* Build up of water in storage tanks raises capacity concerns
* Fisheries union: no choice but to agree to groundwater release
* Any contaminated leak could end Fukushima fishing industry
* Fishermen request third party checks (Adds comment from fisheries federation)
By Mari Saito
TOKYO, March 25 (Reuters) - Fishermen working near Japan's destroyed Fukushima nuclear plant agreed on Tuesday to allow the release of uncontaminated groundwater around the facility into the ocean, a fisheries union official said, a rare victory for the operator.
Tokyo Electric Power Co (Tepco), the operator of the Fukushima station that suffered triple nuclear meltdowns after the March 2011 earthquake and tsunami, is trying to contain radioactive water at the site. It has lobbied local fishermen to allow a "groundwater bypass" for nearly two years.
"The final consideration was based on the fact that we cannot allow them to release contaminated water. We realised that if the situation continued as it was, the whole system will fall down," said Kenji Nakada, an official at the Fukushima fisheries federation.
"In such a case, the fisheries industry in Fukushima would be completely finished."
Tepco has built a thousand tanks at the Fukushima plant that hold more than 431,000 tonnes of radioactive water. Nearly 90 percent of available capacity in the tanks are already filled with radioactive water.
Contaminated water accumulates at a rate of 400 tonnes a day at Fukushima as groundwater flows downhill into the destroyed basements of the reactor buildings and mixes with highly radioactive water used to cool melted fuel. Radioactive water poses a long-term risk to the shutdown of the Fukushima Daiichi station, a task expected to span more than three decades.
Tepco's bypass will release 100 tonnes of groundwater a day that flows downhill towards the devastated plant and funnel it to the sea before it reaches the reactor buildings.
Both Japan's Nuclear Regulation Authority and the International Atomic Energy Agency have said controlled release of low-level water should be considered to make storage space at the facility for irradiated water.
Local fisheries unions had been bitterly opposed to Tepco's proposed bypass after irradiated water leaked from tanks that were just uphill of the proposed groundwater drains last year. The leaks sparked international alarm and led to a boycott of Fukushima fish by South Korea.
Last month Tepco found another leak of highly contaminated water from one of its hastily built tanks at the plant.
A Tepco spokesman said on Tuesday recent tests of groundwater at Fukushima showed little impact from past tank leaks.
As part of its approval of the bypass, local media reported that fishermen requested a third party organisation to check radiation levels of groundwater before it is released and any released water to have less than 1 becquerels per litre of Cesium-134, a radioactive element that has a half life of around two years.
The legal limit of releasing Cesium-134 into the ocean is 60 becquerels per litre.
A fishing ban along the coast of Fukushima after the nuclear accident pushed most fishermen out of a job except for occasional work catching certain types of fish deemed safe. (Reporting by Mari Saito; Editing by Jeremy Laurence)