The operator of the plant said it had stopped the operation because a part of the system that absorbs radioactive caesium needed to be replaced sooner than expected.
Engineers speculated that highly radioactive mud might have entered the treatment system or that waste water was more radioactive than previously measured.
The world’s worst nuclear disaster since Chernobyl in 1986 has cased radioactive material to leak into the air, ground and sea around the plant and forced the evacuation of 80,000 people in a 20 kilometre radius.
Water has been pumped into reactor cores and fuel rod pools, leaving more than 100,000 tonnes of contaminated water in basements, drains and ditches, some of which has leaked into the ocean. The water needs to be decontaminated before it can be stored or recycled back through the reactors to cool them,
Pumping out the highly radioactive water will allow the task of long-term repair work to the cooling system to begin.
Of Japan’s 54 commercial nuclear reactors, 17 are currently in operation. Two other units are in test operation but offline. The remaining 35 reactors, including six at the Fukushima plant, have remained shutdown due to quake damage or routine inspections.