In all water moderated reactors, to operate the reactor requires the presence of water to moderate (slow down) the neutrons produced by the nuclear reaction and allow the reaction to proceed. If the reaction becomes too great, the water turns to steam, quickly and automatically removing the water moderator and shutting down the nuclear reaction.
As a small reactor, the SL-1 was designed with a main central control rod which was able to produce a very large excess reactivity if it was completely removed. The excess reactivity is a measure of how much more capacity there is to accelerate the nuclear reaction than is required to start a controlled nuclear reaction for power generation. The potential for excess reactivity is always required because the fuel becomes less reactive over time. A greater excess reactivity causes a faster increase of the rate of the nuclear reaction. In normal operation, the control rods are withdrawn only enough to cause sufficient reactivity for a sustained nuclear reaction and power generation.
At 9:01 pm on January 3, 1961, after a shutdown of eleven days for the holidays, and during maintenance procedures, the SL-1 went prompt critical. In four milliseconds, the heat generated by the resulting enormous power surge caused water surrounding the core to explosively vaporize. The water vapor caused a pressure wave to strike the top of the reactor vessel. This propelled the whole reactor vessel upwards and bounced it off the ceiling of the containment building, which killed the operator who had been standing on top of the vessel, leaving him pinned to the ceiling. The other two military personnel who were supervising restart operations were also killed.
There were no other people at the reactor site. The ending of the nuclear reaction was caused solely by the design of the reactor and the basic physics of water turning to steam and its moderator removal effect.
One of the required maintenance procedures called for the main control rod to be manually withdrawn approximately three inches in order to attach it to its automated control mechanism, from which it had been disconnected. Post accident calculations estimate that the main control rod was actually withdrawn approximately twenty inches, causing the steam explosion. The three most common theories proposed for this discrepancy are sabotage by one of the operators, inadvertent withdrawal of the main control rod, which was known to be "sticky," or an intentional attempt to "exercise" the sticky rod, to make it travel more smoothly within its sheath. The maintenance logs do not address what the technicians were attempting to do and thus the actual cause of the accident is unlikely to ever be known. There were signs that at the time of the accident the operator was attempting to reinsert the control rod and prevent the runaway reaction.
The remains of the SL-1 reactor are now buried near the original site of the reactor.
The accident caused this design to be abandoned and future reactors to be designed so that a single control rod removal wouldn't have the ability to produce the very large excess reactivity which was possible with this design. The reduced excess reactivity limits the size of the power surge which is possible. The accident also showed that in a real extreme accident the water to steam conversion would shut down the nuclear reaction, demonstrating the inherent safety of the water moderated design against the possibility of a nuclear explosion.