(UPDATE) TEPCO says the steam has stopped.
In their update of the steam seen coming from between the reactor well and the DSP (device storage pool/pit) of Reactor 3 at Fukushima I Nuclear Power Plant, TEPCO says it was probably the rainwater that evaporated on contact with the Containment Vessel.
From TEPCO's email notice for the press, 7/18/2013:
Considering the result of the measurement [dust sampling, which showed no elevated levels of cesium or iodine] and plant parameters, we presume that the steam was generated when rainwater seeped in through the gap of the reactor well cover, got heated by the Containment Vessel head.
So, there is the CV head (cap), and it is warm enough to turn water into vapor, according to TEPCO. The temperature at the bottom of the Pressure Vessel inside the Containment Vessel, as of July 18, 2013, is about 40 degrees Celsius. (Hmmm... How warm could the top of the CV be?)
(UPDATE: TEPCO added the information of the temperatures at the top of Pressure Vessel inside the CV. See below.)
The company released the photos of the location on the operating floor of Reactor 3 (in Japanese only so far, so I put English labels). It's hard to see the steam in the still photos, but it is evident in the video (to be uploaded here later):