Workers at a tsunami-crippled Japanese nuclear plant are scrambling to find the cause of a highly radioactive water leak from a brand-new storage tank amid concern that the problem is hampering cleanup.

The Fukushima Dai-ichi plant suffered multiple meltdowns after the March 2011 tsunami knocked out power and is still running on a fragile makeshift cooling system.

Operator Tokyo Electric Power Co. said Thursday the leak has stopped after water was moved to two adjacent containers but the cause remained unknown. The leak occurred at one of nearly 40 steel tanks TEPCO hastily assembled last month to receive radioactive water from several underground pools that had also leaked.

Radioactive water leakage from the wrecked reactors and groundwater runoff has been a headache at the plant.