The operator of the wrecked Fukushima nuclear power plant says it is struggling with its latest efforts to stop contaminated underground water leaks from running into the sea.

Tokyo Electric Power Co. said Tuesday that some of the water was seeping over or around “chemical walls” it had created by injecting chemicals into the soil that solidify into a wall.

The latest problem involves underground water which has built up over the last month, since the company started creating the chemical walls in an embankment to stop leaks after it detected radiation spikes in water samples.

TEPCO has said it was slow to tackle the underground water problem as it devoted itself to cooling its melted reactors, which pose greater risks.