and as it was revealed at the fall conference of the Atomic Energy Society of Japan on September 20, the water had to be boiling for that to happen.
The article by Mainichi Shinbun on September 13 (linked below) summarizes the findings well enough. According to the article, the government researchers at Japan Atomic Energy Research and Development Agency and at Tokyo University announced the result of their analysis that the explosion of Reactor 4 reactor building at Fukushima I Nuclear Power Plant on March 15 may have been caused by a large amount of hydrogen gas produced by water radiolysis - dissociation of molecules by nuclear radiation in the Reactor 4 Spent Fuel Pool.
Here's from Mainichi Shinbun Japanese (9/13/2011):
An analysis by a team of researchers from Tokyo University and Japan Atomic Energy Agency shows that part of the reason for the explosion of Reactor 4 reactor building at Fukushima I Nuclear Power Plant was the large amount of hydrogen gas generated in the Spent Fuel Pool of Reactor 4. Radiation splits water and creates hydrogen. The analysis will be presented in the Atomic Energy Society of Japan's fall conference that will start in Kitakyushu City from September 19.
The Reactor 4 Spent Fuel Pool contained 1,535 fuel rods, the most in the reactors at Fukushima that broke down. At the time of the March 11 earthquake/tsunami, the reactor had been stopped for the regular maintenance. But the tunami knocked out the power supply, which caused the loss of cooling, and the reactor building exploded on March 15, 4 days after the quake.
In Reactors 1 and 3 which blew up in hydrogen explosions, it is considered that the fuel rods inside the reactors were damaged, which generated hydrogen. However, the fuel rods from Reactor 4 did not have noticeable damages. TEPCO suspects that hydrogen gas from Reactor 3, which shares the exhaust stack with the Reactor 4, flew through to Reactor 4, causing the explosion in Reactor 4.
However, the team of researchers from Tokyo University and JAEA looked at the 20-hour gap between the explosions of Reactor 3 and 4, and suspected there are some other causes for the explosion [of Reactor 4]. The team irradiated the water in a flask at room temperature, at 97 degrees Celsius, and at boiling temperature and measured the density of hydrogen generated. At 97 degrees Celsius, the density was 1.5 times as much as at room temperature, and at boiling temperature the density was 100 times as much as at room temperature.
If the density of hydrogen in the air exceed 4%, there's a danger of explosion. Vapor that went to the upper floors of the reactor building would have been cooled by the walls and condensed into water again, but hydrogen may have remained gaseous, increasing the ratio of hydrogen in the air.
Professor Yosuke Katsumura of Tokyo University (radiation chemistry), one of the team members, says "Water radiolysis may have happened in addition to the hydrogen flow from Reactor 3. We would like to test whether it can happen in an actual reactor building with a real-size spent fuel pool".
TEPCO says, "It is possible in theory, but we do not know whether a large enough amount of hydrogen is generated that way to cause an explosion".
There seem to be many interesting papers presented at the conference, highly scientific and technical, detailed insights into what may have happened and how at Fukushima I Nuke Plant. A nice departure from muddling through by TEPCO and NISA for the past 6 months. I only wish I had studied physics and chemistry more...