By RFE RL
By Charles Recknagel for RFE/RL
After three weeks of efforts to regain control of Japan’s damaged reactors at the Fukushima Daiichi nuclear power plant, several bits of bad news this week showed the job could be even more difficult than originally thought.
Rescue teams working inside the power station found pools of contaminated seawater forming in the complex’s basement areas. That was a sign that the cold water the teams have been pouring into the reactors to head off a meltdown is itself leaking out and spreading radioactive material.
At the same time, investigators detected higher levels of plutonium in soil samples taken outside the power plant. The find raised fears that the fuel rods in at least one of the station’s six reactor cores have already undergone a partial meltdown and heightened the urgency of keeping the reactor cores cool.
Nuclear experts say it is now clear that a substantial number of the fuel rods that powered the nuclear plant were damaged.
“The source of the plutonium is reasonably clear, it’s come from the inside of the fuel rods themselves, there is no other place in the reactor that could come from,” says Tim Abram, a nuclear expert with the Dalton Nuclear Institute at Manchester University in Britain.
“It confirms what we already strongly suspected, which is that in at least one of the units there is at least some damage to some of the fuel rods,” Abram adds, “meaning that the fission products, the fuel material itself, has escaped at least into the primary containment and, more than likely, beyond that.”
Much Damage, But Still No Meltdown
The exact extent of the damage is still unknown. Tokyo Electric Power, which runs Fukushima, has released data only sparingly. Its president, Masataka Shimizu, is currently in the hospital being treated for hypertension, further slowing communication.
Another nuclear expert, Lars Gunsell, with the Swedish Radiation Safety Authority in Stockholm, says estimates of the damage range from 10 to 16 percent of the fuel rods in the most damaged reactor, Unit 2.
But while the fuel rods have suffered what Gunsell calls a “large” amount of damage, he says there is no indication yet that the partial meltdown could trigger the most catastrophic scenario associated with nuclear-reactor accidents: the full meltdown of the reactor core. That occurs when temperatures in the reactor core rise to such a high degree that the nuclear fuel itself dissolves and melts through the reactor vessel, spilling out directly into the environment:
“In our opinion, there is large damage of the fuel core. But so far there has not been any information indicating that there is a melt-through of the reactor vessel,” Gunsell says. “We believe from the information we have that the damaged core is still in the reactor vessel.”
Not all nuclear experts agree. Because crucial control-room functions in the reactors remain disabled, no one can be exactly sure what is happening inside the reactor cores.
One expert, Richard Lahey, who was head of safety research for General Electric, which installed the Fukushima reactor units, told Britain’s “The Guardian” newspaper this week that data from Unit 2 “suggest that the core has melted through the bottom of the pressure vessel,” adding, “I hope I am wrong.”
Can The Cores Be Cooled?
The current heat in the reactor cores comes from the natural decay process of the radioactive fuel rods, a process that is independent of whether or not the nuclear reactor is on or off. The Fukushima plant automatically switched off seconds after it was hit by a combined earthquake and tsunami on March 11. Yet reducing the decay heat — and with it the danger of a further meltdown — remains a massive challenge.
Japanese rescue teams have focused on trying to cool the reactor fuel rods with emergency infusions of water into the reactor system. Abram says that strategy, combined the gradual natural slowing down of the decay rate, seems to be succeeding in limiting the danger of further damage to the reactor core.
“Firstly the amount of heat being introduced into the system by decay heat is naturally falling away with time, it is reducing, and secondly the rate at which heat is being removed from the reactors is increasing because at least some measure of cooling has been restored,” the nuclear expert says. “If we take both of those things together, then the likelihood of even further core damage occurring is, I would not say it’s gone entirely, not at all, but it is certainly diminishing.”
But efforts to cool the reactor cores this way are complicated by the unexpected leakage of the injected water out of what normally should be a closed-loop cooling system. Abram says the rescue teams are still trying to determine whether the leakage is due to cracks in the reactors’ structure or due to a failure of the normal drainage system to fully close.
So far, the leaks have produced several Olympic-size swimming pools’ worth of water in the reactor plant’s basement area and in a tunnel leading to the sea. Because the water leaking through the reactor picks up radiation along the way, the leakage is threatening to disperse more radiation into the environment just as the earlier explosions that blew the roofs off two of the reactor buildings spread radioactive particles into the air.
‘Not Weeks Or Months, But Years’
The leakage has created its own containment crisis, with Japanese engineers this week scrambling for ways to prevent a large-scale spill into the sea.
Japan’s “Asahi Shimbun” newspaper reported on March 30 that one plan under discussion is to drape the nuclear reactor buildings with special covers to limit radiation and pump the contaminated water into a tanker anchored offshore. That comes as the level of radioactive iodine in the sea off Japan’s disaster-hit Fukushima nuclear plant today reached its highest reading yet at 4,385 times the legal limit.
Nuclear safety official Hidehiko Nishiyama: “We will have to continue cooling for quite a long period. We should be thinking years.”
And it also comes as the UN’s International Atomic Energy Agency (IAEA) this week recommended that the Japanese authorities double their current 20-kilometer exclusion zone around Fukushima, after high levels of cesium-137 radiation were detected in a village twice that distance from the plant.
Unlike radioactive iodine, which dissipates relatively quickly, cesium-137 has a half-life of 30 years, posing a long-term contamination risk.
Among the biggest questions for the weeks ahead are whether the rescue team can seal off the leaks so that the fuel rods can be maintained in a contained cooling system that essentially mimics what nuclear-reactor operators due with spent fuel. That is, put the fuel in a cooling pond where it remains for years until it can be disposed of safely.
But whether the damaged nuclear reactors can be repaired to contain and cool the nuclear fuel or whether entirely different solutions will be needed still remains unknown. The only certainty is that the solution will have to last not weeks, or months, but years.
As top Japanese nuclear official Hidehiko Nishiyama, the deputy director-general of the Nuclear and Industrial Safety Agency, told reporters this week: “We will have to continue cooling for quite a long period. We should be thinking years.”