The SimplyInfo research team discusses a possible explosion inside Unit 1′s vessel:
From this NRC document, the existence of damaged concrete debris is explained:
“the case that the containment has many sub-compartments, a local deflagration or detonation may occur that damages the sub-compartment and through this may generate missiles (concrete blocks from the disintegrated compartment walls) that can endanger the containment integrity. … The resistance of a concrete containment to such objects is larger than that of a steel containment: upon impact, the missile may generate cracks rather than gross failure.”
Explosive events are known to be possible inside reactor buildings, inside containment structures and inside the reactor pressure vessel. Unit 1′s accident progression was rapid. By the early hours of March 12th, 2011 radiation levels were rising in the turbine building of unit 1. At the point unit 1 lost cooling the meltdown took place in less than an hour. This would also cause the rapid build up of hydrogen as fuel melts. The hydrogen explosion of unit 1 could have occurred in locations beyond just the refueling floor. This would explain the charred appearance of the containment structure and possibly some of the damage found inside so far.
Workers were able to briefly obtain some pressure readings in unit 1, The recordings show pressures close to, but not exceeding, the setpoint of safety/relief valves. Intriguingly, there is no evidence that safety/relief valves opened. This lack of pressure buildup in the reactor vessel indicates either the pressure indicators were not working properly or there was already an opening in the reactor vessel allowing pressure to escape. There has long been an assumption unit 1 suffered from a LOCA (Loss of Coolant Accident). [...]
[...] In the case of a LOCA fuel is uncovered, interactions and fuel melt, maximum hydrogen generation and a possible detonation in vessel are possible. A pipe break low enough in the vessel which would still allow fuel melt but trap the hydrogen in the reactor vessel, especially if the safety relief valves don’t lift could create conditions where an in vessel hydrogen explosion could happen.
See also: NRC officials suspected a 'detonation' at Reactor No. 1 weeks before Gundersen postulated that scenario at No. 3
Published: October 16th, 2012 at 6:37 am ET