The operator of Japan's battered Fukushima nuclear power plant said Tuesday it was having trouble with the early stages of an ice wall being built under broken reactors to contain radioactive water.

Tokyo Electric Power (TEPCO) has begun digging the trenches for a huge network of pipes pipes under the plant through which it intends to pass refrigerant.

This will freeze the soil and form a physical barrier that is intended to prevent clean groundwater flowing down mountainsides from mixing with contaminated water underneath the leaking reactors.

TEPCO said Tuesday a smaller, inner ice wall whose pipes it sank earlier to contain the already-contaminated water was proving difficult.

"We have yet to form the ice stopper because we can't make the temperature low enough to freeze water," a TEPCO spokesman said.

"We are behind schedule but have already taken additional measures, including putting in more pipes, so that we can remove contaminated water from the trench starting next month."

The coolant being used in the operation is an aqueous solution of calcium chloride, which is cooled to -30 degrees Celsius (-22 Fahrenheit).

The idea of freezing a section of the ground, which was proposed for Fukushima last year, has previously been used in the construction of tunnels near watercourses.

However, scientists point out that it has not been done on this scale before nor for the proposed length of time.

Coping with the huge -- and growing -- amount of water at the tsunami-damaged plant is proving to be one of the biggest challenges for TEPCO, as it tries to clean up the mess after the worst nuclear disaster in a generation, in which three reactors went into meltdown.

As well as all the water used to keep broken reactors cool, the utility must also deal with water that makes its way along subterranean watercourses from mountainsides to the sea.

Full decommissioning of the plant at Fukushima is expected to take several decades. An area around the plant remains out of bounds, and experts warn that some settlements may have to be abandoned because of high levels of radiation.