Japan warns of radiation leak from quake-hit plants

While some radiation leakage could be expected,Naoto Sekimura,a professor at the University of Tokyo,said a major radioactive disaster was not likely.

No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise,but it also will stop the reaction,he said.

What happens when a reactor loses coolant>>

Even in the worst-case scenario,that would mean some radioactive leakage and equipment damage,but not an explosion. If venting is done carefully,there will be little leakage. Certainly not beyond the 3 km radius.

Kyodo news agency reported that authorities had begun evacuating about 20,000 people from the vicinity of one of the plants,the Daini plant. Prime Minister Naoto Kan,who was flying by helicopter to view the plant by air,had earlier ordered that residents within a 10 km radius to be evacuated from the other plant,the Daiichi plant.

Chief Cabinet Secretary Yukio Edano said amount of leaked radiation would likely be small.

It’s possible that radioactive material in the reactor vessel could leak outside but the amount is expected to be small,and the wind blowing towards the sea will be considered,he told a news conference.

TEPCO said it had lost ability to control pressure in some of the reactors at its Daini plant as it had with the Daiichi plant. Pressure was stable inside the reactors of the Daini plant but rising in the containment vessels,a spokesman said.

Pressure at one Daiichi reactor may have risen to 2.1 times the designed capacity,the trade ministry said.

The cooling problems at the Japanese plant raised fears of a repeat of 1979’s Three Mile Island accident,the most serious in the history of the US nuclear power industry. However,experts said the situation was,so far,less serious.

Equipment malfunctions,design problems and human error led to a partial meltdown of the reactor core at the Three Mile Island plant,but only minute amounts of dangerous radioactive gases were released.

The situation is still several stages away from Three Mile Island when the reactor container ceased to function as it should,said Tomoko Murakami,leader of the nuclear energy group at Japan’s Institute of Energy Economics.

Japan informed the International Atomic Energy Agency (IAEA) that the quake and tsunami cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.

(It’s) a sign that the Japanese are pulling out all the stops they can to prevent this accident from developing into a core melt and also prevent it from causing a breach of the containment (system) from the pressure that is building up inside the core because of excess heat,said Mark Hibbs,a nuclear expert at the Carnegie Endowment for International Peace.

Radiation levels detected at the control unit of the reactors at the Daiichi plant were not ones that would require workers at the plant to evacuate,a trade ministry official said,adding that radiation at the control unit has risen to about 1,000 times the normal level.

The Union of Concerned Scientists,a U.S.-based nonprofit organisation,said this power failure resulted in one of the most serious conditions that can affect a nuclear plant — a station blackout — during which off-site power and on-site emergency alternating current (AC) power is lost.

Nuclear plants generally need AC power to operate the motors,valves and instruments that control the systems that provide cooling water to the radioactive core. If all AC power is lost,the options to cool the core are limited.

If the core overheats,then the fuel would become damaged and a molten mass could melt through the reactor vessel,releasing a large amount of radioactivity into the containment building surrounding the vessel,the UCS said.

It added that it was not clear if the quake had undermined the containment building to contain pressure from any meltdown and allow radioactivity to leak out.

Power supply systems that would provide emergency electricity for the plant were being put in place,the World Nuclear Association said,with a source in the organisation saying the situation is improving.

The reactors shut down due to the earthquake account for 18 percent of Japan’s nuclear power generating capacity.

Nuclear power produces about 30 percent of the country’s electricity. Many reactors are located in earthquake-prone zones such as Fukushima and Fukui on the coast.

The IAEA estimates that around 20 percent of nuclear reactors around the world are currently operating in areas of significant seismic activity.

It said the sector began putting more emphasis on external hazards after an earthquake hit TEPCO’s Kashiwazaki-Kariwa plant in July 2007,until then the largest to ever affect a nuclear facility.

When the earthquake hit the Kashiwazaki-Kariwa nuclear power plant,four reactors shut down automatically. Water containing radioactive material was released into the sea,but without an adverse effect on human health or the environment,it said.

TEPCO had been operating three out of six reactors at the Fukushima Daiichi nuclear plant at the time of the quake,all of which shut down.

A spokesman said that there were no concerns of a leak for the remaining three reactors at the plant,which had been shut for planned maintenance.