Analysis of a Heat Balance Experiment in the TREAT Reactor with SPARTA and OpenMC

This article presents the interpretation of a heat balance experiment conducted at the Transient Reactor Test Facility (TREAT). A reference OpenMC model of TREAT is introduced to evaluate the effects of the control rod positions and temperature variations on the effective multiplication factor (k_eff) and correction factors. Building on prior work, OpenMC is also used to investigate key hypotheses underlying the signal processing algorithm for reactivity and transient analysis (SPARTA) methodology. A critical hypothesis of SPARTA—that reactivities and correction factors can be obtained by linearly combining individual effects—was validated. However, the temperature effect on the correction factors, previously deemed negligible, was found to be significant, necessitating updates to the SPARTA methodology. By fitting an exponential curve to the mean temperature data, SPARTA’s accuracy and convergence were further enhanced. Finally, an experiment interpretation was performed by comparing the SPARTA and OpenMC results, which demonstrated consistent reactivity removal due to temperature increases. While this study confirmed SPARTA’s capability to interpret complex transients independently of an OpenMC model, it also highlights that simpler methods may be sufficient when the correction factors have minimal impact on reactivity calculations.