TEPCO announced that while workers were conducting debris removal operations with remotely operated cranes positioned atop the crippled Unit 3 reactor building at Fukushima Daiichi steam began emanating from near the primary containment structure and equipment storage pool on the 5th floor. The primary containment vessel surrounds the reactor vessel and is made of steel four to eight inches thick.
The steam was first spotted by a subcontractor who was filming the building while preparing to remove rubble from the site. The steam was accumulating and hovering just above the primary containment vessel when it was discovered. Workers immediately halted their work and were evacuated from the area while emergency stockpiles of boric acid were readied in the event that the situation may worsen. The steam, which started around 0800 on July 18th, was visible all day and into the night.
TEPCO has not announced the source of the emission and has reported no rise in temperature and radiation levels, but were unable to stop the stream of steam from pushing itself out of the reactor building. The utility will continue to monitor the temperature and radiation levels in the reactor while also measuring dust near the building to gauge radiation levels.
Some, including TEPCO, have hypothesized that rainwater leaking into the disabled containment vessel or temporary equipment storage pool may have generated the vapor due to the high temperatures, but that hypothesis does not explain why the steam was visible all day long and into the night. While the spent fuel pool is also located on the 5th floor, it has been covered during debris removal operations to prevent debris from being knocked into the pool. It is more likely that the steam is an indication that the melted nuclear fuel in the reactor building is relocating, heating up, losing coolant water, or all of the above. If TEPCO really wanted to better know the conditions atop the primary containment vessel they could use an IR camera to demonstrate the hotter and cooler areas in the reactor building from above.
Source: New York Times