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March 2011, Week 3

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Tue, 15 Mar 2011 21:46:19 -0400
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Second Chances: Containment of a Reactor Meltdown

By Frank Von Hippel | 
14 March 2011
Bulletin of the Atomic Scientists

http://thebulletin.org/node/8616

One aspect of the drama that we have been witnessing at
Japan's Fukushima Nuclear Power Plants since the March 11
earthquake and tsunami is the efforts of Tokyo Electric Power
(TEPCO) to prevent the over- pressurization of the
containments of its boiling water reactors at plants I and
II. These reactors lost power from the grid due to the
earthquake and their backup emergency diesel generators
failed, apparently due to damage from the tsunami.

As a result, there was no power for pumping outside water
through the heat exchangers used to remove the heat generated
by the residual radioactivity in the fuel. The pressure in
the reactors therefore began to rise and their pressure-
relief valves were opened to relieve this pressure. The
reactor containments did have backup pressure-suppression
pools of water in their basements and steam-powered pumps
that could circulate water from the pools to the reactors.
After some time, however, the pressure suppression pools
began to boil and were no longer useable for cooling.

In three of the reactors (Fukushima-Daiichi units numbers 1,
2, and 3 -- the units that were operating at the time of the
earthquake and therefore hot) the water levels in the
reactors fell, uncovering their fuel, which overheated and
released some of its reactivity into the containment. The
steam pressure also built up in the containments to the point
where they were on the verge of bursting.

TEPCO therefore received permission from Japan's government
to release some of the gas from the containments. Because
this gas contained radioactivity, the surrounding population
was evacuated out to a distance of 20 kilometers (12 miles).
Fortunately, the release was orders of magnitude smaller than
occurred at Chernobyl.  Much of the radioactivity is still
trapped in the fuel and most what was released probably
dissolved in the water. Most importantly, the wind blew the
plume to sea.

As a result of the overheating of the reactor fuel, its
zirconium cladding reacted with the steam to oxidize the
zirconium and release hydrogen from the steam. The hydrogen
didn't explode in the containments because they had been
"inerted" by being filled with pure nitrogen (i.e. no oxygen)
but, when the hydrogen was vented into the outer containment
buildings, it mixed with the air there and exploded, blowing
the roofs off the outer buildings but (except perhaps for
unit number two) leaving the inner containments and massive
spent fuel pools intact.

In 1979, during the accident at reactor number two at Three
Mile Island, the fuel was similarly partially uncovered and
its fuel cladding failed, releasing a great deal of
radioactivity to the containment atmosphere, and there was a
hydrogen explosion.  But the containment was not over-
pressured and no significant amount of radioactivity was
released to the atmosphere. The main difference was that
electricity was available to power the reactor's containment
cooling systems and condense the steam within it.  If the
power had failed, the containment of reactor number two would
have been over-pressured as well.

The problem of containment over-pressurization and the
potential need to vent has been a long-term issue in the
nuclear reactor safety community. In 1977, a group of nuclear
engineers at the University of California suggested that a
robust filtration system be installed in reactors to remove
the radioactivity from the vented gases. Some countries
picked up the idea. Sweden installed a filtered vent system
at the Barseback reactors (subsequently shutdown) across the
strait from Copenhagen and France installed filtered vent
systems at all of its reactors, which interested both Germany
and Japan.

In 1982, after the accident at Three Mile Island, Jan Beyea
and I wrote an article on filtered vents in the Bulletin of
the Atomic Scientists in 1982 (page 52). As we reported in
that article, the US Nuclear Regulatory Commission (NRC) was
negative about the idea. They had a number of arguments but
none of them seemed very strong to us. Those interested can
read our summary of the arguments pro and con in the 1982
article.

The unspoken argument against requiring that US nuclear power
plants be retrofitted with filtered vents was that the
industry thought that they were already safe enough and that
the expense would be wasteful. And, as today, the commission
did not want to force the industry to do more than it was
willing to do.

In 2002, the NRC, despite alarming evidence that a pressure
vessel had almost corroded through, refused to force an owner
to shutdown the reactor for inspection before its regular
refueling shutdown. After a review, the NRC's own inspector
general concluded:

     "NRC appears to have informally established an
     unreasonably high burden of requiring absolute proof of
     a safety problem, versus lack of a reasonable assurance
     of maintaining public health and safety."

We failed after Three Mile Island in 1979 to reform the
Nuclear Regulatory Commission or force improved containment
designs. The tragedy in Japan may have given us another
opportunity.

___________________________________________

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