Conditions at the striken Fukushima nuclear power plant in Japan worsened this week as engineers battled to contain a new and highly radioactive water leak.
Engineers discovered contaminated water, used to cool down the reactors, outside the containment buildings and in the turbine halls last weekend.
Japanese prime minister Naoto Kan said the incident had put the government on a “state of maximum alert” over the crisis at the plant.
Since the plant suffered tsunami damage on 11 March engineers’ had focused on cooling the reactors to prevent a meltdown after back-up electricity failed (NCE 17 March).
However, they now have to balance continually pumping in water to cool down the reactor with attempts to prevent radioactive water leakage.
Plant owner Tokyo Electric Power Company (Tepco) detected water in a vertical shaft outside the turbine buildings for reactors 1,2 and 3 on Sunday.
Radioactive puddles ‘inside buildings’
On Monday, officials then discovered radioactive material in puddles inside the turbine buildings of Reactors 1,2,3 and 4 (see box).
The radiation dose on the surface of the water amounted to 0.4 millisieverts (mSv) per hour in Reactor 1 and over 1,000 mSv per hour in Reactor 2. Tepco has been unable to confirm radiation levels in Reactor 3.
Experts believe that the high radioactive values mean that the water must have leaked from the primary containment buildings for each reactor, or from their associated pipework.
Tepco is now looking at the water paths within the turbine building to identify the leak, thought to have been caused either by the earthquake or by subsequent high pressures within the piping as result of the emergency cooling operation.
“This is a 1970s-era power plant that could still use cast iron piping, which is more brittle than more modern materials,” said one nuclear expert.
Test of ingenuity for engineers
Independent nuclear expert Tony Roulstone added that attempts will now be made to try to immobilise the contaminated water, which will “test the ingenuity of engineers on site.”
Roulstone said that ideally engineers would want to build a concrete store, in which the contaminated water could be pumped. This would help contain the radiation and buy time so that the water could be treated later.
But, the International Atomic Energy Agency said that the presence of contaminated water could “hamper recovery efforts”.
Any clean-up operation will not be easy. It took engineers at Dounreay four years to create an underground grout curtain capable of isolating a contaminated waste shaft that was leaking into the sea (NCE 25 May 2006).
But the Dounreay work was completed in a controlled manner, while work at Fukushima is being carried out under emergency conditions. Three nuclear plant workers attempting to cool the reactors are now under hospital observation following exposure to radiation doses of 2,000 MSV to 3,000 MSV.
Japan faces serious health risks
By Jo Stimpson
Radiation discovered in water and soil close to the Fukushima nuclear power plant could pose serious health risks depending on the geography and weather of the region, one radiation expert told NCE.
The biggest risks are from radioactive material entering the food chain through water, or becoming airborne and being inhaled, said former Society for Radiological Protection president Rick Hallard.
On Monday, Tokyo Electric Power Company (Tepco) reported that water pooling in a trench outside Reactor 2 was emitting radiation at over 1,000mSv/h. “That’s a high level,” said Hallard. “It’s a very serious event.”
A dose of 1,000mSv of radiation would produce nausea and vomiting. A 3,000mSv to 4,000mSv dose is potentially fatal.
Hallard said the dose received from normal background radiation would be 2mSv per year, and the European annual limit for safe “occupational exposure” at work is just 20mSv per year. Tepco said the radiation levels at Reactor 1 were just 0.4mSv/h, but levels at Unit 3 are unknown and could not be measured.
Hallard said if the radioactive material entered the sea, there was an immediate risk that it would be absorbed by fish and plants which then enter the food chain. The amount of wave activity will determine how radioactivity will dissipate. “Once the activity gets into the sea it can dissipate quickly,” he said. “Then you’ve got the same amount of radioactivity [overall] but dissipated into a large volume.”
Radioactive Iodine from the plant has a half life of just eight days so radioactivity is expected to decay before affecting anyone outside the immediate area. But roughly half of the material being leaked now is understood to include radioactive isotopes like Caesium with half lives ranging from one year to 30, meaning the material remains dangerous for much longer.
Tepco also found three types of radioactive Plutonium with half lives ranging from 87.7 years to 24,200 years in soil near the plant. But Tepco said the density of the material found was “equivalent to the density in the soil under normal environmental conditions and therefore poses no major impact on human health”.
Hallard said Plutonium is hazardous in much smaller quantities than radioactive Iodine and Caesium and could be inhaled or taken in through food. “It’s only really a hazard when it gets inside the body, and inhalation is the worst way.”
But he said if the Plutonium has come from mixed oxide (MOX) nuclear fuel − which is believed to be used in Reactor 3 - it will have become a particulate ceramic which moves more slowly through water or air. It would take an extreme fire or explosion to make the particles airborne”.