Workers at the crippled Fukushima nuclear plant in Japan have reported steam inside a battered reactor building for the second time in less than a week, the operator said Tuesday.

Steam was seen around the fifth floor of the building housing the No. 3 reactor shortly after 9 a.m., Tokyo Electric Power Co (TEPCO) said, adding workers were continuing with the operation to inject cooling water into the reactor and a pool storing nuclear fuel.

TEPCO said monitoring equipment showed no significant changes, including in the levels of toxic substances the broken reactor is releasing.

Steam was spotted in the same area on Thursday last week but had disappeared by the next day, with TEPCO saying it did not know for sure what had caused it.

It said it was looking at the possibility that accumulated rainwater had been the source.

The reactor, devastated by a massive tsunami in March 2011, is too dangerous to approach, and workers had seen the steam on a camera feed, the utility said.

The roof of the building was blown off in a hydrogen explosion days after meltdowns that were sparked when cooling systems were flooded as the tsunami swept ashore.

© (C) 2013 AFP