“I am a control room nuclear operator. I woul like to clarify the statement about the reactor core not being involved in the explosion and that it was a hydrogen explosion. When a reactor core is over heated/uncovered by the loss of cooling water or by steam blanketing the nuclear fuel over heats and possibly melts. When this happens a uranium and or zirconium steam reaction occurs. This reaction creates hydrogen. Emergency system try to burn this off before it reaches a level that will cause a damaging explosion. It looks like this failed to occur. While the explosion may not have caused any damage to the core it does indicate that the core has been greatly overheated and most likely is very damaged.”—"Kayakhomer," comment left at CNN.COM 2 minutes ago.