Abstract: First nuclear fuel elements in which a fissionable material in which a burnable poison is incorporated is loaded and second nuclear fuel elements in which a fissionable material in which no burnable poison is incorporated is loaded are disposed in a nuclear fuel assembly. Content of the fissionable material in the first nuclear fuel element is less than about 72% of that of the fissionable material in the second nuclear fuel element adjacent to the first nuclear fuel element.

Abstract: A method of starting operation of a nuclear reactor which contains fresh fuel rods comprising increasing a fuel power P(Kw/ft) produced by the fresh fuel rods to an amount determined by the following inequality;P< 366.times.(G/D)- 1.38,where D is a diameter of a fuel pellet of the fresh fuel rod and G is an initial gap between a fuel cladding of the fresh fuel rod and the fuel pellet, maintaining the fuel power at the amount for a predetermined period and thereafter increasing the fuel power to a desired maximum power level.BACKGROUND OF THE INVENTIONThe present invention relates to a method of starting operation of a nuclear reactor having fresh fuel rods of the type containing nuclear fuel covered with fuel rod claddings. (The term "fresh fuel rod" is utilized herein to designate a fuel rod which has not been irradiated or used.