Sensitivity and Uncertainty Analysis of Multiphysics Nuclear Reactor Core Depletion

Nuclear reactor simulation is a complex process described by the neutronic, thermal-hydraulic, and fuel thermo-mechanical behavior of the core components. In current generation reactor physics analysis these three areas are at best loosely coupled. Within this work, a methodology for tightly coupling the core neutronics code PARCS, thermal-hydraulics code PATHS, and fuel rod simulator code FRAPCON was developed. This coupled code package was applied to two fuel depletion problems. The behavior of the fuel-cladding gap and associated temperature drop was found to be important. The code package was then applied to the pin cell calculation to evaluate the uncertainty and sensitivity of the nuclear performance of the core due to the influence of fuel thermo-mechanical models available for manipulation in FRAPCON. A sensitivity study was conducted to determine which fuel models were influential on the neutronics outputs; we determined that fuel thermal conductivity, fuel thermal expansion, cladding creep, fuel swelling and heat transfer coefficient had an important influence on some neutronics parameters. The package was integrated within the DAKOTA uncertainty package. Two sampling-based methods and two stochastic expansion methods were used to evaluate the uncertainty in the nuclear parameters throughout core depletion. We found that the uncertainty in the core reactivity was approximately 60 pcm at the beginning of depletion, reducing to approximately 15 pcm by the end of life, due to the effect of plutonium buildup reducing the importance of fuel uncertainty on high-burnup fuel. We found that the response statistics could be well-estimated by a first-order tensor-product PCE expansion, only requiring 32 calculations. Using variance-based decomposition, we found that initially the most important models contributing to nuclear variance were the thermal conductivity and fuel thermal expansion models. Once the fuel-clad gap closes, however, fuel thermal conductivity uncertainty dominates the overall variance in the output. We also found that the importance of input interactions on overall variance is negligible; at worst case, interaction effects contribute ~3% of the overall variance in both Doppler temperature and reactivity.PhDNuclear Engineering and Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111444/1/asbiele_1.pd