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Figuring out the flow of groundwater essential

August 21, 2013


August 20, 2013


Tepco yet to track groundwater paths
Liquefaction threat adds to Fukushima ills

Staff Writer

Tokyo Electric Power Co. has radioactive water problems at its crippled Fukushima No. 1 nuclear plant that seem uncontainable, as seen in the latest case to beset the utility: tanks used to store highly tainted water that was used to cool its melted reactors but now must be safely stored are leaking.

The plant, part of which was built on filled-in land, also faces the risk of liquefaction if another big temblor hits.

The tank leaks come as Tepco struggles to halt the flow, some 300 tons a day, of highly radioactive groundwater into the Pacific, where it is believed wreaking environmental havoc.

And the groundwater problem will probably dog Tepco over the next four decades or so as it tries to scrap and entomb the spent fuel from reactors 1 to 4 and the melted fuel inside units 1 to 3.

A key to minimizing the exposure of groundwater to the highly radioactive coolant water leaking into the basements of the buildings housing the crippled reactors is to learn the courses the groundwater takes from the landward mountains to the sea under the plant, experts say.

“Figuring out the flow of the groundwater is vital, as it will help spot where the sources of contamination are and stop it from spreading further in the future,” said Atsunao Marui, a groundwater expert and principal senior researcher at the National Institute of Advanced Industrial Science and Technology, a Tokyo- and Ibaraki-based semi-public research body.

At the moment, Tepco has a rough idea of where the groundwater is flowing.

About 1,000 tons of groundwater flows from the mountains under the complex daily, and about 400 tons of it penetrates the basement walls of the buildings housing reactors 1 to 4 of the six-reactor plant, thus mixing with the highly radioactive coolant water leaking from the containment vessels.

The remaining 600 or so tons apparently flows to the sea and Tepco suspects about half of it gets contaminated somewhere else under the plant.

But the exact paths the groundwater takes have yet to be pinpointed.

Tepco compiled a groundwater flow simulation for an Aug. 12 meeting with experts from the Nuclear Regulation Authority, but the utility said the simulation was inaccurate.

According to Marui, the simulation was not difficult to draw up, but Tepco needs to collect more data from a wider area, even outside the plant, where it doesn’t have monitoring wells to check groundwater flows.

“If the government is really planning (to take a step forward to reducing the tainted groundwater), it needs to (support Tepco in) widening the monitoring points to outside the plant,” including from the source of the groundwater and the mountainside, said Kazunari Yoshimura, an expert in water-related matters, who runs Global Water Japan, a water-consulting company.

To reduce the amount of groundwater flowing under the reactor buildings, Tepco dug a number of wells on the nearby mountains and plans to pump the water up before it has a chance to mix with radioactive water.

Aware of Tepco’s struggles, Prime Minister Shinzo Abe said earlier this month that taxpayer money will be pumped into a project to build barriers around the reactor buildings to prevent tainted groundwater from flowing to the sea.

Considering Tepco will have to deal with radioactive groundwater for about 40 years until the reactors are safely neutralized, getting accurate data about the groundwater, including the amount, depth and detailed flow paths, is essential if it is to take effective measures, said Yoshimura, who has advised Tepco on the problem.

Tepco must be aware that more monitoring wells need to be built but it is probably hesitant to do so outside the complex, in part because it lacks manpower, Yoshimura said.

The large volume of groundwater flowing under the plant is creating another problem — the possibility that the land it stands on will liquefy if another major earthquake hits.

The east side of the reactor buildings, in an area close to the sea where land was filled in, appears more vulnerable to liquefaction. Marui said the reclaimed land consists of clay and crushed rocks, through which water can easily pass.

Tepco recently injected liquid glass into the filled land, thereby forming an underground barrier to help prevent groundwater from reaching the sea.

Due to technical reasons, the barrier had to be built 1.8 meters below ground, meaning tainted groundwater can flow to the sea above it. Tepco officials believe that is happening now.

And because the wall is blocking a certain amount of groundwater, the level of groundwater has risen in the fill area, raising the risk of liquefaction if and when another earthquake hits, Tepco said. However, the plant’s turbine buildings are likely to withstand any earthquake because they are built on the bedrock, it said.

Before the nuclear crisis started on March 11, 2011, Tepco pumped up about 850 tons of groundwater per day from sub-drains to prevent it from flowing under the complex.

It recently started pumping groundwater from under the fill area to reduce the accumulation.

“(Tepco) is seeing a danger that the area near the sea might become like mud, so it is pumping up the groundwater,” said Marui.

Yoshimura of GWJ said if a another big quake hits the Fukushima plant, there is a risk that the highly radioactive water that is presently flooding the basements of the reactor buildings could flow out and further contaminate the groundwater.

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