nuclear reactor or nuclear power plant, as a light water reactor (LWR), including a high-converting LWR, a heavy water reactor, a gas cooled reactor, a high temperature reactor, a fast breeder, with given maximum and effective output power (installed or nominal capacity) and given cooling capacity of the emergency cooling system is safely operated with increased maximum or effective power output which exceeds the values permitted by present nuclear standard and safety regulations. Further, a method for design and construction of a nuclear reactor with reduced cost of the emergency cooling system which is more economical than present methods are disclosed. The improvement is based on a new and more precise way of predicting the decay heat produced in the core of a nuclear reactor after shutdown and makes use of new, more reliable data which show that the decay heat of nuclear reactors is overestimated by present nuclear standards and nuclear safety regulations such as, e.g., ANSI/ANS-5.