As the scram continued, the reactor output jumped to around 30,000 MW thermal, 10 times its normal operational output, the indicated last reading on the power meter on the control panel. Some estimate the power spike may have gone 10 times higher than that. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion, like the explosion of a steam boiler from excess vapour pressure, appears to have been the next event. There is a general understanding that it was explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the explosion that destroyed the reactor casing, tearing off and blasting the upper plate called the upper biological shield, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard.