FKBN - April 5, 1968

On April 5, 1968, an accident occurred at the Russian Federal Nuclear Center (VNIITF), which is located in the southern Ural Mountains. The FKBN, a Russian acronym for “a physics neutron pile”, was a vertical lift assembly with a natural Uranium reflected core. The FKBN was developed to study radiation tolerance. The configuration of the assembly consisted of a thick natural uranium reflector with a large internal cavity.

            At the time of the accident, the FKBN was being used to research the effects of a polyethylene sphere on a reactor system. The image below shows how the components were configured at the time of the accident.

            The design consisted of three parts: a stationary core, an upper reflector and a lower reflector. The core was made from Uranium with a polyethylene sphere at its center. The upper reflector was a natural uranium piece and the lower reflector was also a natural uranium piece with the same dimensions as the upper reflector.

            The accident occurred on the afternoon of April 5, 1968.

            The operation of lowering the upper reflector could not be carried out under remote control so a senior specialist operated a lift while a junior specialist stood next to the FKBN reflector to guide it in to place. The assembly became critical as the upper half approached the core and was about to make contact with it. The emergency instrument system responded by dropping the lower reflector returning the system to a subcritical level.

            The accident was caused by a series of miscalculations and errors in judgment. The first error that occurred was the lack of realization of the lower reflector’s position before lowering the upper reflector. The lower reflector was placed was in a higher position than the specialists realized. The lower reflector was at a high location due to a failure to reposition the half after a morning test was concluded. The second oversight was the senior specialist’s expectation that the polyethylene sphere in the middle of the core would have a small effect on the system’s reactivity. 

            Several additional violations of procedure were identified as contributing to the accident. These violations include the switching off of the instrument system that alerted the specialists that the system was becoming critical. There were also two operators that were missing, a third specialist who would’ve manned the control room and a health physics specialist.

            Following the accident, the two specialists remained conscious and were able to inform administrative officials and request an ambulance. The junior specialist, who was positioned close to the assembly, died three days after the excursion. The senior specialist, who was farther, received an accumulated dose in the 5-10 Sv range and passed away 54 days after the accident.

           Once again, the moral of the story is to make sure everyone is there. There are additional lessons of make sure you have all of the equipment in the right position and that your safety systems and radiation detectors are on.