Kyodo News noted earlier that Reactor Number 4 has caught fire:
The Herald Sun reported:
RADIATION levels near a quake-stricken nuclear plant are now harmful to human health, Japan's government says after explosions and a fire at the facility.
"There is no doubt that unlike in the past, the figures are the level at which human health can be affected," said chief government spokesman Yukio Edano.
Although the number-four reactor was shut for maintenance when the quake and tsunami struck last Friday, "spent nuclear fuel in the reactor heated up, creating hydrogen and triggered a hydrogen explosion".
He said radioactive substances were leaked along with the hydrogen.
"Please keep in mind that what is burning is not nuclear fuel itself," Mr Edano said. "We'll do our best to put out or control the fire as soon as possible."
AP now says the fire has now been put out, although the Japanese government says that high levels of radiation are being released:
Prime Minister Naoto Kan said radiation has spread from four reactors of the Fukushima Dai-ichi nuclear plant in Fukushima province, one of the hardest-hit in Friday's 9.0-magnitude earthquake and the ensuing tsunami that has killed more than 10,000 people. "The level seems very high, and there is still a very high risk of more radiation coming out,"
The fire was put out. Even though it was unoperational, the fourth reactor was believed to be the source of the elevated radiation release because of the hydrogen release that triggered the fire.
"It is likely that the level of radiation increased sharply due to a fire at Unit 4," Edano said. "Now we are talking about levels that can damage human health.
Hopefully, Edano is right, and the high levels of radiation were due to a temporary fire, which has been put out.
However, high radiation levels were reported before the fire, when reactor number 2 exploded earlier today, and the government said that its containment core had been breached.
The Christian Science Monitor notes that a design feature of the Fukushima reactors may mean that spent fuel rods release far more radiation than the reactors themselves:
A particular feature of the 40-year old General Electric Mark 1 Boiling Water Reactor model – such as the six reactors at the Fukushima site – is that each reactor has a separate spent-fuel pool. These sit near the top of each reactor and adjacent to it, so that cranes can remove spent fuel from the reactor and deposit it in a swimming-pool-like concrete structure near the top of the reactor vessel, inside each reactor building.
If the hydrogen explosions damaged those pools – or systems needed to keep them cool – they could become a big problem. Keeping spent-fuel pools cool is critical and could potentially be an even more severe problem than a reactor meltdown, some experts say. If water drains out, the spent fuel could produce a fire that would release vast amounts of radioactivity, nuclear experts and anti-nuclear activists warn.
"There should be much more attention paid to the spent-fuel pools," says Arjun Makhijani, a nuclear engineer and president of the anti-nuclear power Institute for Energy and Environmental Research. "If there's a complete loss of containment [and thus the water inside], it can catch fire. There's a huge amount of radioactivity inside – far more than is inside the reactors. The damaged reactors are less likely to spread the same vast amounts of radiation that Chernobyl did, but a spent-fuel pool fire could very well produce damage similar to or even greater than Chernobyl."
But another scientist said while the spent-fuel pools have capacity for high volumes of radioactive material, the amount of fuel currently in the spent-fuel pool might be less than widely believed, based on data he has seen showing only about as much spent fuel in the vulnerable pool as contained in the reactor.
"The inventory numbers I've seen for the spent-fuel pool [that was losing coolant] is well below capacity," said Edwin Lyman, a physicist with UCS, which describes itself as neither pro- nor anti-nuclear power, but which says nuclear safeguards today are not adequate. "That could limit the damage."
It is stunning that the reactors were so poorly designed.
BBC reports (scroll down on left side):
Japanese engineer Masashi Goto, who helped design the containment vessel for Fukushima's reactor core, says the design was not enough to withstand earthquakes or tsunami ...
Goto was speaking about the reactor core. I'm certain he would say the same thing about
the spent-fuel pool.
As MSNBC notes, there are 23 virtually-identical reactors in the U.S.