The guy who got pinned to the ceiling was from the SL-1 accident, and it was due to a poor reactor design which allowed the reactor to go prompt critical by removing only one control rod. Normally the rods were in the down position when the reactor was shut off, but to bring it back online the rods had to be lifted by hand a few inches so they could be attached to the drive cables. Another flaw was that the control rods had a habit of getting stuck, so the leading theory is that the operator accidentally pulled it out too far when trying to get it free. We’ll never know exactly what happened though, since all three operators died in the explosion.