Modeling and validation of dosimetry measurement assumptions within the Armed Forces Radiobiology Research Institute TRIGA Mark F reactor and associated exposure facilities using Monte Carlo techniques

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The TRIGA Mark F reactor at the Armed Forces Radiobiology Research Institute in Bethesda Maryland is a 1 megawatt steady state reactor which can also be operated in pulse mode at a power of up to 2500 megawatts. It is characterized by a moveable core and two large exposure rooms, rather than a thermal column or beam ports, as found in most research reactors. A detailed model of the reactor and the associated exposure facilities was developed using the Monte Carlo N-Particle (MCNP) and Monte Carlo N-Particle Extended (MCNPX) software programs.

The model was benchmarked against operational data from the reactor, to include total core excess reactivity, control rod worths, and nominal fuel element worths. The model was then used to model burnup within individual fuel elements within the core to determine the effect of core movement within the reactor pool on individual element burnup. Movement of the core with respect to the two exposure rooms was modeled to determine the effect of movement of the core on the radiation fields and gamma and neutron fluxes within the exposure rooms. Additionally, the model was used to demonstrate the effectiveness of gadolinium paint used within the exposure rooms to reduce thermal neutron production and subsequent Ar41 production within the exposure rooms.

The model showed a good approximation to measured benchmark data across all applied metrics, and additionally provided confirmation of data on dose rates within the exposure rooms. It also showed that, while there was some variation of burnup within individual fuel elements based on core position within the reactor pool, the overall effect was negligible for effective fuel management within the core. Finally, the model demonstrated explicitly that the use of gadolinium paint within the exposure rooms was, and remains, an effective way of reducing the thermal flux, and subsequent Ar-41 production within the exposure rooms. It also demonstrated that the gadolinium paint also resulted in a much steeper neutron flux gradient within the exposure rooms than would have been obtained had neutrons been allowed to thermalize within the wood walls lining the rooms and then reenter the exposure facilities.