Criticality Calculations for a Nuclear Fuel Fabrication Plant

The operations with the fissile materials such as U²³⁵ introduce the risk of a criticality accident that may be lethal to nearby personnel and can lead the facility to shutdown. Therefore, the prevention of a nuclear criticality accident should play a major role in the design of a nuclear facility. The objectives of criticality safety are to prevent a self-sustained nuclear chain reaction and to minimize its consequences. Sixty criticality accidents occurred in the world. These accidents are divided into two categories, 22 accidents occurred in process facilities and 38 accidents occurred during critical experiments or operations with research reactors. About 21 criticality accidents including Japan Nuclear Fuel Conversion Co. (JCO) accident took place with fuel solution or slurry and only one accident occurred with metal fuel. In this work, the nuclear criticality calculations have been performed for a typical nuclear fuel fabrication plant producing nuclear fuel elements for nuclear research reactors with low-enriched uranium (LEU) up to 20 %. The calculations were performed for both normal and abnormal conditions. The effective multiplication factor (k_eff) during the nuclear fuel fabrication process (Uranium hexafluoride – Ammonium Diuranate (ADU) conversion process) was determined. A proposed liquid control system was designed to stop the process and protect the system from reaching the set point initiating the critical conditions. A proposed Safety Logic Control Safety System (SLCSS) was designed to stop and shut down the process in the precipitator tank during abnormal conditions. The function of SLCSS is protecting and prevents the precipitation process of ADU from reaching the set point initiating the critical conditions. A Criticality Accident Alarm System (CAAS) was installed as part of criticality safety management for use in reducing the radiation workers could be exposed to in the rare case of a criticality accident. The results show that the effective multiplication factor (k_eff) was subcritical during normal and abnormal conditions in the precipitation process. Several accident scenarios were postulated and the criticality of these accidents were evaluated. The computer code Monte Carlo N-Particle Transport Code MCNP-4B which based on Monte Carlo methodology was used to calculate neutron multiplication factor. The criticality calculations were performed for the cases Of, change of moderator to fuel ratio, solution density and concentration of the solute in order to prevent or mitigate criticality accidents during the nuclear fuel fabrication process. The calculation results were analyzed and discussed