Progress on Implementing a Subcritical Assembly at Oak Ridge National Laboratory for Nuclear Criticality Safety Training

A subcritical assembly is being implemented at Oak Ridge National Laboratory (ORNL) to expand hands-on nuclear criticality safety training capabilities for the US Department of Energy Nuclear Criticality Safety Program (NCSP). The subcritical assembly will be designed using retired Aerojet General Nucleonics (AGN)-201M fuel in storage at the Y-12 National Security Complex. The AGN-201M fuel contains UO2 powder enriched to near 20 wt. %²³⁵U that is matrixed in polyethylene cylindrical plates. The AGN-201M reactor can achieve a critical configuration with 665 g²³⁵U when fissile control rods are inserted into the core. This fuel is ideal for creating simple experiments with high neutron multiplication and minimal fissile mass while meeting administrative fissile limits built into ORNL operations. The first phase of design was a reconfiguration of the AGN-201M assembly using graphite reflection. This resulted in a heavy system that would not be conducive to performing simple hands-on experiments. The second phase utilized the fuel available from four AGN-201M reactors and different reflecting materials. This effort resulted in a configuration that can be performed as a table-top experiment. Using the new configuration, four proposed experiments will focus on training students on the parameters that influence neutron multiplication in a system. These parameters are typically taught using the acronym MAGICMERV (Mass, Absorption, Geometry/shape, Interaction of units, Concentration/density, Moderation, Enrichment, Reflection, and Volume) as a mnemonic device. The proposed experiment topics include the increase of fissile mass to the assembly, assembly reflection, and introduction of materials into the system that would behave as neutron absorbers (e.g., B4C) or neutron moderators (e.g., graphite). The availability of this subcritical assembly at ORNL will facilitate training beyond that provided in the typical NCSP courses by allowing hands-on training for regional fissile operators, university students, and regulators, and it will also be adaptable to the needs of the student base. This paper focuses on the redesign efforts and design of the experimental station.