The operator of Japan's disabled Fukushima nuclear plant announced Thursday it had detected "steam-like gas" in one of the reactor buildings but said no significant change in radiation levels had been observed.

Tokyo Electric Power Company said in a statement that one of its workers noticed on video camera early Thursday that the steam was "wafting through the air near the central part of the fifth floor (equipment storage pool side) of Unit 3."

That reactor was seriously damaged by a hydrogen explosion and partial meltdown in the wake of the devastating magnitude-9.0 earthquake and subsequent tsunami that hit Japan on March 11, 2011, and crippled the plant.

Despite the steam, TEPCO technicians confirmed that the processes of injecting water into the reactor and of cooling its spent fuel pool were proceeding stably.

In addition, the temperature of reactor No. 3's containment vessel remained at 38 C (100 F), unchanged from Wednesday, the operator said.

"We will continue to monitor the status closely," TEPCO said, adding that the steam may be due to water from heavy rains on Wednesday having fallen into an additional containment pool installed near the area where the steam was observed.

The high levels of radiation in reactor No. 3 since the nuclear disaster, the worst since Chernobyl in 1986, prevents technicians from getting closer to verify the status of the reactor and forces them to carry out their work using remote-controlled machinery.

Nearly 3,500 employees currently are working to completely dismantle the Fukushima nuclear plant, a process that could take at least 40 years.

With the reactors having already been brought to cold shutdown, the priority now is to begin withdrawing spent fuel from reactor No. 4's storage pool late this year, a step prior to removing its fuel rods.

Meanwhile, the main problem TEPCO faces is the enormous amount of contaminated waste water that has collected in the basements of the reactors.

This is due to water being pumped into the reactors to cool them, although the problem has been compounded by a massive inflow of groundwater. EFE