TOKYO, Jan 23 (Reuters) - The operator of Japan's crippled Fukushima nuclear power plant has found that a coolant solution, used to create an ice wall halting the seepage of groundwater into reactor buildings, has leaked from two storage tanks.

The leakage has had no impact on the wall or environment, said Tokyo Electric Power Co Holdings Inc (Tepco).

Still, it underscores the unpredictable challenges in the clean-up of the site, nearly 11 year after an earthquake and tsunami ravaged Japan's northeastern coast, causing the world's worst nuclear disaster since Chernobyl, Ukraine, in 1986.

Only last year, Japan's government approved https://www.reuters.com/world/asia-pacific/japan-says-release-contaminated-fukushima-water-into-sea-2021-04-12 the release of over 1 million tonnes of irradiated water from the site after treatment, starting around spring 2023. Tepco last month said https://www.reuters.com/markets/commodities/japans-tepco-build-underwater-tunnel-fukushima-water-release-2021-12-21 it would build a tunnel reaching into the sea for the operation.

On Sunday, Tepco spokesperson Tsuyoshi Shiraishi said about four tonnes of a calcium chloride solution used to maintain the ice wall had leaked in what was the eighth such leakage.

"We're now confirming the reason," he said.

The last leak in December 2019 saw 16 tonnes spilled, likely due to metal fatigue resulting from vibrations caused by construction vehicles, Shiraishi said.

There was no immediate impact on the wall's function as it takes several months for the wall to thaw in the absence of coolant, he said.

Separately, a group of six men and women is set to file on Jan. 27 a lawsuit against Tepco claiming they developed thyroid cancer due to exposure to radiation from the Fukushima disaster, the Mainichi newspaper reported.

The plaintiffs, who were minors living in Fukushima prefecture at the time of the 2011 disaster, are seeking 616 million yen ($5.42 million) in compensation from the electricity provider, the Mainichi said.

If the complaint was served, Tepco would respond in good faith after hearing the contents of the claims and arguments in detail, the firm said in a statement.

($1 = 113.6800 yen) (Reporting by Daniel Leussink; Editing by Christopher Cushing)