According to Tokyo Electric Power Company (TEPCO), the leak of untreated water from a piece of filtering equipment at the crippled Fukushima Daiichi power plant occurred last week because personnel failed to notice that some valves were left open by mistake.

The company, TEPCO, revealed the information at a news conference on Thursday.

The discovery is based on the company's ongoing investigation into the source of the leak, which occurred on February 7. TEPCO has started questioning workers who were there when the incident occurred.

The company told NHK that workers had failed to notice that of the 16 valves on the device's pipes that needed to be closed, 10 were left open.

The company now claims that 1.5 tonnes of water containing approximately 6.6 billion becquerels of radioactive material spilled. It had earlier stated that there had been a leak of roughly 5.5 tonnes of water containing 22 billion becquerels of cesium-137 and other radioactive materials.

The utility admitted that the operation manual lacked clear instructions for workers regarding the closure of the valves. It promised to take steps to prevent a recurrence.