Isotopic and reactor physics characterization of a gas-cooled, pebble-bed microreactor

Pebble-bed High Temperature Gas-Cooled Reactor (HTGR) designs present a unique modeling challenge. Pebble-bed reactors can have a variety of pebble compositions due to varying levels of burnup. In addition, the pebbles are mobile in the core — entering from the top and exiting through the bottom — and may fall in a haphazard arrangement. This work introduces a 20 MWth pebble-bed HTGR reactor design (which will be referred to as Sangamon20) that is representative of current pebble bed reactors, and investigates not only the neutronics of the base model, but the changes to core neutronics after making modifications to the simulation. These modifications include: heterogenous versus homogenous pebble centers, imposing a universal symmetry assumption, and changing the arrangement of pebble fuel compositions, using Serpent and Python. This is in support of the ultimate goal of this project: to establish a baseline source term and to determine what simplifications, if any, can be made in the model to strike a balance between computational cost and precision of the results. This is crucial for any future licensing effort, safety analysis, or accident analysis involving pebble-bed reactors. The model of Sangamon20uses a random dispersal of seven different pebble compositions, each corresponding to a different burnup level.The heterogeneous tests compare k_eff; thermal and fast flux profiles; and the neutron lethargy-adjusted energy spectra in the core, reflector, coolant, a random fresh, and a random discharge-burnup pebble. Shuffling and symmetry tests monitor changes to k_eff and the outgoing neutron current at the outer reflector boundary; the former because it is an important reactor physics parameter, and the latter because it can be used to find the anticipated neutron flux the Reactor Pressure Vessel (RPV) would experience. This informs the level of radiation damage one could expect the RPV to experience each year - which is useful from a design and safety perspective.Neither the symmetry test nor the shuffling test showed a major difference in either the k_eff or the outward neutron current at the outer edge of the reflector. This would suggest that there is no need to simulate all possible pebble placements to characterize a reactor. However, for the heterogeneous tests, k_eff differed by 4.45%, and the pebble spectra at certain higher energies disagreed by a factor of 2-4. A complete fuel isotopic composition at each burnup step is accessible at [37]. This thesis discusses select isotopic inventories of interest. These can be used to inform source term determination or spent fuel compositions.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Richter, Zoe

Contributors dc:contributor

• Munk, Madicken
• Kozlowski, Tomasz

• HTGR
• pebble-bed
• microreactor

Statement dc:rights

• Copyright 2022 Zoe Richter

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115804

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115804