On 5 April workers employed by operator Tepco discovered a leak in one of seven Fukushima Daiichi underground reservoirs that store contaminated water. Quality tests of water between the outer bentonite layer and inner shielding sheet detected radioactive contamination concentrations in the millions of Bq/cm3. It is assumed that the water has leaked into the soil, not the ocean, since there is no drainage ditch near the reservoir.
On 6 April the amount of water lost from reservoir 2 was estimated to be 120 cubic metres. The water had a gamma concentration of 1.5 Bq/cm3 and beta concentration of 5900 Bq/cm3. Samples taken of the water in reservoir 2 showed beta radioactive concentration ranging from 3.3×10^-2 Bq/cm3 to 6.9×10^3 Bq/cm3.
Although no decline in water levels in two nearby underground reservoirs, 1 and 3, has been detected, Tepco suspects that what it calls a ‘small’ amount of water has leaked between their internal shielding sheets and the outer bentonite layer. Reservoirs 1, 2 and 3 contain seawater concentrated in the current circulating water treatment process, and have capacities of 13 000, 14 000 and 11 000 cubic metres. As of 7 April, before much pumping began, reservoir 1 was one-third full, 2 was two-thirds full and 3 was almost completely full. Of the four other underground reservoirs, two (5 and 7) are empty, 6 contained concentrated seawater amounting to less than 10% of its 10 000 cubic metre capacity, and 4 contains low radioactivity floodwater from units 5 and 6.
Initially, Tepco workers planned to pump the greater part of the water into reservoir 1, and the balance into reservoir 6. That plan was altered once leaks were suspected at reservoir 1. As of 10 April, reservoir 2 water had declined to about 20% capacity after pumping out to reservoir 1 and 6. The level of reservoir 1 increased to about 55% capacity until 8 April, and since then has remained constant. Water in reservoir 6 has increased to about 50% capacity. A plan for transferring a relatively small amount of water from reservoir 3 (containing 10 400 cubic metres, about 95% full) to reservoir 6 stalled when a leak was discovered in a transfer hose on 11 April.
The short-term plan is to maintain water levels in underground reservoirs at about 80%, which is the height of the reservoir leakage detection hole in the upper part of the reservoir. This is assumed to be the leakage location. Also, plans to install a further 120 000 cubic metres of water tank storage during the April – September 2013 period will be expedited. A new comprehensive treatment system to remove many radionuclides from the water (the ALPS system) is currently under hot testing.
New data sampling procedures to collect samples twice a day at drain holes and leakage detection holes have begun; on 8 April Tepco said that ‘the water level, etc., of the concerned reservoir will continue to be intensively monitored.’ Results of beta radioactivity measurements from samples taken on April 10 showed no significant changes from April 9.
The soil, gravel and the sheet around the leakage detection hole of reservoir 2 has been removed for visual inspection. Temporary pumps were installed on 10 April to move leaked water back into the underground reservoir.
Tepco is also planning to bore sampling holes in the ground around the underground reservoirs and toward the sea to collect groundwater to asses the extent of the soil radiation plume.
Spare capacity remains in underground reservoirs 5 and 7 (about 14 000 cubic metres), and in above-ground tanks (22 000 cubic metres) as of 9 April.