Tokyo Electric Power Co. on Tuesday started transferring radioactive water from one of the three leaky tanks at the Fukushima No. 1 nuclear plant to a more reliable container.
Tepco plans by the end of June to empty four sunken tanks holding a total of about 23,000 tons of heavily contaminated water.
The water, which has been seeping into the soil, will be transferred both to existing containers and tanks that will be newly installed.
Tepco continues to face the risk of running out of storage capacity because the amount of radioactive water continues to increase. Water is needed to cool the facility's stricken reactors.
The cistern that Tepco started pumping water from on Tuesday appeared to be in the worst shape of the three.
The utility also plans to remove water from other sunken tanks, including those that have not been found to have problems.
Tepco will continue to use one tank beyond June because it stores about 3,000 tons of much less contaminated water, but it intends to eventually stop using all of the sunken cisterns.