Editor's Note: This is part of IEEEÂ Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. John Boyd is an IEEEÂ Spectrum contributor reporting from Kawasaki, Japan.
The radioactive materials from Japan's damaged Fukushima Dai-1 nuclear power plant continue to spread.
Late Monday afternoon local time, Tokyo Electric Power Co. (TEPCO) suddenly announced it was preparing to release more than 11 500 tons of stored "low-level radioactive waste water" into the Pacific Ocean. The water had been used to cool the reactors, and was contaminated with radiation due to still-mysterious leaks in the reactors or the emergency cooling systems. TEPCO's decision to dump water into the ocean came in response to the urgent need to prevent flooding in buildings controlling the No. 5 and No. 6 reactors and to create space to store more highly radioactive water, which has pooled in the basements of the other reactors' turbine buildings.
This announcement came on top of Saturday's news that a worker at the plant had discovered contaminated water pouring directly into the sea (pictured above); efforts to stem the leak have been unsuccessful as of Monday evening, local time. Government officials have also acknowledged it will now "take months" to fully restore the crippled reactors' cooling systems, which raises questions about the need to extend the 20-kilometer evacuations zone.
In a press release announcing the water dump, TEPCO said there is still a "great amount of radioactive waste water in the turbine buildings … and especially the turbine building of Unit 2 has extremely high levels of radioactive waste water." In addition, "as low radioactive subsurface water is piling up in sub-drain pits of Unit 5 and 6 and … running into buildings, we are concerned that important equipment to secure the safety of the reactors will be submerged." The company said that 1500 tons of the water to be released would be "low-level radioactive subsurface water stored in the sub-drain pits of Unit 5 and 6."
The government has no alternative but to agree to the emergency discharge, although the water contains roughly 100 times the government limit of radioactive iodine-131. TEPCO estimated the impact of the discharge would approximate 0.6 millisieverts "of effective radioactive dose per year for adults … if they ate adjacent fish and seaweeds every day," an amount equaling "one-fourth of the annual radioactive dose to which the general public is exposed in nature."
In addition to the radioactive water that TEPCO will deliberately release into the ocean, the company also has to deal with a leak. Over the weekend, TEPCO discovered highly radioactive water spewing into the sea through a 20-centimeter crack or hole in a concrete pit near the sluice gate of reactor No. 2. The pit is near the intake pipe used to feed seawater to the reactor's cooling system, and it is connected by a pipe to a tunnel extending from the No. 2 turbine building. This tunnel is also connected to the turbine building's trench, which houses electric cables and pipes. Last week, the trench was found to be flooded with contaminated water (as were the trenches for the No. 1 and No. 3 turbine buildings). It's thought that overflow water from cooling the reactor and spent fuel storage pool has found its way into the turbine building basement, from there into the trench, and then on into the tunnel connected to the cracked pit.
NHK, Japan's national broadcaster, showed a photograph of the contaminated water gushing into the sea and estimated the rate at 2 liters per second. Last week, Japan's nuclear agency found seawater containing thousands of times the legal limit of radiation 330 meters south of the plant.
TEPCO is now battling to plug the crack near the No. 2 reactor's sluice gate. Over the weekend, the first attempt failed when concrete poured into the tunnel was washed away by the flowing water before it could set. Next, absorbent polymer was poured in. The polymer is said to soak up to 50 times its volume and stiffen in two to three minutes, but this too failed to stop the flow. The third attempt had TEPCO workers mixing sawdust and newspaper with the polymer to give it more bulk, but this has also been ineffective.
In a press briefing Monday morning, Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency (NISA), said workers poured in 13 kilograms of white dye into the trench at around 7 a.m. local time to try and pinpoint the source of leaking water, but no trace of the dye has been seen flowing through the crack as of 10:30. He also said TEPCO was preparing to set up "silt fences" in the sea immediately around where the contaminated water is leaking into the sea. The fences, normally used in civil engineering projects, would be suspended from floats and hung with weights. But with more details scarce it was not immediately clear how such barriers would prevent the radioactive water from spreading.
In his morning press conference, Chief Cabinet Secretary Yukio Edano reiterated the point he made Sunday that TEPCO "must stop the radioactive water from leaking into the sea as quickly as possible." He added that even though it was being diluted in the ocean, over time it would have an impact on marine resources. He said this before the emergency decision had been made to dump the stored low-level radioactive water into the ocean.
Echoing comments made by an official of the independent Nuclear Safety Commission on Sunday, Nishiyama told reporters that it would probably take months before the reactors' cooling systems could be fully restored, given the various challenges still facing TEPCO—including the need to pump water from various tanks to make storage space for the highly contaminated water pooled in turbine basements.
"Contaminated water needs to be moved from the turbine buildings first, before work can even begin on the electronic circuitry controlling the cooling system," says Akio Yamamoto, a professor in the Department of Materials, Physics and Energy Engineering of Nagoya University's Graduate School of Engineering. "And due to the considerable volume of contaminated water, draining it will take time."
He added that only then can TEPCO "repair the pumps in the residual heat removal system. And due to the high radiation in the environment, this too may take some time."
PHOTO: TEPCO
