Workers at the Fukushima Daiichi site are working to prevent further spread of contaminated water following a leak of 300 cubic meters of partially-treated water from a storage tank on site. The leak has been rated by the Japanese regulator as Level 3 on the International Nuclear Event Scale.

TEPCO discovered the leak during a routine inspection on 19 August. An employee found water leaking from a rainwater drain valve in a dike surrounding tanks of contaminated water. A puddle of approximately 1-2 cm was found inside the dike, with two significantly larger puddles (3 m x 3 m x 1 cm and 0.5 m x 6 m x 1 m) found outside, TEPCO said.

On 20 August, TEPCO identified the source of the leak as Tank No. 5 (H4-I-5), where the water level was found to be around 3 meters lower than in neighbouring tanks. TEPCO said that the fall in water level corresponded to around 300 cubic meters of water. Dose readings taken at the site of the leak were high, reaching 100 mSv/hour (gamma and beta) and 1.5 mSv/hour (gamma only), according to TEPCO data.

There was no trace of water having flowed into a drainage ditch, which means that it is unlikely it flowed into the sea, according to TEPCO.

Following the discovery of the leak, TEPCO immediately began taking countermeasures to prevent further spread of contaminated water. On 19 August it shut off all the drain valves in the dike, and placed absorption mats at locations where there were ‘remarkable’ amounts of leaked water. It also launched an investigation to determine the source of the leak, and said that it would collect soil samples to determine how far the water had spread.

TEPCO plans to pump the leaked water from the dike into an empty storage tank (installed on 20 August), with the operation expected to take around two days. The water will be first transferred to a drainage pump to a temporary tank, then to an underwater pump and finally to the newly-installed tank. It has also removed contaminated soil found outside the dike and installed sandbag dikes to prevent the leaked contaminated water flowing into drainage channels. Impermeable sheets have been installed at two locations to prevent spread of contaminated soil/water.

The leak, initially rated at Level 1 (an anomaly) on the International Nuclear Events Scale (INES) has been upgraded to Level 3 (serious incident) by the Japan Nuclear Regulatory Authority. NRA said 21 August, that the high levels of beta radioactivity in the water (80 million Bq per litre) and the amount of contaminated water correspeond to the INES level 3 criteria.

Photo: Water Leak at a Tank in the H4 Area in Fukushima Daiichi Nuclear Power Station (Source: TEPCO)