Modeling and Validation of Approach to Criticality and Axial Flux Profile Experiments At the Missouri S&T Reactor (MSTR)

An MCNP model of the Missouri University of Science and Technology Reactor (MSTR) has been in development since 2007. The MCNP model is based on the best available configuration and composition data for the reactor and is flexible, allowing easy re-configuration to accommodate multiple core layouts. Validation of the model included the simulation of experiments routinely performed at the reactor and comparing the simulation results with the experimental data. These validation experiments include the approach to criticality (predicting critical control rod height) and the determination of the axial flux profile in the core. There is good agreement between the experimental data and the model predictions. The most accurate flux profile was produced when the model included an axial temperature variation. The modeled normalized profile accounting temperature variations matches within 10% at 34 of 49 points with the experimental data and within 30% at 46 of 49 points. The simulated approach to criticality matched with the experimental data with an average difference of 0.59 ± 0.08 in. at all points. One significant source of difference between the simulated and experimental data is that the model uses fresh fuel compositions and does not take into account the 20 years of operation and the consequential burnup and poison buildup. Moreover, the initial model does not consider detailed temperature variations and effects, causing a systematic difference between the simulated critical control rod position and the experimental values