The Tokaimura Accident
Part VI: Lessons Learned
Shunsuke Kondo, a nuclear safety expert from the University of Tokyo who has done an independent analysis of the accident, indicated that the technicians assigned to process the Joyo fuel that day were under time pressure. Apparently the lead technician was anxious to complete the current batch before a new team of workers arrived. Furthermore, the workers were apparently not aware of the mass limitations on the uranium to be added to the precipitation tank. Over an extended time and with changes in staff, the 2.4-kg (5.3-lb) limit became interpreted as a batch limit. The operators strictly adhered to this limit; however, they had no idea that batches could not be combined.
Since the possibility of a critical event was deemed implausible with the JCO design and enforcement of the handling limits, training and priority regarding critical mass (that is usually provided to workers in fissionable fuel processing operations) was minimized. The operators had no real understanding of the relationship between quantity, concentration, and vessel shape with regard to the accumulation of a critical mass. It appears that there were serious failures in communications and operator training, and that regulatory agencies and plant managers were not diligent in following approved procedures.
Besides acting as a neutron deflector, a secondary effect of the water jacket may have been to prolong the chain reaction. Per Peterson and Joonhong Ahn of the University of California, Berkeley, have pointed out that, without the water jacket, the heat generated by the chain reaction would have expanded the solution, decreasing its density and slowing its reaction rate. With the water jacket in place to remove the fission heat roughly as fast as it was generated, however, the solution may have been kept just above the critical density.