Japan considers wider evacuation zone, but water leak is stopped at reactor site
The Wall Street Journal reports that the Japanese government is considering extending the evacuation zone around the devastated Fukushima nuclear reactor complex.
According to the newspaper, the government is recalculating the risk of radioactivity that continues to be found in areas outside the plant four weeks after Japan's devastating earthquake and tsunami.
Some good news comes via NucNet which reports that workers at the Fukushima nuclear power plant have used 6,000 litres (8 cubic yards) of coagulant to stop a leak from a trench next to the unit 2 inlet point that has been causing highly radioactively contaminated water to flow into the sea since March 29th.
No decision on new evacuation limits yet
The government's top spokesman, Chief Cabinet Secretary Yukio Edano, said April 8 the current 20-kilometer (12-mile) evacuation zone around the Fukushima Daiichi plant may need to be expanded to a larger perimeter. He said the original boundaries were set to protect against short term exposure to radiation, but with continued releases of radioactive material, the government wants a to look at a bigger protection zone.
Edano said the larger zone is expected to prevent cumulative exposure to radiation than a nuclear plant worker is limited to in a year—50 millisieverts. A one year dose clearly linked to cancer would be 100 millisieverts. At 400 millisiverts of short-term exposure, the acute symptoms of radiation poisoning can become readily apparent.
"Current evacuation orders apply to areas where people are in danger of having received 50 millisieverts [of cumulative exposure]," Mr. Edano said. "We are now looking into what to do with other areas where, with prolonged exposure, people may receive that
The WSJ also reports that two weeks ago, a Japanese government agency released a computer simulation that showed that in the first 12 days after the tsunami, some areas outside the evacuation zone exceeded Japan's recommended cumulative exposure limits.
It isn’t clear whether the government trusts the information sufficiently to make a decision. TECPO has been plagued with reporting errors in radiation measurements.
The government is clear about local contamination problems within a few miles of the Fukushima plant.
"The [hydrogen] explosions sent radioactive materials flying to areas far outside the nuclear complex," Mr. Nishiyama, of Japan's Nuclear and Industrial Safety Agency, said at a news conference. "Radioactive materials, once spread, cannot be put back. The best we can do is to stabilize the damaged reactors and prevent further emissions of radiation."
Workers Use Coagulant To Stop Fukushima-Daiichi Leak
(NucNet): [April 6] Workers at the Fukushima-Daiichi nuclear power plant have used coagulant to stop a leak from a trench next to the unit 2 inlet point that has been causing highly contaminated water to flow into the sea since March 29th.
Tokyo Electric Power Company (Tepco) said that they used liquid tracer to find the source of the leak in the concrete utility trench and then used the coagulant to seal it. The company said the leak had stopped.
Iodine-131 and Caesium-137 were both detected in water sampled in the trench and in the sea near the water discharge. While the radioactive iodine has a half life of eight days, and degrades to background levels in a few weeks, the half life of cesium-137 is about 30 years.
The water leak was discovered on April 2nd when workers detected water releasing a radiation dose rate of more than 1,000 millisieverts per hour in the trench and found a crack about 20 cm wide on the trench’s concrete wall, from where water was thought to be flowing into the sea. Severe radiation poisoning, in some cases, fatalities, occur at 2,000 millisieverts per hour.
Meanwhile, Tepco has begun discharging into the sea low-level radioactive wastewater stored in sub-drain pits at units 5 and 6 and in a reservoir of the central radioactive waste disposal facility.
Tepco plans to discharge approximately 10,000 tonnes of low-level radioactive water and about 1,500 tonnes of the low-level radioactive subsurface water into the sea. [Water weights 8.4 pounds/gallon. A metric tonne is 2,240 pounds. Therefore, 10,000 metric tonnes of water = 22,400,000 pounds / 8.4 equals 2.7 million gallons of radioactive water.
The discharge is necessary because workers need to use the reservoirs for storing highly radioactive water from the unit 2 turbine building, where dangerously high levels of radioactivity in water have been found in the turbine building basement. There is a risk that this radioactive water might also flow into the sea.
Tepco said it is concerned that vital equipment needed to secure the safety of the reactors might be submerged if this water is not drained.
The utility is also monitoring an accumulation of hydrogen gas in the primary containment vessel of unit 1 Measures are being taken to avoid a hydrogen explosion similar to the explosion in the primary containment vessel of unit 2.
Removing Contaminated Water Might Take Another Week, NISA Says
(NucNet): Moving highly radioactive water from a turbine building basement at the Fukushima nuclear power plant might not start for another week, Japan’s Nuclear and Industrial Safety Agency (NISA) has said.
The water needs to be moved from the turbine building basement to a storage facility. The water has been hampering work to restore the reactor's cooling systems.
NISA said the facility must be checked for cracks that might have been caused by the earthquake, and that this could take up to a week. The agency said the facility was designed to store low-level radioactive water.
The International Atomic Energy Agency said today that overall, the situation at the plant remains “very serious” although there are early signs of recovery in some functions such as electrical power and instrumentation.
Nitrogen Injection Continues At Unit 1 PCV
(NucNet): The operator of the Fukushima nuclear power plant says it is continuing to inject nitrogen gas into the primary containment vessel (PCV) at unit 1 without any problems.
Injecting nitrogen is intended to displace oxygen inside the PCV, thereby reducing the risk of explosion caused by the combustible combination of hydrogen and oxygen.
Because the containment has already been damaged and pressure inside might drop below atmospheric pressure, there is a risk that outside air containing oxygen could leak into the PCV resulting in the build-up of an explosive hydrogen-oxygen gas mix.
Tepco says it plans to continue the injection for about six days and will also consider taking similar measures at units 2 and 3.
Meltdown no longer seen as likely
The LA Times reported April 8 that Japan's nuclear crisis ebbing. The newspaper indicated that although the situation at the Fukushima nuclear plant is far from stabilized, evidence suggests that a complete meltdown is unlikely.
There is no evidence that overheating during the last month has resulted in any melting of the reactor vessels or their containment structures, Obama administration officials said Thursday.
The assessment, provided to The Times on background, suggests that the plant is unlikely to suffer a complete meltdown.
"We are a long way from a point where anybody would say this is stable," a senior administration official said. "But it is not a runaway. For a long time, we will be at a declining level of risk."
Separately, the La Times reported the staff of the NRC came under heavy questioning April 7 by the Advisory Committee on Reactor Safeguards, which is a panel of experts, academics and nuclear industry officials that provides guidance to the agency.
At a committee meeting, NRC officials were asked about the scientific basis for their agency's advice that Americans evacuate a 50-mile zone around the plant. According to the LA Times, NRC officials said they couldn't provide an explanation and would have to get back to the committee.
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