12 April last updated 11:09
In some quarters, Japan's swift decision to establish an exclusion zone, initiate monitoring of people and food, and dispense iodine tablets has received praise - and the volume of advice and warnings issued to the public has been very different from the silence that pertained after Chernobyl, despite criticisms from other quarters over the quality and timing of some of the Japanese advice.
Nuclear experts are cautioning, however, that the situation at Fukushima is still very much in play.
Where most attention has centred on reactors 1, 2 and 3, there is also concern about the spent fuel pond in reactor building 4.
In the week following the tsunami, it became clear that the pond was running dry, meaning that fuel rods would have heated up.
This meant likely degradation of their zirconium alloy cladding, the possible release of hydrogen, and - by Tepco's admission - the risk that a nuclear chain reaction could begin.
"The potential for atmospheric release is very large, with 250 tonnes of material sitting in that pond," said Dr Large.
"If you were to have an energetic event you could have a very large release indeed."
Where most attention has centred on reactors 1, 2 and 3, there is also concern about the spent fuel pond in reactor building 4.
In the week following the tsunami, it became clear that the pond was running dry, meaning that fuel rods would have heated up.
This meant likely degradation of their zirconium alloy cladding, the possible release of hydrogen, and - by Tepco's admission - the risk that a nuclear chain reaction could begin.
"The potential for atmospheric release is very large, with 250 tonnes of material sitting in that pond," said Dr Large.
"If you were to have an energetic event you could have a very large release indeed."
The priority is, as it has been all along, to restore adequate coolant to the fuel ponds and the reactors themselves - while hoping that earthquake aftershocks and bad weather do not hamper operations any more.
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12 April 2011 Last updated at 11:09 GMT BBC |
------------------------------------------------------------ AP News
AP 12 April
＜Questions and answers: Japan, Chernobyl disasters＞
AP 12 April
＜Questions and answers: Japan, Chernobyl disasters＞
Q. So is Japan's crisis as bad as Chernobyl's?
A. Not yet. Chernobyl was a fast-moving crisis. A routine shutdown went awry, causing a reactor to overheat, explode and burn. For 10 days, the reactor spewed high levels of radiation into the air and only cooled after helicopters dropped sand, clay, lead and other materials on it. By contrast, Fukushima crisis has been a slow cascade of problems over a month. Explosions occurred at three of Fukushima's reactors and one may be leaking. But the two plants' reactor designs are different. Unlike Chernobyl's reactors, Fukushima's have pressure vessels of steel six inches (15 centimeters) thick that may have helped contain the damage.
Q. Then why the same severity level rating?
A. The IAEA defines a level-7 accident as one in which a large amount of radiation is released into the atmosphere, likely harming human health and damaging the environment over the long-term. That threshold is set at several tens of thousands of terabecquerels - a unit of radiation - of iodine-131, a radioactive element commonly released in nuclear accidents. Leaks at both plants have exceeded that limit, but the Japanese government says Fukushima's are still one-tenth of those released by Chernobyl. The possibility Fukushima's emissions could surpass Chernobyl's is considered small, but still a risk until Fukushima's cooling systems are restored.
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