Tokyo (AFP) April 7, 2011
Workers at Japan's stricken nuclear plant on Thursday pumped nitrogen into a crippled reactor in a bid to prevent a possible explosion, as the government mulled widening an exclusion zone around the site.
With the crisis at the Fukushima Daiichi plant approaching the end of its fourth week, operator Tokyo Electric Power said it was concerned a build-up of hydrogen gas at the No. 1 reactor could cause another explosion at the site.
And a week after the UN's nuclear watchdog said the 20-kilometre (12-mile) evacuation zone around the stricken plant should be increased, chief government spokesman Yukio Edano indicated Tokyo was considering widening the zone.
"The existing safety standards for local residents are that an evacuation order is issued if there is a possibility that they might receive radiation 50 millisieverts or above," he said.
"The standard assumed that a high level of radiation is emitted temporarily."
"We are discussing how best to issue evacuation orders based on data and standards for accumulative radiation," Edano said.
He added a procedure to inject nitrogen gas into the housing around reactor No. 1 was "proceeding smoothly".
TEPCO said it was concerned that massed hydrogen inside the vessel could mix with incoming oxygen, creating an explosion.
Experts say the risk of a detonation could rise as the nuclear fuel rods cool and as the steam inside the containment vessel condenses into water, reducing pressure inside the unit and drawing air in through cracks.
Workers began pumping in nitrogen, an inert gas abundant in the atmosphere, which they hope will displace the oxygen. The process to inject 6,000 cubic metres (210,000 cubic feet) will take around six days, TEPCO said.
"Workers started injecting nitrogen gas at 1:31 am (1631 GMT Wednesday). Since the pressure level went up, they confirmed that the gas was successfully going into the container," said a spokesman with Japan's nuclear safety agency.
TEPCO said it was also planning to inject nitrogen gas into reactors number 2 and 3 as a preventative measure.
In the days after the earthquake and tsunami crippled the plant, large explosions resulted from hydrogen accumulation near the reactors, damaging the outer buildings housing them.
Around 3,400 people are unaccounted for along the 40-kilometre stretch of coast covered by the exclusion zone and on Thursday, around 300 police began searching for bodies in the the outer 10-kilometre band of the zone.
Television pictures showed officers in full body suits entering the area, while a police spokesman said all officers were armed with radiation meters.
The plant has emitted radioactive material into the air, contaminating drinking water and farm produce, with radioactive iodine above legal limits detected in vegetables, dairy products and mushrooms.
Nuclear concerns continue to distract from the March 11 disaster that has left more than 12,500 dead and over 15,000 missing. Around 160,000 people remain in evacuation centres.
A huge relief operation continues, with tens of thousands of Japanese troops assisted by around 15,000 US personnel.
Reports said US Secretary of State Hillary Clinton was to visit Japan, a long-time security ally, although relations have been strained since the centre-left Democratic Party of Japan came to power in September 2009.
Support from allies will be welcomed in Japan, where concern was rising over the possible contamination of seafood following a spike in levels of radioactive iodine-131 in seawater outside the plant.
"If the situation worsens we don't know what the outlook will be," a manager at a popular sushi chain told AFP, saying the number of customers was down by about a third compared with normal times. "It's very scary to think about."
The Bank of Japan on Thursday warned of pressures as a result of the triple disaster and bolstered funding for quake-hit areas, unveiling a 1.0 trillion yen ($11.7 billion) scheme to keep banks in affected areas liquid.
The BoJ also downgraded its view of the economy due to last month's disasters.
"Japan's economy is under strong downward pressure, mainly on the production side, due to the effects of the earthquake disaster," the central bank said.
The government estimates the total cost from collapsed or damaged houses, factories and infrastructure such as roads and bridges at 16-25 trillion yen over the next three years.
earlier related report
In the days after the March 11 earthquake and tsunami that crippled the plant, an accumulation of hydrogen caused large explosions that badly damaged the buildings housing them. Attention is now focused on the reactor containment vessel.
The operation at reactor No. 1 is expected to take six days. Attention will then turn to reactors No. 2 and 3.
Here is an explanation of how the process works:
WHAT IS THE PROBLEM?
The overheating of the nuclear fuel rods inside the reactor and their subsequent contact with cooling water has generated large amounts of hydrogen.
Hydrogen reacts explosively with oxygen, which is not normally present inside a reactor, which is airtight. However, suspected damage to the containment vessel is thought to have caused it to leak, so workers are concerned that air from the atmosphere -- containing oxygen -- could be drawn in.
Japan's nuclear safety agency says explosions become likely if the concentration of hydrogen in a given space exceeds four percent and oxygen increases to five percent.
Work continues to cool the cores, to try to halt the nuclear reaction, but paradoxically, as the cores cool, the risk of a hydrogen-oxygen explosion could increase.
This is because as it cools, the steam will condense into water, reducing the pressure inside the container and sucking air in.
WHAT'S AT STAKE?
If an explosion occurs at the facility, large amounts of radiation could be released into the environment. This could make the area around the facility even less hospitable to humans and greatly increase the difficulty of containing the disaster.
HOW WILL NITROGEN HELP?
Nitrogen is a largely inert gas, making up around 80 percent of the atmosphere.
Workers hope that by pumping in 6,000 cubic metres (210,000 cubic feet) of nitrogen over the next six days they will be able sufficiently to dilute levels of both oxygen and hydrogen to keep them below critical levels.
The introduction of a large volume of nitrogen will also help to prevent outside air from being drawn into the casing.
ARE THERE ANY POSSIBLE DANGERS?
Officials from Japan's nuclear safety agency say there are no possible dangers and that the nitrogen will not react in any way with the hydrogen.
However, the Yomiuri Shimbun reported that there was a possibility that as the nitrogen is injected, radioactive air inside the container could be forced out through any cracks in the casing.
An official from TEPCO said workers will carefully calibrate the pressure inside the reactor casing, to ensure it does not exceed the level at which air could be forced out.
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