The giant ice wall employs the same technology as the trench project and involves the same contractor, Kajima Corp.
A Tokyo Electric (Tepco) spokesman said fluctuating water levels in the tunnel under the No. 2 reactor building were preventing the water from freezing over.
In the three years since a massive earthquake and tsunami set off nuclear meltdowns at Fukushima, Tepco has been fighting a constant battle to pump out, treat and store hundreds of thousands of tonnes of contaminated water.
Tepco flushes huge amounts of water over the reactors to keep them cool, but that water mixes with groundwater that seeps into basements, requiring more pumping, treatment and storage.