Fire trucks and police riot control water canons are being used to flood the containment vessel with water as temperatures continue to rise.

Staff dousing reactors with seawater were forced to evacuate part of the building for a period this morning after radiation levels surged. These later receded and staff returned.

“If these efforts fail, then that will lead to an increased risk of uncontrolled criticality (and uncontrolled chain reaction) beginning,” said Bellona nuclear physicist Nils Bøhmer. “They must do everything possible to avoid a melt of the fuel.”

Rising temperatures at reactors No 5 and 6 also continue to be a problem as higher temperatures will evaporate cooling water there as well. Water canons will also be trained on reactor six beginning at about 6 p.m Japanese time, or around 10 a.m. central European time, Bøhmer said citing official reports from the Japanese nuclear safety authority.

In all there are 1000 tonnes of spent nuclear fuel in the pond at reactor No 4 and a total of 200 to 250 tonnes distribured among the fuel ponds at reactor Nos 1, 2 and 3, according to official reports.

The Russian environmental group Ecodefence that there was a total of 1760 tonnes of spent nuclear fuel located among all the spent fuel storage ponds at Fukushima Daiichi.

A rupture to the containment vessel of reactor No 2 has also been reported, which causes yet another area of concern as fuel continues to melts and possible expose radioactive gasses to the environment.

The plutonium danger

It has been suggested that the fuel in reactor No 3, which is a mix of uranium and plutonium oxide, or MOX, could pose a far greater risk were it to melt down and release radiation into the air or, at worst, explode.

Bøhmer said that any core meltdown at any of the reactors which run on regular 20 percent enriched uranium fuel would also pose a risk of plutonium discharges into the environment because plutonium is also a by-product of burning uranium.

Vladimir Slivyak, co-chair of Russia’s Ecodefence, has said that releases of plutonium, which is a heavier radioactive element, poses a far higher risk of radiological diseases in people and longer-term damage to the environment than just a release of uranium because of its longer half life, or period of decay until it is safe.

Bøhmer said that the success of efforts to bring the overheating in reactor 3 to heel are to early to predict.

Water dump on fuel storage ponds

The Japanese government also opened a new front of attack – so far with little effect – against rising heat and levels of radiation at the Fukushima Daiichi plant’s spent fuel storage ponds as it resorted to dumping seawater from helicopters on reactor No 3’s spent fuel storage facility.

Radiation levels taken a half hour after the early Thursday morning dump, however, failed to bring about any reduction, Tokyo Electric Power (TEPCO) said, according to Kyodo news agency.

The goal of the water dumps, which will be ongoing according to Japanese officials, is twofold: to cool the No 3 reactor, which burns a mixed oxide of uranium and plutonium, or MOX, fuel as well as to replenish the water supply in the spent fuel cooling pond. Though TEPCO has been unable to take precise measurements, the pool is thought to be empty or nearly empty of water.

Dumping water on them however is a gamble. With the rods exposed to air, their zirconium tubes begin to react with oxygen and heat up even more. The material inside the tubes melts and can release highly radioactive isotopes such as cesium-137 and iodine-131. The biggest danger is of an uncontrolled chain reaction – as occurs under controlled circumstances in the core of a nuclear reactor – in the open air.

But dumping water, which is a neutron moderator, directly onto the spent fuel also causes a slight chance of a chain reaction, according to nuclear safety specialist Lawrence Williams, who spoke with the BBC.

TV footage showed the CH-47 Chinook helicopters dousing the No 3 reactor in an attempt to cool an overheating pool for spent fuel rods and prevent it from releasing dangerous radioactive steam, the Guardian reported.

Two helicopters, flying at less than 300 feet, dumped four loads of water on the reactor, although the footage suggested a significant quantity was missing the target.

Belona’s Bøhmer confirmed this and said this tactic was ultimately ineffective as the as the pilots simply have to fly too high to avoid radiation to accurately hit the their target.

Radiation levels above reactor No 3 are reported at 87.7 millisieverts per hour at 300 feet above the reactor, decreasing to 4.13 millisieverts per hour at 1000 feet, Kyodo news reported.

The Japanese Defence Ministry said it planned to release at least 12 more loads in the 40 minutes that each crew can remain in the area before experiencing limited radiation exposure.

US NRC issues harsh assessment

In the early morning hours of Thursday in Japan, Greg Jaczko, chairman of the US Nuclear Regulatory Commission, called attention to the problems at the fuel ponds at Fukushima Daiichi at a congressional committee meeting in Washington.                          

He said there was the possibility of a leak in the spent fuel pool in reactor No 3, “which could lead to a loss of water in that pool”, as well as a falling water level in the spent fuel level at the No 2 reactor.

His starkest comments were about the spent fuel pond atop reactor No 4, which were later protested by the Japanese cabinet.

“We believe at this point that Unit 4 may have lost a significant inventory, if not lost all, of its water,” Jaczko told the committee in remarks reported by US media. “There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures.”

He added that radiation levels around the site could give emergency workers “lethal doses” of radiation, forcing them to stay away.

“We believe that around the reactor site there are high levels of radiation,” Jaczko said. “It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time.”

Chief Cabinet Secretary Yukio Edano said he was “baffled” by Jaczko’s remarks about the fuel pond at reactor No 4, and attributed the comments to “a slight delay conveying to the US side the information about whether or not there is water” at the fuel pond at No 4.

Indeed, Bøhmer confirmed that media reports from Japan indicated that there was no water in No 3’s pond, thus making it the most urgent cause of concern.

He said that helicopter fly-overs of the fuel pond at No 4 revealed the presence of water, but because of the distance pilot had to maintain because of radiation and steam, they could not determine exactly how much water remained in the pond.

EC Energy Commissioner: ‘Practically everything is out of control

In developments late last night and early this morning,  the European Commission’s Energy chief Guenther Oettinger told European Parliament that, “[t]here is talk of an apocalypse and I think the word is particularly well chosen,” which has enraged Japanese officials, who have been criticized both in and out of Japan for withholding crucial information, world media reports have indicated.

“Practically everything is out of control,” Oettinger added.

“I cannot exclude the worst in the hours and days to come.”

In response to Japanese criticism of this remarks, Oettinger said he was only responding to what he had been told and had seen on media reports.

The US NRC’s Jaczko went on in his statements to the congressional committee to recommend a sizeable expansion of the evacuation zone around the embattled nuclear power plant from the Japanese government set 20 kilometre radius, and urged Americans living within an 70 kilometre radius of the plant to evacuate.

Late last night, reports began to circulate that TEPCO was attempting to lay power cables from a different electicity provider to restore power, and, hence, coolant circulation, to Fukushima Daiichi. But efforts to do so have been continually hampered by high radiation levels.