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Cracks occurring in new stainless reactor shrouds.

TOKYO, Jan. 22 Kyodo

A spate of cracking is occurring in nuclear reactor core shrouds made of an improved stainless steel material, government officials said Wednesday.

The Industrial and Nuclear Safety Agency, which is under the Ministry of Economy, Trade and Industry, also found that the new shrouds suffer from the same flaw as the ones they replaced, in that the cracks tend to get bigger once they occur.

These findings run counter to power companies' earlier expectations that with the introduction of the new stainless steel material, which was thought to have a built-in tolerance to stress corrosion cracking, they would be able to prevent shroud cracks.

Power utilities will now have to take other measures to forestall cracking, such as removing the strains to which the stainless steel is subjected, experts said.

The stainless steel in question is SUS316L. Power companies had previously used a type called SUS304, but since stress corrosion cracking occurred, the composition was changed and the new product developed.

But cracks were found in the Fukushima No. 2 and Kashiwazaki-Kariwa nuclear plants of Tokyo Electric Power Co. and in the Hamaoka plant of Chubu Electric Power Co., both of which use SUS316L.

An agency study of a piece of the new shroud material found that the way the surface is trimmed and stiffened during processing makes it crack-prone, the officials said.

''We had not expected cracks to occur in this way,'' an expert said. ''This happened because strains were not removed.''

Shotpeening and laser peening are among the ways to remove steel strains, the expert said.
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Publication:Japan Energy Scan
Date:Jan 28, 2003
Words:262
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