A third explosion in four days has rocked the earthquake-damaged Fukushima Daiichi nuclear plant in Japan.
The Tokyo Electric Power Company (TEPCO) has confirmed there was a blast at Fukushima Daiichi number two reactor unit at 6.14am Japanese time (10.14am NZT).
Today's blast at Daiichi Unit 2 follows two hydrogen explosions at the plant - at Unit 1 and Unit 3 - as authorities struggle to prevent the catastrophic release of radiation in the area devastated by a tsunami.
After the blast, radiation levels increased over the legal limit and some staff at the site were evacuated, a TEPCO spokesman said.
The cause of the blast is not yet known.
The suppression pool, which contains radioactive materials, lost pressure by two-thirds after the incident and may have been damaged in the incident.
At a media conference, officials from the Tokyo Electric Power Company could not confirm whether there had been damage to the suppression pool.
"We have only confirmed the pressure has gone down - we do believe there is that possibility but there could possibly be a problem with the pressure gauge," a spokesman said.
"The blast was heard near the containment vessel. We checked the parameters and we found out the pressure was falling in the suppression pool, indicating there was some damage to the suppression pool, so that caused the evacuation of operators."
The number two reactor has been the cause of great concern for TEPCO, who have struggled to inject water into the reactor's containment, leaving the fuel rods exposed.
The water is being injected to cool the fuel and there are fears the reactor's core will melt at a faster pace if the exposed rods are not covered.
TEPCO officials said the water level had not changed following the blast and about half of the fuel rods remain exposed. Water is continuing to be injected into the containment.
"In terms of the seriousness of this incident, we apologise for causing concern to the public," the spokesman said.
While the officials were repeatedly apologetic for the concern caused by the incident, they had little details on what had happened at the site.
"These are the facts. What does that mean? We are trying to grasp that."
Kyodo News reported Tokyo Electric Power Company officials as saying radiation leaking from the Fukushima plant had reached eight times its annual limit in an hour.
The 8,217 micro sievert per hour reading was detected after a third blast hit the plant this morning, Kyodo reported.
If the suppression pool is damaged, it is possible radioactive liquid or air could leak out and get into the atmosphere, depending on where the damage is, an expert told NHK World.
However, he said radiation levels are not high enough to be a risk to people at this stage.
Increased radiation levels have also been detected in the Ibaraki province, between the damaged Fukushima plant and Tokyo, says Kyodo News.
The cascading troubles in the Fukushima Daiichi plant compounded the immense challenges faced by the Tokyo government, already struggling to send relief to hundreds of thousands of people along the country's quake-and tsunami-ravaged coast where at least 10,000 people are believed to have died.
No one knows how much damage has been done to the fuel rods, either in this reactor, number 2, or in reactors number 1 and number 3, where engineers began pumping in seawater over the weekend.
Officials have called the situation a partial meltdown because they have detected minute quantities of radioactive caesium and iodine - byproducts of the nuclear fission that powers the reactor - outside the plant.
Fukushima prefectural officials said that 190 people have been exposed to some radiation from the plant.
Officials of the Tokyo Electric Power Co said conditions are stable at reactors number 1 and number 3 and that the cooling seems to be working.
An explosion at reactor number 3 on yesterday destroyed the outer building at the number 3 reactor and injured 7 workers, but did not damage the reactor containment vessel.
A similar explosion at reactor number 1 earlier in the weekend damaged that building and released what is said to be small amounts of radiation into the environment.
A US warship sailing off the coast of Japan reported that it sailed through a small plume of radiation from the plant, but has successfully decontaminated both the ship and sailors.
Japanese authorities have so far reported no radiation release from the explosion at the number 3 reactor.
Japan nuclear crisis not Chernobyl-like - watchdog
Earlier, the head of the UN atomic watchdog IAEA said it was "very unlikely" that the crisis at Japan's Fukushima nuclear power plant would turn into a Chernobyl-like situation,
"Let me say that the possibility that the development of this accident into one like Chernobyl is very unlikely," Yukiya Amano told a news conference at the headquarters of the International Atomic Energy Agency in Vienna.
The current crisis at the Fukushima nuclear plant in Japan was caused not by human error or a design fault, as in the case of Chernobyl in Ukraine in 1986, but by a "huge natural catastrophe beyond imagination," Amano said.
In addition, the reactors at Fukushima had been automatically shutdown when the earthquake hit, so "there is no chain reaction going on," the IAEA chief said.
Furthermore, the Chernobyl reactor did not have a reactor vessel, while Fukushima does "and that reactor vessel is still contained" even after two explosions there, he said.
The "design is different and the structure is different. Based on this, it is very unlikely that Fukushima would develop into an accident like Chernobyl," Amano said.
- Paul Harper, NZ Herald staff, AAP, AFP, AP