At about 4 am Saturday, March 12, operators determine that water level in Reactor #3 has dropped sufficiently to expose the core and in another hour they begin to vent the reactor vessel in unit 1 to the torus suspension pool. By 11 am, they begin venting on unit #2.
Suddenly, at 3;56 PM, the reactor building of unit #1 is rocked with a violent explosion. This was the result of the ignition of hydrogen which had been released into the structure. The evolution of hydrogen was undoubtely due to the oxidation of the zirconium cladding surrounding the fuel pellets.
Where this hydrogen came from has not been discovered. My best guess is that a previous hydrogen burn had occurred within and breaching the reactor containment, releasing further hydrogen gas into the reactor building. Previous studies were of the opinion that such an event in spent fuel pool would not occur before 100 hours or so after loss of cooling.
It is noteworthy to note that a similar hydrogen burn occurred some 10 hours into the incident at Three Mile Island . However, because of the enormous free volume within the containment, the resultant pressure rise reached only to a maximum of 28 psig. Reactor containment building are built to a design pressure of about 60 psig, but should be able to withstand twice that. By contrast with the PWR plants with a free volume of about 2 million cu. ft., the BWR containment here fits rather tightly, like a skirt around the reactor system.
