U.S. Nuclear Regulatory Commission spokesman David McIntyre on steam coming from Fukushima Daiichi Unit 3, Jan. 9, 2014: “This also happened last year at this time, and periodically since the tsunami in 2011 […] We are in touch with the Japanese regulator and TEPCO, and from what we’ve seen and heard there is no reason to suspect that this steam is an indicator of anything bad happening.”
Scientific American, Jan. 9, 2014: The radioactive detritus at Fukushima is still throwing off roughly one million watts worth of heat, according to Fairewinds Energy, a nuclear safety advocacy group based in Burlington, Vt. That heat turns water into steam—and when the air is cold enough, as it is in winter in Japan, that steam is visible. […] Nor is this plume of steam—sometimes visible, sometimes not—only apparent in winter. When the atmospheric conditions are right, with relatively low temperature and high humidity, the steam is visible even in summer, as happened in July 2013. It is fortunate that physics suggests such steam is nothing to worry about, because it is impossible to check firsthand. Due to the meltdown in that reactor, radiation levels are too high for any human to enter without receiving an unacceptable dose.
Nuclear expert Arnie Gundersen of Fairewinds Energy Education was asked about the steam coming from Unit 3 during a Jan. 6, 2014 interview on Coast to Coast AM (at 1:22:45 in): Fission products continue to be hot for 5, 6, 7 years so the plant is going to continuously steam even then. I have another theory […] there’s a large blob of nuclear material underneath the nuclear reactor, and this is from the meltdown. […] When all the uranium gets close together — especially in Unit 3, the plutonium emits what’s called spontaneous neutrons — neutrons you need to perpetuate a chain reaction, so the spontaneous emission of these delayed but spontaneous neutrons can cause other fissions inside this blob of material there. It’s something called sub-critical multiplication. […] The net effect is scientists aren’t looking at all the heat sources. You’ve got this radioactive rubble and that’s the focus, but it’s possible that we can also be having some additional fissions that are occurring in that blob that are keeping the remnants of the core hotter than scientist would believe that is occurring.