The incident on Wednesday highlights the dangers Japan still faces in decommissioning the plant that was knocked out by an immense tsunami in 2011 in the world's worst atomic disaster since Chernobyl in 1986.

Five workers were cleaning pipes at the ALPS system filtering wastewater for release into the sea when two were splashed when a hose came off accidentally, a spokesman for operator TEPCO told AFP.

Two others were contaminated when they were cleaning up the spill, the spokesman said.

The radiation levels of the two hospitalised men's bodies were at or above 4 becquerels per square centimetre, the threshold considered safe.

The possibility that the two men sustained burns due to radiation exposure is low, according to a doctor, TEPCO said.

"We've been told the condition of the two workers being hospitalised is stable," the spokesman said.

"Both workers will stay in hospital for about two weeks for follow-up examinations," he said.

TEPCO is analysing how the incident happened and reviewing measures to prevent a repeat, he said.

The incident came a few days after TEPCO completed releasing the second batch of wastewater from the plant, and as United Nations inspectors visited the facility for a safety review.

Tokyo insists that the water being released is harmless and is heavily diluted with seawater, a view backed by the International Atomic Energy Agency, but China and Russia have criticised the release and banned Japanese seafood imports.

The release of the 540 Olympic swimming pools worth of water is meant to clear space for the much more hazardous task of removing radioactive fuel and rubble from three stricken reactors.