TOKYO  | Wed Apr 8, 2015 11:34am BST

Japan is considering evaporating or storing underground tritium-laced water from the wrecked Fukushima nuclear plant as an alternative to releasing it into the ocean, Tokyo Electric Power Co's chief decommissioning officer told Reuters on Wednesday.

The removal of hundreds of thousands of tonnes of water containing tritium, a relatively harmless radioactive isotope left behind in treated water is one of many issues facing Tokyo Electric as it tries to cleanup the wrecked plant.

Tokyo Electric wants to release the tritium laced water to the ocean, a common practice at normally operating nuclear plants around the world, but is struggling to get approval from local fisherman, who are concerned about the impact on consumer confidence and have little faith in the company.

With the release to the ocean stalled, the government task force overseeing the cleanup is looking at letting the water evaporate or storing it underground, chief decommissioning officer Naohiro Masuda, told Reuters at the close of a seminar on decommissioning.

Masuda said he didn't know when the discussions would be completed and a decision made.

Time and space is running out for Tepco, which has been forced to build hundreds of tanks to hold contaminated and treated water.

The evaporation method was used after the Three Mile Island disaster but the amounts were much smaller, Dale Klein, an outside adviser to Tepco told Reuters last week.

"They have huge volumes of water so they cannot evaporate it like they did at Three Mile Island," Klein said. "If they did it would likely be evaporated, go out over the ocean, condense and fall back as rainwater. There's no safety enhancement."

Tepco has been fighting a daily battle against contaminated water since Fukushima was wrecked by an earthquake and tsunami in March 2011 and three reactors underwent meltdowns.

Water flushed over the wrecked reactors to keep them cool enough to prevent further radioactive releases is treated but current technology can't remove tritium.

"They really do need to make a decision," Klein said. "Storing it in all those tanks, you are just asking for failure."

Missteps and leaks have dogged the efforts to contain the water, slowing down the decades-long decommissioning process and causing public alarm.

"I think they will need to make that decision," U.S. Nuclear Regulatory Commission Chairman Stephen Burns, said when asked should Japan release the tritium laced water at a media briefing at the U.S. Embassy on Wednesday.