Dark Light Dwellers (UN-D-7983)
The safety procedures were immediately initiated, but the boron control rods designed to absorb excess neutrons and thus lower core temperate were unable to cope with the unexpected increase of neutrons and temperatures continued to rise.
Fortuitously the Daemon chose to return to it's own dimension moments before the reactor's temperature exceeded fatal levels.
Professor Bob Baladene, the technician in charge and the first civilian to clearly have seen the Daemon, as it lingered in the reactor chamber, gave the order to lower all the control rods in an effort to shut the reactor down, moments later the Daemon 'blurred' through the reactor casing and disappeared from sight.
With the creature gone, the rods were able to absorb the remaining neutrons and narrowly avoid meltdown.