Nuclear criticality safety lessons learned in the design of the uranium processing facility at the Y-12 national security complex

The Uranium Processing Facility (UPF) is being built by the National Nuclear Security Administration (NNSA) to replace aging uranium processes at the Y-12 National Security Complex (Y-12 NSC) in Oak Ridge. This paper will summarize some of the more important lessons learned for the NCS (Nuclear Criticality Safety) practitioner working in a design environment rather than in a production or experimental environment. As the Chief NCS Engineer for the UPF Project I have been responsible for ensuring the technical evaluations produced on the project are high quality, technically accurate, and conform to the Y-12 NCS Program expectations. Ensuring nuclear criticality safety for processes that do not yet exist in reality is a challenge very different than that of ensuring subcriticality for processes in an operating facility. The biggest difference being that in design space - everything is imaginary whereas in the operating facility you have the benefit of being able to go out and look at the equipment and interview the operators. Ensuring that Nuclear Criticality Safety is integrated into design is a process that requires constant communication between the NCS staff and the broader engineering design team. Until the facility is constructed - nothing is physically stable and so rigorous documentation of decisions - to include any assumptions - is necessary to minimize errors. Non-NCS engineers will not understand the hazard or the physics of nuclear criticality. It is up to the NCS staff to provide training and communication to the team. The NCS staff must be patient and prepared to explain themselves many times and be as flexible as possible.