One hundred tonnes of contaminated concentrated salt water has leaked out of an aboveground tank at Fukushima Daiichi into the H6 tank farm area on the plateau to the west of the damaged reactors, outside of protective dikes installed around the tank.

The leak was stopped on 20 February. Workers have started to collect the water, and contaminated soil. TEPCO said that no contaminated water had reached the sea because there are no drains in the vicinity. The water was highly contaminated with beta-radiation: 2.3×10^8 Bq/L, compared with Cs-137 contamination of 9.3×10^3 Bq/L.

Essentially the problem was that the tank overflowed; contaminated water escaped out of the flange at the top of the tank, flowed down the rainwater gutter on the outside of the tank to the outside of the protective dike.

The leak was most likely not caused by tank structural failure, but more likely human error and/or valve failure.

TEPCO reported that a high water level alarm (corresponding to 98.9% of capacity) for tank H6N-C1 went off about nine hours before the leak was detected, but was assumed to be measurement error because there was no water transfer, inspection or gauge maintenance work.

Three valves link the tank to the supply of contaminated salt water. According to the TEPCO report, a visual inspection revealed that although the principal valve to the tank farm was closed, two downstream tank inlet valves were open. Once they were closed, the volume of overflowing water decreased. TEPCO said that the first valve "may have let through contaminated water for some reason, whose cause is currently under investigation."

However, according to a report on antinuclear website Enformable, TEPCO admitted yesterday that all of the valves might have been mistakenly opened whilst attaching valve tags. It said that as a result TEPCO is now interviewing workers, considering clarifying procedures and improving the design of the rainwater collection system.

There were several high-profile leaks on the tank farm in 2013. Although the leaking water is highly contaminated, the risk of it reaching the sea is limited because of the distance and physical barriers.

Photo: Water Overflow from the flange of the top panel on the upper part of H6 tank area (Credit: TEPCO)