The fire which followed the latest setback was extinguished, but not before a release of radioactive materials, writes DICK AHLSTROM,Science Editor
THE THREAT of a reactor core meltdown continues at the Fukushima Daiichi nuclear power plant.
Workers at the beleaguered facility have been pumping seawater through three overheating reactors in the hope of cooling them and eliminating the possibility of the uranium/zirconium fuel rods melting.
Their work has been made more difficult following a third hydrogen explosion on the Fukushima Daiichi site, followed by a serious fire last night at a spent fuel rod storage pond.
Workers at the plant managed to extinguish the fire, but it is believed to have triggered a release of radioactive materials into the atmosphere.
Radiation exposure is measured in milliSieverts (mSv). Reports from Japan indicate that at one point, radiation levels reached 400mSv an hour – recorded adjacent to the fire.
After the blaze, though, they quickly dropped to 0.6 mSv an hour at the plant periphery.
In comparison, a person living in Ireland might expect to receive three to four mSv from natural sources over an entire year.
Primary cooling systems for the plant’s three reactors were knocked out by the quake, to be replaced by standby electricity generators. These in turn were overwhelmed by the tsunami that quickly followed in the wake of the earthquake. A third back-up system powered by batteries kicked in but was unable to extract enough heat out from the three reactors.
Meanwhile, the emergency committee that meets in the event of nuclear radiation threats to Ireland has had two informal meetings over the past few days. The full national emergency planning body has not met, however, in response to the ongoing nuclear risk in Japan.
The committee, comprising representatives from Government departments and State agencies, convenes at times of nuclear threat. This is when the National Emergency Plan for Nuclear Accidents takes effect.
The unfolding Japanese nuclear crisis did not represent a significant threat to citizens here however, according to a spokesman from the Radiological Protection Institute of Ireland.
“It hasn’t been invoked because there is no risk to Ireland,” he said yesterday.
“There was a mini-meeting on Saturday of a few of the agencies but the full National Emergency Plan for Nuclear Accidents hasn’t been invoked,” the institute spokesman added.
A second informal meeting was held on Monday, he said. This included members of the emergency Response Co-ordinating Committee including representatives from the institute, Met Éireann, the Garda and at least two departments, Environment and Foreign Affairs, he said.
The idea was to keep the channels open and to exchange information on the unfolding events taking place in Japan.
The institute has already issued two statements indicating that the limited radiation releases from Japan were too far away to pose any health risk here.
There are six nuclear reactors on the Fukushima Daiichi site, said the institute’s Dr Claire McMahon, the director of environmental surveillance and assessment.
Three of these were fully shut down for routine maintenance, but reactors 1, 2 and 3 were operating when the quake struck.
All three were automatically shut down when the tremor was detected, with control rods stopping the nuclear reactions that normally take place inside a reactor.
The loss of primary cooling prevented the loss of heat in the reactors and this worsened as the tsunami wave struck the complex, once again halting the essential cooling process, Dr McMahon said.
Since then there has been a constant struggle to cool the fuel rods. Cooling is usually handled by circulating fresh water in a closed loop that bathes the fuel rods.
This system failed, forcing the workers to use seawater in the cooling circuit. The decision to use seawater meant, however, that the nuclear reactors could never be used again, Dr McMahon said.
The first explosion occurred at reactor 1 on Saturday, prompting a public evacuation to beyond 10km (six miles). As attempts to cool the reactors continued a second explosion occurred on Sunday at reactor 3.
Japanese safety authorities said yesterday that fuel rods had been left partially uncovered by cooling seawater in reactor 2 for an extended period. This in turn triggered an explosion or some damage to equipment as noise was detected from beneath the reactor.
This incident “may have affected the integrity of its primary containment vessel”, according to a statement on the International Atomic Energy Agency website.
Reactor cores are encased within steel and concrete containment vessels which are built to withstand explosions and cope with reactor malfunctions including a full meltdown of the reactor fuel, according to Dr McMahon.
The situation deteriorated when a third explosion and subsequent fire occurred adjacent to the shut-down reactor 4 at a spent fuel cooling pond.
Used reactor fuel must be kept in cool water to prevent it overheating.
The fire was extinguished after two hours, but it triggered a significant release of radioactive material, according to the agency, which also confirmed that all three explosions were due to an accumulation of hydrogen gas.
The weather seems to be co-operating in the clean-up operation, however, in that westerly winds are now blowing emissions from the stricken plant out to sea.