TOKYO (AP) — Radioactive debris from melted fuel rods may have seeped deeper
into the floor of a Japan’s tsunami-hit nuclear reactor than previously
thought, to within a foot from breaching the crucial steel barrier, a new
simulation showed Wednesday.
The findings will not change the ongoing efforts to stabilize the reactors
more than eight months after the Fukushima Dai-ichi plant was disabled, but
they harshly depict the meltdowns that occurred and conditions within the
reactors, which will be off-limits for years.
The plant operator Tokyo Electric Power Co. said its latest simulation
showed fuel at the No. 1 reactor may have eroded part of the primary
containment vessel’s thick concrete floor. The vessel is a beaker-shaped
steel container, set into the floor. A concrete foundation below that is the
last manmade barrier before earth.
The fuel came within a foot of the container’s steel bottom in the
worst-case scenario but has been somewhat cooled, TEPCO’s nuclear safety
official Yoshihiro Oyama said at a government workshop. He said fuel rods in
the No. 1 reactor were the worst damaged because it lost cooling capacity
before the other two reactors, leaving its rods dry and overheated for hours
before water was pumped in.
The nuclear crisis following the March 11 earthquake and tsunami caused
massive radiation leaks and the relocation of some 100,000 people.
Another simulation on the structure released by the government-funded Japan
Nuclear Energy Safety Organization, or JNES, said the erosion of the
concrete could be deeper and the possibility of structural damage to the
reactor’s foundation needs to be studied.
JNES official Masanori Naito said the melting fuel rods lost their shape as
they collapsed to the bottom of the vessel, then deteriorated into drops
when water pumping resumed, and the fuel drops spattered and smashed against
the concrete as they fell, Naito said.
TEPCO and government officials are aiming to achieve “cold shutdown” by the
end of the year — a first step toward creating a stable enough environment
for work to proceed on removing the reactors’ nuclear fuel and closing the
The government estimates it will take 30 years or more to safely
decommission Fukushima Dai-ichi.
Wednesday’s simulations depict what happened early in the crisis and do not
mean a recent deterioration of the No. 1 reactor. Oyama said, however, the
results are based only on available data and may not match the actual
conditions inside the reactors, which cannot be opened for years.
Some experts have raised questions about achieving the “cold shutdown,”
which means bringing the temperature of the pressure vessel containing
healthy fuel rods to way below the benchmark 100 Celsius (212 Fahrenheit).
They say the fuel is no longer there and measuring the temperature of empty
cores is meaningless, while nobody knows where and how hot the melted fuel
Kiyoharu Abe, a nuclear expert at JNES, said it’s too early to make a
conclusion and more simulations should be done to get accurate estimates.
“I don’t think the simulation today was wrong, but we should look at this
from various viewpoints rather than making a conclusion from one
simulation,” Abe said. “It’s just the beginning of a long process.”