This story has been rumbling away on various dubious websites for a few days. Here it is from RT.com, which I consider reasonably reliable: Plumes of mysterious steam rise from crippled nuclear reactor at Fukushima. The link below will take you to a Japanese-language report from TEPCO, which Google can translate into passable English. Excerpt:
Fresh plumes of most probably radioactive steam have been detected rising from the reactor 3 building at the crippled Fukushima nuclear plant, said the facility’s operator company.
The steam has been detected by surveillance cameras and appeared to be coming from the fifth floor of the mostly-destroyed building housing crippled reactor 3, according to Tokyo Electric Power Co (TEPCO), the plant’s operator.
The steam was first spotted on December 19 for a short period of time, then again on December 24, 25, 27, according to a report TEPCO published on its website.
The company, responsible for the cleanup of the worst nuclear disaster since Chernobyl, has not explained the source of the steam or the reason it is rising from the reactor building. High levels of radiation have complicated entry into the building and further inspection of the situation.
Three of the plant’s reactors suffered a nuclear meltdown in March 2011 after the Great East Japan Earthquake and resulting tsunami hit the region. The plant is comprised of six separate water boiling reactors. At the time of the earthquake, reactor number 4 had been de-fueled and reactors 5 and 6 were in cold shutdown for planned maintenance, thereby managing to avoid meltdowns.
Unlike the other five reactors, reactor 3 ran on mixed core containing both uranium fuel and mixed uranium and plutonium oxide, or MOX nuclear fuel. The Reactor 3 fuel storage pond still houses an estimated 89 tons of the plutonium-based MOX nuclear fuel composed of 514 fuel rods.