Hitachi, Ltd.

Katsutoshi Shinbo

Takanori Hosokawa

Hideo Maki

Yuichiro Yoshimoto

Takashi Fukumoto

Shozo Saito

Takao Igarashi

Satoshi Sugawara

Kenichi Itoh

Zenichiro Endo

Minoru Watahiki

A control rod has blades each of which includes a neutron absorption region in which neutron absorbers each loaded to the core of a reactor from below and made of absorption nuclear chain type neutron absorber are disposed. The neutron absorption region is divided into two, upper and lower, regions in an axial direction; the boundary between the upper and lower regions is positioned within the range of from 3/8 to 5/8 of the full length of the neutron absorption region in the axial direction from the lower end of the neutron absorption region; and the quantity of the absorption nuclear chain type neutron absorber contained in the lower region in the section perpendicular to the axial direction is smaller than the quantity of the absorption nuclear chain type neutron absorber contained in the upper region in the section perpendicular to the axial direction.

Control rod

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.

Nuclear fuel assembly

In raising the power of a nuclear reactor, before the linear heat generating rate of nuclear fuel elements arranged in the core of the nuclear reactor reaches 240 W/cm, the power rise of the reactor is suspended at least once and the reactor power is held at the fixed level. The raise of the power of the nuclear reactor before the arrival of the linear heat generating rate at 240 W/cm is performed by pulling out control rods inserted into the core. When the linear heat generating rate exceeds 240 W/cm, the power of the nuclear reactor is gradually raised in such a way that the linear heat generating rate is increased in a proportion of below about 1.8 W/cm/hour by increasing the flow rate of a coolant supplied to the core.

Method of operating a nuclear reactor

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;

Method of starting operation of a nuclear reactor

The opposite ends of a fuel cladding are sealed with end plugs. A plurality of UO.sub.2 pellets are charged into the fuel cladding. Defined in the lower portion of the fuel cladding is a gas plenum which houses fission products therein. Charged in the plenum defined in the nuclear fuel element is a material (such as boron carbide which absorbs neutrons and produces helium.

Nuclear fuel element and method for fabricating the nuclear fuel element

A nuclear fuel assembly having a bundle of longitudinal fuel elements vertically disposed in a reactor vessel, an upper and a lower tie plate supporting the top and the bottom of the bundle of the fuel elements, a fully ended tubular channel covering around the bundle of the fuel elements to define a coolant flow passage. The channel is removably supported by the upper tie plate. After a predetermined operating period, the channel is pulled out from the fuel assembly, turned upside down and thereafter again inserted over the fuel assembly in a upended state so that the life time of the channel is elongated.

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Katsuta, Japan

Katsutoshi Shinbo