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 concerns that the problem is hampering cleanup efforts.

The Fukushima Dai-ichi plant suffered multiple meltdowns after the March 2011 tsunami knocked out power and is still on a fragile makeshift cooling system that produces massive amounts of highly radioactive water coming out of the wrecked reactors.

Plant operator Tokyo Electric Power Co. said Thursday that the leak stopped after some of the water inside the faulty tank was moved to two adjacent containers, but that its cause was still 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 turned out to be defective and had also leaked.

A worker spotted water dripping down from a seam on the 500-ton cylindrical tank built last month, but the amount of total leakage was about 1 liter (a quarter of a gallon), according to TEPCO spokesman Masayuki Ono.

TEPCO has been hit by a series of mishaps over the last few weeks, including a rat-induced blackout, adding to concerns about the plant’s ability to safely complete the decades-long process of decommissioning.

The massive radioactive water is among the most pressing issues affecting the plant’s cleanup process.

Runoff from the three reactors melted in the aftermath of the March 2011 disasters and a steady inflow of groundwater seeping into the basement of their damaged buildings produce about 400 tons of contaminated water daily. TEPCO says that about 300,000 tons of contaminated water has been stored in tanks at the plant, and that the amount will double within a few years.