The operator of Japan's crippled nuclear plant said Saturday that deteriorated seams and a possible contortion of a reassembled storage tank might have caused a massive contaminated water leak that has triggered fears over the plant's radioactive water management. Tokyo Electric Power Co. said that after the foundation of the tank, which was storing radioactive water, partially collapsed two years ago, it was moved and reassembled. A 300-ton water leak from the tank was discovered Monday. The massive leak was the fifth and worst from a Fukushima Dai-ichi tank since the plant suffered triple meltdowns after the massive earthquake and tsunami in 2011. All five of the plant's tanks are collapsible and are seamed with rubber seals. TEPCO spokesman Noriyuki Imaizumi said the tank passed a water-tightness test and other safety requirements after being reassembled. The leak might have started when rubber seals degenerated, failing to cushion the tank's possible contortion, he said, adding that the company was further investigating the cause. Nuclear regulatory officials have raised concerns over a design flaw of the rubber seam tanks and urged a switch to more durable welded-seam tanks. TEPCO said that it believes the leaked water seeped mostly underground, but that some might have escaped into the Pacific. About one-third of the plant's 1,000 tanks storing contaminated water use the same design. The water had been used to cool the wrecked reactors. The latest leak was another example of how TEPCO has repeatedly failed to acknowledge problems it could have foreseen and acted on to mitigate before they got out of control. The tank problem also compounds TEPCO's ongoing battle with other radioactive water leaks from elsewhere in the plant that have already escaped into the ocean.