Sensitivity and uncertainty analysis of a fixed-source criticality accident alarm system benchmark experiment

The Department of Energy (DOE) Nuclear Criticality Safety Program (NCSP) funded the development of a first of its kind criticality accident alarm system (CAAS) benchmark to be published by the International Criticality Safety Benchmark Evaluation Project (ICSBEP) in the International Handbook of Evaluated Criticality Safety Benchmark Experiments (handbook). The experimental data for this effort was obtained from an experiment conducted by Oak Ridge National Laboratory and the Commissariat à l' Énergie Atomique et aux Énergies Alternatives (CEA) on the SILENE reactor located at CEA Centre de Valduc. This paper discusses the results obtained from using the MAVRIC sequence in the SCALE code package to computationally model the first of three pulses conducted during the experimental effort. First, a high fidelity 3-D model was developed and used to simulate the neutron activation measurements from the pulse. Second direct perturbation was used to reduce the High Fidelity model to a simplified model of the experiment. Finally, a sensitivity and uncertainty analysis was performed using the simplified 3-D model. The high fidelity and simplified 3-D models both produced estimates of activation in good agreement with the experimental results. The ratio of computed to experimental (C/E) results ranged from 0.95 to 1.28. Computed sensitivity coefficients revealed that the model was most sensitive to the thickness of the activation foils. The overall uncertainty in the computed responses due to uncertainties in the input information was in all cases less than 10% and was the major driver of uncertainty in the final results.