anybody know if by meltdown, they mean the tubes the fuel pellets are in melted? or something different??? anybody know if by meltdown, they mean the tubes the fuel pellets are in melted? or something different??? It is statistically probable that all the Fuel Pellets have melted into a molten core within reactor #1. Could have occurred on day 1 or any other day for a few weeks afterward. Odds now favor it happened very close to day 1. During TMI the molten core section caused a breakout from the RPV that was contained within a control rod housing box assembly. There was not enough force to break out completely. We have to assume that there was enough force this time. Also, the presence of 1,000 of tons of highly radioactive water that was supposed to be running within a closed loop internal cooling system, does not favor 100 % containment.