A POOL containing spent fuel rods at Fukushima's No. 4 reactor "is the major concern" in Japan's nuclear crisis, presenting the risk of radioactivity being released directly into the air, a French safety agency says.

The deep tank at the reactor unit contains used fuel rods which are extremely radioactive and normally kept immersed in cooling water.

Unlike the fuel rods that are used in the reactor vessel, the spent rods are not surrounded by a steel-and-concrete containment vessel, which is designed to confine leaks of radioactive gas and particles.

Instead, they are housed in the overall building covering the No. 4 unit, which has been "badly damaged", the Nuclear Safety Authority (ASN) said last night.

Fires broke out yesterday and again early today in the area of the fuel-rod pool before being extinguished, the agency said.

"The state of the fuel rods stored in this pool, which have been potentially affected by the fire, is not known," the ASN said.

"The evaporation of water in the pool is continuing."

If the tanks run dry, the rods can overheat and the metal sheaths that surround the fuel can be ruptured.

"The Japanese authorities have consequently indicated there is the possibility of releases of radioactivity directly into the atmosphere."

Japan has asked the National Police Agency to send a water cannon to the Fukushima plant to help pour water into the pool, broadcaster NHK said.

The water cannon truck, held by Tokyo's Metropolitan Police Department, may start operations overnight.

A Japanese military helicopter was deployed late yesterday on a mission to dump water from a huge bucket onto the fuel rods, but returned due to a high level of radiation above the reactors and containment pool before night fell.



White fumes were seen coming from the plant's No. 3 reactor, with the plant's owner Tokyo Electric Power company (TEPCO) saying the clouds might be steam.

TEPCO said it was unable to check the smoke-like clouds, which began about 10.00am local time yesterday, as the radiation levels were too high.

The rise in radiation levels which saw an evacuation order issued for workers at the Fukushima plant was likely caused by the No. 2 reactor, the nation's nuclear safety agency said.

The evacuation order was issued at 10.40am local time and about an hour later workers were allowed to return when the radiation spike subsided, Kyodo News reported.

Government spokesman Yukio Edano said there was no need to expand the evacuation area around the plant, located 250km northeast of Tokyo, based on current data.

The nuclear safety agency said the level around the plant peaked at 6.4 millisieverts at 10.45am local time. Within 10 minutes it had fallen to 2.9 millisieverts, AFP reported.

A spike in radiation levels near the reactors yesterday ranged from 30 to 400 millisieverts.

A single dose of 1000 millisieverts - or one sievert - causes temporary radiation sickness such as nausea and vomiting.

The Japanese Government said that it was ready to seek cooperation with the US military to avert a nuclear catastrophe in the wake of Friday's devastating quake and subsequent tsunami.

"US military cooperation is needed when necessary. They have already cooperated in providing logistic materials," Mr Edano said.

He added: "We are preparing ourselves" to arrange further cooperation.

Two Australian search and rescue crew members suffered low-level exposure to radiation after the helicopter they were travelling in was forced to land near Fukushima.

South Korea steps in

South Korea said it would send an emergency shipment of cooling material to Japan to help control damaged nuclear reactors.

Tokyo has asked for 52 tonnes of boron, a key material used for regulating nuclear chain reactions, as it is running short of the metalloid to cool the overheated Fukushima reactors, Seoul said.

"We've sent boron samples. Now, we are scraping up all we got. We will ship 52 tonnes of boron as soon as Japan finds the sample compatible with their system," an official of the Ministry of Knowledge Economy said.

Boron is an essential ingredient in control rods used to halt or slow down fission reactions at nuclear reactors.

Japan also asked for an emergency fuel shipment from South Korea.

S-Oil Corporation, SK Energy Company and GS Caltex Corporation said they planned to ship refined petroleum products such as petrol, diesel and kerosene to Japan after many refineries were forced to shut down due to the devastating quake.





Toll climbs

The official toll of the dead and missing has climbed to 11,000, with 3676 confirmed dead, police said.

The total number of people unaccounted for rose by more than 800 to 7558, the national police agency said in its latest update.

The number of injured stood at 1990.

The crisis at the power plant now rates six on a seven-point international scale of gravity for nuclear accidents.

France's Nuclear Safety Authority says the Fukushima crisis - rating six - is now just one point below the 1986 Chernobyl nuclear disaster in the Ukraine.

The 1979 Three Mile Island disaster in Pennsylvania, US, registered as five. Japan's nuclear watchdog previously rated the situation at Fukushima as four.

But the Australian Radiation Protection and Nuclear Safety Agency says there's "a very low to negligible chance of contamination'' to Australians in affected areas.

However, the agency has warned Australians to continue to stay away from the plant and Miyagi Prefecture on the east coast of Japan.

Australians should not enter the 20-30km exclusion zone around the Fukushima facility, the Department of Foreign Affairs updated travel advice says.

As of 8pm last night, 3901 Australians were registered in Japan. Of these, 3442 have been confirmed as safe, including 151 in the worst affected areas, while 94 remain unaccounted for, DFAT said.

- with NewsCore, AFP

If you cannot contact family and friends in the region, ring the Department of Foreign Affairs on 1300 555 135

Originally published as Nuclear pool biggest concern