The radioactive core in a reactor at the crippled Fukushima nuclear power plant appears to have melted through the bottom of its containment vessel and on to a concrete floor, experts said, raising fears of a major radiation release at the site.
The warning follows an analysis by a leading US expert of radiation levels at the plant. Readings from reactor two have been made public by the Japanese authorities and Tepco, the utility that operates it.
Richard Lahey, who was head of safety research for boiling-water reactors at General Electric when the company installed the units at Fukushima, said that workers at the site appeared to have “lost the race” to save the reactor, but there was no danger of a Chernobyl-style catastrophe.
Workers have been pumping water into three reactors at the stricken plant in a desperate bid to keep the fuel rods from melting down, but the fuel is at least partially exposed in all the reactors. At least part of the molten core, which includes melted fuel rods and zirconium alloy cladding, seemed to have sunk through the steel “lower head” of the pressure vessel around reactor two, Lahey said.
“The indications we have, from the reactor to radiation readings, suggest that the core has melted through the bottom of the pressure vessel in unit two, and at least some of it is down on the floor of the drywell,” Lahey said. “I hope I’m wrong, but that’s certainly what the evidence is pointing towards.”
The major concern when molten fuel breaches a containment vessel is that it reacts with the concrete floor of the drywell underneath, releasing radioactive gases into the surrounding area. At Fukushima the drywell has been flooded with sea water, which will cool any molten fuel that escapes from the reactor and reduce the amount of radioactive gas released.
Lahey said: “It won’t come out as one big glob; it’ll come out like lava and that’s good because it’s easier to cool.” The drywell is surrounded by a secondary steel-and-concrete structure designed to keep radioactive material from escaping into the environment. But an earlier hydrogen explosion at the reactor may have damaged this.
“We’re concerned because we’re detecting water outside the containment area that’s highly radioactive and can only have come from the reactor core,” Lahey said. “It’s not going to be anything like Chernobyl, where it went up with a big fire and steam explosion, but it’s not good news for the environment.”
Exposure guidelines
The radiation level at a pool of water in the turbine room of reactor two was measured recently at 1 000 millisieverts an hour. At that level workers could remain in the area for just 15 minutes under current exposure guidelines.
A less serious core meltdown happened at the Three Mile Island nuclear plant in Pennsylvania in 1979. During that incident engineers managed to cool the molten fuel before it penetrated the steel pressure vessel. The task is a race against time, because as the fuel melts it forms a blob that becomes increasingly difficult to cool.
In the light of the Fukushima crisis, Lahey said, all countries with nuclear power stations should have “Swat teams” of nuclear reactor safety experts on standby to give swift advice to the authorities in times of emergency, with international groups coordinated by the International Atomic Energy Authority.
The warning came as the Japanese authorities were being urged to give clearer advice to the public about the safety of food and drinking water contaminated with radioactive substances from Fukushima.
Robert Peter Gale, a US medical researcher who was brought in by Soviet authorities after the Chernobyl disaster, in 1986, has met Japanese Cabinet ministers to discuss establishing an independent committee charged with taking radiation data from the site and translating it into clear public health advice.
“What is fundamentally disturbing the public is reports of drinking water one day being above some limit and then a day or two later it’s suddenly safe to drink. People don’t know if the first instance was alarmist or whether the second one was untrue,” said Gale.
“My recommendation is they should consider establishing a small commission to independently convert the data into comprehensible units of risk for the public so people know what they are dealing with and can take sensible decisions,” he said.
Proposal to transplant cells of plant workers
Japanese authorities are considering plans to collect and freeze cells from engineers and water cannon operators at the Fukushima nuclear power plant in case they are exposed to dangerous levels of radiation.
The proposal has been drawn up as a precautionary measure that could potentially save the lives of workers if they receive high doses of radiation while battling to bring the damaged nuclear reactors under control.
High levels of radiation can cause serious illness and death from bone marrow failure, but the condition can be treated if patients are seen quickly enough and given transplants of blood stem cells collected before they are exposed.
The procedure requires workers to take a drug for several days that causes their bone marrow to release stem cells into the blood. They are then hooked up to a machine through which their blood is passed and filtered to extract the stem cells. The procedure is already used to treat cancer patients whose bone marrow is destroyed by chemo or radiotherapy.
Alejandro Madrigal, the president of the European group for blood and marrow transplantation, said the plan made sense given the risk to workers at Fukushima. He said more than 50 hospitals in Europe had agreed to help the Japanese if required.
But Robert Peter Gale, an American medical researcher advising the Japanese government, had doubts. “These cells can reconstitute bone marrow function. That is not the only target of high-dose radiation.
They would have damage elsewhere, to their lungs, gastro-intestinal tract and their skin. I, and a number of colleagues, feel it’s not an appropriate thing to do.” —