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Updated The Tokyo Electric Power Co (TEPCO) is pumping seawater through two ailing reactors at the Fukushima Daiichi (Fukushima No 1) nuclear plant in an attempt to cool radioactive materials in the plant, while simultaneously attempting to prevent an explosion in a third.

Cooling of the reactors was disrupted by Friday's 9.0 earthquake (more details here), and although they were shut down, began to overheat. On Saturday, an explosion in the No. 1 reactor destroyed the external structure but left the steel containment vessel intact.

Early this morning, a hydrogen explosion rocked the No. 3 reactor, but once again the containment vessel held. Plant workers have since battled to maintain an emergency supply of seawater flowing around the rods, with the supply cut off for a couple of hours earlier today "due to the shortage of water left in tanks". The concern is that if the fuel rods are not sufficiently cooled, they could melt the container that houses the core.

At around 7:30 GMT this morning, Kyodo news agency reported that the top of the fuel rods in No 2 reactor were "about to be above cooling water", and that the government would do its "best" to prevent another explosion - again by pumping seawater through the reactor.

National strategy minister Koichiro Genba assured there was "no possibility" of a Chernobyl-style disaster, according to local media. Dr Philip Lloyd, a nuclear physicist at Cape Town's Enery Research Institute, told the BBC's World Service that the explosions were "a matter of great concern", but "seen against the totality of the disaster that Japan is facing, I think things are standing up extremely well".

He added: "This is exactly the same as happened at Three Mile Island, when we had a meltdown there but radiation was contained. It seems in this particular case the reactors withstood the seismic event extremely well."

The government has evacuated over 200,000 people from the 20km exclusion zone around Fukushima Daiichi, although there are still "about 475 people in hospitals and nursing care facilities within the radius". Twenty-two people are confirmed to have suffered radiation contamination following Saturday's blast, with as many as 190 exposed.

Quite how much radiation was leaked by the explosions is unclear.

Kyodo reports: "On Monday, radiation at the plant's premises rose over the benchmark limit of 500 micro sievert per hour at two locations, measuring 751 micro sievert at the first location at 2:20 a.m. and 650 at the second at 2:40am, according to the report.

"The hourly amounts are more than half the 1,000 micro sievert to which people are usually exposed in one year.The maximum level detected so far around the plant is 1,557.5 micro sievert logged Sunday."

The US's aircraft carrier USS Ronald Reagan, which had been assisting rescue efforts from a position around 100 miles east of Fukushima, moved "away from the downwind direction of the plant" after it detected "low-level" radiation.

7th Fleet commander Jeff Davis elaborated: "The maximum potential radiation dose received by any ship's force personnel aboard the ship when it passed through the area was less than the radiation exposure received from about one month of exposure to natural background radiation from sources such as rocks, soil, and the sun." ®

Update

TEPCO said at around 08:30 GMT that No 1 and No 2 reactors had "pulled out of emergency". However, at just before 12:00 GMT, Kyodo noted: "Steam being released at No. 2 reactor of Fukushima nuke plant."

An hour before, news reports suggested the water inside the reactor had evaporated, before Kyodo said TEPCO had managed to raise the level to 30cm. The operator admitted the fuel rods were damaged, but ruled an explosion "unlikely".

Kyodo reports: "Fuel rods at the quake-hit Fukushima No. 1 nuclear power plant's No. 2 reactor were fully exposed at one point after its cooling functions failed, the plant operator said Monday, indicating the critical situation of the reactor's core beginning to melt due to overheating.

"The rods were exposed as a fire pump to pour seawater into the reactor to cool it down ran out of fuel, Tokyo Electric Power Co. said. The firm had reported the loss of cooling functions as an emergency to the government."

TEPCO is examining the possibility of "opening a hole in the wall of the building that houses the reactor to release hydrogen" and prevent an explosion.

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