The problem was traced to the incorrect installation of control rod electrical cables.
From the en.wikipedia.org
One man pulled out the main control rod, and angry neutrons did the rest.
From the newscientist.com
Eller wouldn't speculate as to whether the control rod problems were related to the new equipment.
From the chron.com
Once its control rod is cut, it would take a year and a lot of effort and expense to restart the reactor.
From the economist.com
The largest lump shows an intact control rod channel.
From the en.wikipedia.org
The lithium acts as a liquid control rod.
From the newscientist.com
Some of the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck at one-third insertion.
From the en.wikipedia.org
Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired.
From the en.wikipedia.org
About an hour and 20 minutes later, the control rod and pipes used to gauge neutrons started melting and falling onto the bottom of the pressure vessel.