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发帖数: 7918 | 1 【 以下文字转载自 Military 讨论区 】
发信人: powerforward (今天你敏感詞了沒有), 信区: Military
标 题: 紐約時報：GE設計的冷卻系統有缺陷
发信站: BBS 未名空间站 (Tue Mar 15 14:15:39 2011, 美东)
Reactor Design in Japan Has Long Been Questioned
By TOM ZELLER Jr.
Published: March 15, 2011
The warnings were stark and issued repeatedly as far back as 1972: If the
cooling systems ever failed at a Mark 1 nuclear reactor, the primary
containment vessel surrounding the reactor would probably burst as the fuel
rods inside overheated. Dangerous radiation would spew into the environment.
Now, with one Mark 1 containment vessel damaged at the embattled Fukushima
Daiichi nuclear plant and other vessels there under severe strain, the
weaknesses of the design — developed in the 1960s by General Electric —
could be contributing to the unfolding catastrophe.
When the ability to cool a reactor is compromised, the containment vessel is
the last line of defense. Typically made of steel and concrete, it is
designed to prevent — for a time — melting fuel rods from spewing
radiation into the environment if cooling efforts completely fail.
In some reactors, known as pressurized water reactors, the system is sealed
inside a thick, steel-and-cement tomb. Most nuclear reactors around the
world are of this type.
But the type of containment vessel and pressure suppression system used in
the failing reactors at Japan’s Fukushima Daiichi plant — and in 23
American reactors at 16 plants — is physically less robust, and it has long
been thought to be more susceptible to failure in an emergency than
competing designs.
G.E. began making the Mark 1 boiling water reactors in the 1960s, marketing
them as cheaper and easier to build — in part because they used a
comparatively smaller and less expensive containment structure.
American regulators began identifying weaknesses very early on.
In 1972, Stephen H. Hanauer, then a safety official with the Atomic Energy
Commission, recommended in a memo that the sort of “pressure-suppression”
system used in G.E.’s Mark 1 plants presented unacceptable safety risks and
that it should be discontinued. Among his concerns were that the smaller
containment design was more susceptible to explosion and rupture from a
buildup in hydrogen — a situation that may have unfolded at the Fukushima
Daiichi plant.
“What are the safety advantages of pressure suppression, apart from the
cost saving?” Mr. Hanauer asked in the 1972 memo. (The regulatory functions
of the Atomic Energy Commission were later transferred to the Nuclear
Regulatory Commission.)
A written response came later that same year from Joseph Hendrie, who would
later become chairman of the N.R.C. He called the idea of a ban on such
systems “attractive” because alternative containment systems have the “
notable advantage of brute simplicity in dealing with a primary blowdown.”
But he added that the technology had been so widely accepted by the industry
and regulatory officials that “reversal of this hallowed policy,
particularly at this time, could well be the end of nuclear power.”
In an e-mail on Tuesday, David Lochbaum, director of the Nuclear Safety
Program at the Union for Concerned Scientists, said those words seemed
ironic now, given the potential global ripples on the nuclear industry from
the Japanese accident.
“Not banning them might be the end of nuclear power,” said Mr. Lochbaum, a
nuclear engineer who spent 17 years working in nuclear facilities,
including three that used the G.E. design.
Questions about the G.E. reactor design escalated in the mid-1980s, when
Harold Denton, an official with the N.R.C., asserted that Mark 1 reactors
had a 90 percent probability of bursting should the fuel rods overheat and
melt in an accident. A follow-up report from a study group convened by the
commission concluded that “Mark 1 failure within the first few hours
following core melt would appear rather likely.”
In an extreme accident, that analysis held, the containment could fail in as
little as 40 minutes.
Industry officials disputed that assessment, saying the chance of failure
was only about 10 percent.
Michael Tetuan, a spokesman for G.E.’s water and power division, staunchly
defended the technology this week, calling it “the industry’s workhorse
with a proven track record of safety and reliability for more than 40 years.”
Mr. Tetuan said there are currently 32 Mark 1 boiling water reactors
operating safely around the globe. “There has never been a breach of a Mark
1 containment system,” he said.
Several utilities and plant operators also threatened to sue G.E. in the
late 1980s after the disclosure of internal company documents dating back to
1975 that suggested the containment vessel designs were either
insufficiently tested or had flaws that could compromise safety.
Paul Gunter, director of the Reactor Oversight Project with Beyond Nuclear,
an organization opposed to nuclear power, says that regulators and utilities
began raising concerns about the containment design as far back as the
1970s.
“The key concern has always been that the containment structure was
undersized, and that a potential accident could overwhelm and rupture it,”
Mr. Gunter said.
The Mark 1 reactors in the United States have undergone a variety of
modifications since these initial concerns were raised. Among these,
according to Mr. Lochbaum, were changes to the doughnut-shaped torus — a
water-filled vessel encircling the primary containment vessel that is used
to reduce pressure in the reactor. In early iterations, steam rushing from
the primary vessel into the torus under high pressure could cause the vessel
to literally jump off the floor.
In the late 1980s, all Mark 1 reactors in the United States were also
ordered to be retrofitted with venting systems to help reduce pressure in an
overheating situation, rather than allow it to build up in a containment
system that regulators were concerned could not take it.
It is not clear precisely what modifications were made to the Japanese
boiling water reactors now failing, but James Klapproth, the chief nuclear
engineer for General Electric Hitachi, said a venting system was in place at
the Fukushima plants to help relieve pressure.
With electrical power cut off in the aftermath of the earthquake and backup
sources of power either failing or exhausted, workers have been struggling
to inject seawater into the reactor to maintain control, but they have had
some trouble venting the resulting steam.
Mr. Gunter argued that in any event, such venting amounts to a circumvention
of the whole notion of containment in the first place. “They essentially
have to defeat containment to save it,” he said.
What role the specifics of the G.E. design is playing in the rapid
deterioration of control at the Fukushima plant is likely to be a matter of
debate, and it is possible that any reactor design could succumb to the one-
two punch of an earthquake and tsunami like those that unfolded last week in
Japan.
Although G.E.’s liability would seem limited in Japan — largely because
the regulatory system in that country places most liability on the plant
operator, the company’s share price was down more than 2 percent at midday
Tuesday as the situation at the Fukushima plant deteriorated.
Still, Mr. Lochbaum said it was important to emphasize that the design
specifications for containment and cooling on any reactor are a matter of
balance. The primary alternate reactor design, the pressurized water reactor
, calls for a thicker and bigger containment structure, for example. A
boiling water reactor design like the one at Fukushima does allow for
scaling back on the size of the containment system while ostensibly
maintaining the requisite safety margins.
In that sense, Mr. Lochbaum said, G.E.’s the boiling water reactors should
be no better or worse in weathering accidents than any other design.
Should the ability to cool the reactor completely fail, however, Mr.
Lochbaum said, “I’d certainly rather have a bigger, thicker containment
structure.” |
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