SL-1 significant event steam explosion
Cause
One of the required maintenance procedures called for Rod 9 to be manually withdrawn about four inches (10 cm) in order to attach it to the automated control mechanism from which it had been disconnected. Post-accident calculations, as well as examination of scratches on Rod 9, estimate that it had actually been withdrawn about twenty inches (51 cm), causing the reactor to go prompt critical and triggering the steam explosion. The most common theories proposed for the withdrawal of the rod are (1) sabotage or suicide by one of the operators, (2) a murder-suicide involving an affair with the wife of one of the other operators, (3) inadvertent withdrawal of the main control rod, or (4) an intentional attempt to "exercise" the 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 will never be known. However, it seems unlikely that it was a suicide.Post-accident experiments were conducted with an identically weighted mock control rod to determine whether it was possible or feasible for one or two men to have withdrawn Rod 9 by 20 inches. Experiments included a simulation of the possibility that the 48-pound (22 kg) central rod was stuck and one man freed it himself, reproducing the scenario that investigators considered the best explanation: Byrnes broke the control rod loose and withdrew it accidentally, killing all three men. When testing the theory that Rod 9 was rapidly withdrawn manually, three men took part in timed trials and their efforts were compared to the energy of the nuclear excursion that had occurred.
A spare SL-1 control rod actuator assembly was used for mock-up on which the speed of manual rod withdrawal was measured for several subjects. The equipment is the same as that on SL-1 except for the control rod, which is simulated by a weight to give a total movable load of 84 lb., the net weight of the SL-1 movable assembly in water. ... The test was conducted by instructing the subject to lift the rod as rapidly as possible, while an electric timer, measured the elapsed time from beginning of rod motion to some predetermined distance of withdrawal. Distances up to 30 inches were measured.
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