Subject: EEE会議（米国の原子炉容器漏水問題）
Date: Mon, 21 Apr 2003 22:03:58 +0900
From: "kkaneko" <kkaneko@eagle.ocn.ne.jp>

各位殿

すでにご存知かもしれませんが、米国の原子力規制委員会（NRC)は、テキサス州
ヒューストンの南西約１４０キロにある南テキサス原子力発電所で、１号機（１９８
８年運転開始）の原子炉容器の底部から冷却水が漏れる事故が起きたことを明らかに
しました。漏出した水の量はわずかながら、原子炉容器の底からの漏水は米国内はも
ちろん、世界でも前例がないということで、同委員会は全米の１０３基の商業用原子
炉運転者に対し注意喚起を行った由です。米国では昨年、オハイオ州オーク・ハー
バー原発で圧力容器の上蓋の腐食問題がクローズアップしましたが、今回のトラブル
はどの程度深刻なものでしょうか。詳細は次のNY Times紙（４月１８日付け）でどう
ぞ。
金子熊夫
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Reactor Vessel Is Leaking Water

By MATTHEW L. WALD

ASHINGTON, April 18 ? A nuclear reactor in Texas is leaking cooling water
from the bottom of its giant reactor vessel, a development that experts view
with concern because they have never seen it before, the Nuclear Regulatory
Commission said today.

Technicians at the South Texas Nuclear Project, 90 miles southwest of
Houston near Wadsworth, have found residues that indicate that cooling water
leaked from the vessel through two penetrations where instruments are
inserted into the core, according to the plant operator.



Operators at all 103 commercial nuclear reactors have been giving closer
attention to their reactor vessels since the discovery last year of
extensive leaks in the vessel head at the Davis-Besse plant in Oak Harbor,
Ohio, near Toledo.

The Texas plant, South Texas 1, shows much smaller signs of leaking than the
Ohio plant. In both cases, technicians found deposits of boron, a chemical
added to the water to control the nuclear reaction, which remains after the
water evaporates.

At Davis-Besse, technicians cleaned out boron with shovels. In Texas,
technicians found an amount about half the volume of an aspirin tablet, said
Ed Halpin, the plant general manager.

No corrosion is visible, but no one is sure what is underneath. At
Davis-Besse, the steel of the vessel was so corroded that a metal part on
the head flopped over like a mailbox post that has been loosened from the
front lawn. Workers have replaced the vessel head, a part intended to last
for the lifetime of the reactor. Davis-Besse has remained closed since the
leak was discovered, 13 months ago.

South Texas 1 is one of the youngest plants in the country. It went on line
in August 1988. South Texas 2, which is adjacent, followed in June 1989. It
shows no sign of leakage.

The two reactors are owned by the Cities of Austin and San Antonio, a
subsidiary of American Electric Power of Columbus, Ohio, and Texas Genco, a
generating company that is a subsidiary of CenterPoint Energy of Houston.

The South Texas leak is unexpected and, so far, unexplained.

"This is the first time it's been seen, either here or abroad," said Victor
Dricks, a spokesman for the regulatory commission.

Representatives of two industry groups are at the plant to study the
problem, and managers have promised to keep the reactor, which is closed for
refueling, out of operation until they find the cause and repair it to the
satisfaction of the nuclear agency, Mr. Dricks said.

The vessel is 14.4 feet wide and 46 feet high, made of steel about six
inches thick. Its bottom has 58 penetrations, where instruments can be
inserted to measure the flow of neutrons, the subatomic particles that
sustain the chain reaction. There are leaks at two penetrations, although
the water volume was apparently small, Mr. Dricks said.

At plants around the country, cracks of some metal parts have been traced to
stresses created in construction. Others have been caused by a phenomenon
called intergranular stress corrosion cracking that occurs in some metals
under stress at high temperature. Mr. Halpin said he would not speculate
about the cause in Texas.

Water in the vessel is at a temperature higher than 500 degrees and pressure
higher than 2,000 pounds a square inch. So even a small hole could release
large volumes of radioactive water into the containment building.

Mr. Dricks said, however, that the pumps in the emergency core cooling
system could inject water faster than it could leak through a hole the size
of the penetration, so that the nuclear core would stay covered. The design
is for contamination in such cases to stay in the containment dome.

A repair problem is that the radiation field under the reactor is about 500
millirem an hour, Mr. Halpin said. At that rate, a worker would absorb in
four hours the radiation dose that most reactor operators set as a limit for
a full year. Repair work in such high fields is usually carried out by large
teams of workers, each spending just a short period at work.