On March 19, Casey Stocking successfully defended his MS thesis, An Investigation on the Impact of Temperature and Density Homogenization on Vessel Fluence Calculations. Casey’s committee consisted of his co-advisors, Maria Avramova and David Kropaczek, and members, Ralph Smith and Tara Pandya.

Abstract

Casey Stocking. An Investigation on the Impact of Temperature and Density Homogenization on Vessel Fluence Calculations. (Under the direction of Prof. Maria Avramova and Prof. David Kropaczek.)

As the reactor fleet continues to age, accurately modeling the reactor pressure vessel fluence becomes necessary for estimating radiation embrittlement. The majority of previous work in this field has been conducted using deterministic neutronic codes. This work focuses on core approximation methods that allow forMonte Carlo vessel fluence results at a reasonable computational expense. This investigation was performed using VERAShift; a code within the VERA code suite. VERAShift is comprised of a coupled eigenvalue calculation between MPACT, a deterministic neutronics code, and CTF, a subchannel thermal-hydraulic code. The fission source, temperatures, and densities are then passed to Shift, a continuous-energyMonte Carlo radiation transport code, which performs a fixed source vessel fluence calculation.

The objective of this project is to investigate core approximation techniques. Specifically, this project investigates the impact of various temperature and density homogenizations in a reactor core on vessel fluence calculations. These homogenizations are intended to reduce memory usage while maintaining accuracy. There are two main homogenization types being considered; assemblyaveraged and ring-averaged. Assembly averaged homogenization performs volume-averaged temperature and density homogenization over each assembly and ring-averaged homogenizations are performed over radial rings about the center of the core. The explicit peripheral fuel pins option allows for explicit temperature and density modeling in the outer most pincells and assemblies. Semi-unique pincells were created to reduce the memory usage for homogenizations by using fewer unique pincell descriptions.

Seven cases were considered for comparing the accuracy of semi-unique pincells and unique pincells. These cases showed good agreement between semi-unique and unique pincell configurations. This showed that semi-unique pincells accurately model the reactor geometry for vessel fluence calculations. Eleven mini-core cases were chosen for investigating the accuracy of temperature and density homogenization in vessel fluence calculations. All of the homogenizations investigated were significantly more accurate than the nonunique pincell model. Pin-ring homogenizations performed inconsistently; depending on ring thickness and location. It was hypothesized this is caused by lumping varying enrichments together. These homogenizations were also shown to provide moderate to significant performance enhancements; up to a factor of 2-3 reduction in memory and a factor of 5 reduction in runtime. All of the homogenizations were also shown to out-performnonunique pincells for a realistic reactor geometry. Overall, temperature and density homogenization using semi-unique pincells appears extremely promising.