"United States government engineers sent to help with the crisis in Japan
are warning that the troubled nuclear plant there is facing a wide array of fresh threats
that could persist indefinitely,
and that in some cases are expected to increase as a result of the very measures being taken
to keep the plant stable,
according to a confidential assessment prepared by the Nuclear Regulatory Commission.
Among the new threats...
mounting stresses placed on the containment structures
as they fill with radioactive cooling water...
...the possibility of explosions inside the containment structures
due to the release of hydrogen and oxygen from seawater pumped into the reactors...
If the fuel continues to heat and melt because of ineffective cooling,
...that could also leave a radioactive mass that could stay molten for an extended period.
...the document raises new questions about whether pouring water on nuclear fuel
in the absence of functioning cooling systems can be sustained indefinitely.
Experts have said the Japanese need to continue to keep the fuel cool for many months
until the plant can be stabilized, but there is growing awareness that the risks
of pumping water on the fuel present a whole new category of challenges
that the nuclear industry is only beginning to comprehend.
The document also suggests that fragments or particles of nuclear fuel
from spent fuel pools above the reactors were blown “up to one mile from the units,”
and that pieces of highly radioactive material fell between two units and had to be “bulldozed over”...
...The ejection of nuclear material,
which may have occurred during one of the earlier hydrogen explosions,
may indicate more extensive damage to the extremely radioactive pools than previously disclosed.
David A. Lochbaum, a nuclear engineer who worked on the kinds of General Electric reactors used in Japan
...said...“I thought they were, not out of the woods, but at least at the edge of the woods,”
...“This paints a very different picture, and suggests that things are a lot worse.
They could still have more damage in a big way if some of these things don’t work out for them.”
...Because slumping fuel and salt from seawater that had been used as a coolant
is probably blocking circulation pathways, the water flow in No. 1
“is severely restricted and likely blocked.”
Inside the core itself, “there is likely no water level,” the assessment says, adding that as a result,
“it is difficult to determine how much cooling is getting to the fuel.”
Similar problems exist in No. 2 and No. 3, although the blockage is probably less severe...
...Margaret Harding, a former reactor designer for General Electric, warned of aftershocks and said,
“If I were in the Japanese’s shoes, I’d be very reluctant to have tons and tons of water
sitting in a containment whose structural integrity hasn’t been checked since the earthquake.”
...Hydrogen explosions in the first few days of the disaster heavily damaged several reactor buildings
and in one case may have damaged a containment structure.
...Nuclear engineers have warned in recent days
that the pools outside the containment buildings that hold spent fuel rods
could pose an even greater danger than the melted reactor cores.
The pools, which sit atop the reactor buildings and are meant to keep spent fuel submerged in water,
have lost their cooling systems.
The N.R.C. report suggests that the fuel pool of the No. 4 reactor
suffered a hydrogen explosion early in the Japanese crisis
and could have shed much radioactive material into the environment,
what it calls “a major source term release.”
Experts worry about the fuel pools because explosions have torn away their roofs
and exposed their radioactive contents.
...“Even the best juggler in the world can get too many balls up in the air,”
Mr. Lochbaum said of the multiplicity of problems at the plant.
“They’ve got a lot of nasty things to negotiate in the future,
and one missed step could make the situation much, much worse.”
JAMES GLANZ and WILLIAM J. BROAD