The plug was just a bit of frozen fuel salt. The research group used to melt the plug to shut down the reactor when they wanted to go home after a day's work. T

The plug was just a bit of frozen fuel salt. The research group used to melt the plug to shut down the reactor when they wanted to go home after a day's work. Then in the morning they would heat up the containment pool to re-melt the fuel, and pump it back up into the reactor.