Hitachi, Ltd.

Shigeto Murata

Yasuhiro Masuhara

Toshitsugu Nakao

Osamu Yokomizo

Shin-ichi Kashiwai

Motoaki Utamura

Koji Nishida

Yoshiyuki Kataoka

Akihito Orii

Yuichiro Yoshimoto

Kotaro Inoue

Hiromasa Hirakawa

Masataka Hidaka

Michio Murase

Shinichi Kashiwai

Akio Tomiyama

Iwao Yokoyama

Shoichiro Kinoshita

Yoshishige Kawada

Akihiko Minato

Yuichiro Maeda

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time, and the inside of a pressure vessel is pressurized by a pressurized tank and at the same time control rods are withdrawn to thereby heat cooling water in a state of single-phase flow to high temperature.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. This prevents the flow fluctuation and the reduction of stability of core due to occurrence of unstable phenomena of low temperature two-phase flow at the start-up time of the reactor to thereby make it possible to perform stable starting-up of the reactor and to shorten the start-up time of the reactor to thereby realize improvement in economy as well as reliability.

Boiling water nuclear reactor and start-up process thereof

In a fuel assembly having fuel rods inserted into regularly arranged cells of a fuel spacer which keeps the fuel rods correctly spaced from one another and has spacer elements disposed in a plurality of stages in the longitudinal direction of the fuel rods, the fuel spacer comprises vanes formed on the cells in such a manner that each vane is bent from a cut formed in a part of the side wall of the cell. The vanes obliquely project into the corresponding spaces between adjacent fuel rods and allow a coolant flowing through the spaces and forming two-phase flows to generate swirling flows toward the fuel rods. The fuel spacer may alternatively comprise either thin-walled cylinders having built-in vanes or spiral vanes, which are fixed to the spacer that has not been subjected to any direct machining. Also disclosed is a fuel assembly provided with any of the above-described fuel spacers. The arrangement of the present invention enables, while assuring a sufficient strength for maintaining fuel rods in their correct position, the transfer of heat from the fuel rods to the coolant to be promoted so as to raise the allowable power level of the fuel assembly, and enables the void ratio to be lowered so as to increase the reactivity.

Fuel assembly and fuel spacer

A fuel assembly according to the present invention is formed in such a manner that the small diameter portion of an upper end plug of a fuel rod is inserted into a boss of an upper tie-plate. The upper end plug includes a small diameter portion and a large diameter portion which is disposed below the former. An expansion spring in disposed in the small diameter portion of the upper end plug. A cover member surrounding this expansion spring is disposed in the large diameter portion of the upper end plug, and extends upwardly. Since the cover member surrounds the expansion spring, pressure loss caused by the expansion spring can be reduced.

Fuel assembly

A nuclear reactor is equipped with a cooling water circulation system which includes a water-feed conduit and a drain conduit to circulate the cooling water to and from a pressure vessel which surrounds a reactor core, the cooling water having a free liquid level maintained above the reactor core. The drain conduit is inserted in the pressure vessel from the upper side toward the lower side relative to the free liquid level. A double tube consisting of an inner tube and an outer tube maintaining a gap therebetween is formed in the end portion of the drain conduit. The inner tube and outer tube being continuous with the drain conduit, the opening at the lower end of the inner tube is located below said free liquid level but above the reactor core. The opening at the upper end of the outer tube is located above the free liquid level. The opening at the lower end of said outer tube is located below the reactor core.

Nuclear reactor

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Katsuta, Japan

Shigeto Murata