Species transfer processes in molten salt reactors

Lee, Joon Hon Alvin

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https://hdl.handle.net/2142/129890 Copy

Description

Title

Species transfer processes in molten salt reactors

Author(s)

Lee, Joon Hon Alvin

Issue Date

2025-07-17

Director of Research (if dissertation) or Advisor (if thesis)

Kozlowski, Tomasz

Doctoral Committee Chair(s)

Kozlowski, Tomasz

Committee Member(s)

• Fischer, Paul
• Brooks, Caleb
• Di Fulvio, Angela
• Munk, Madicken

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Molten Salt Reactors
• Species Transfer
• Plutonium Diversion
• Safeguards
• Adsorption
• Deposition
• Corrosion
• Reprocessing
• Xenon

Abstract

In a molten salt reactor (MSR), the accurate simulation of the fuel salt and salt-facing component compositions is especially important because the nuclear fuel is dissolved within the molten salt and a change in composition can have significant impacts on the neutronics and thermal-hydraulics behavior of the reactor. In this dissertation, a method for modeling the continuous evolution of material compositions in MSRs under various species transfer processes was developed and demonstrated. The method uses the reprocessor function within Serpent 2, which is based on a modified Bateman equation for fuel depletion and is easily extendable to other depletion codes with user-defined species (or material) transfer capabilities. The dissertation focus is on the application of the species transfer method in a graphite moderated MSR to (a) determine the target reprocessing rates of fission products, (b) determine the acceptable graphite adsorption rates and equilibrium constants based on final waste classification, (c) determine the acceptable deposition and corrosion rates of heat exchanger materials, and (d) develop and demonstrate a method for detecting plutonium diversion considering the major and minor species transfer processes in (a), (b), and (c). The results in (a), (b), and (c) can then be used by reactor designers and manufacturers as reference levels to be satisfied in order to fulfil the design requirements, while the results in (d) can be used by safeguards inspectors for a non-proliferation monitoring program. These efforts are aimed to contribute to the successful deployment of MSRs. Relations were derived for converting the physical parameters and measurable quantities of the MSR into a reprocessing rate that is required by depletion codes to simulate continuous reprocessing. Using these relations, the neutronically important elements were identified to consist of xenon and lanthanides, and their reprocessing rates leading to substantial improvements in excess reactivity were determined. Importantly, the results demonstrated that depletion calculations with continuous reprocessing are necessary in order to obtain the correct keff due to contributions from precursors of neutron absorbing species. A refueling scheme with charge balance approximation was also developed and demonstrated to account for the changing salt charge during fuel depletion and the co removal of F- ions, with the approximation producing a notable correction to the reprocessed mass. The relations to simulate graphite adsorption and salt penetration were derived and demonstrated in this dissertation, by considering the rates of adsorption and desorption to be proportional to the species concentration within the materials in contact. Using these relations in the simulation of various adsorption scenarios, the equilibrium constants leading to greater-than-Class-A radioactive waste and their neutronics impact were determined. Simulations were also performed to determine the neutronics and wall-thickness impacts of the intermediate heat exchanger due to deposition and corrosion, and to demonstrate the conversion of postulated heat exchanger and reactor performance criteria into deposition and corrosion bounding rates. A mathematical basis was derived to translate the activities of select gamma emitting fission products into the fissile isotope ratios (or fission rate ratios). Using this set of translation relations, a methodology for the detection of plutonium diversion in MSRs was developed. The methodology consists of three key steps: i) determining the fission product yields due a single fissile isotope (e.g., 235U or 239Pu in this work), ii) determining the fuel evolution of a reference reactor with declared power history and reprocessing and refueling processes, and iii) determining the presence of plutonium diversion by comparing the indicators predicted from measurements of an evaluated reactor with the calculations of the reference reactor. The first two steps calibrated the methodology parameters and identified several pairs of gamma-emitting species that are appropriate for diversion detection due to their good predictive performance and resistance to unwanted species transfer influences. In particular, the 138mCs/134mI pair emerged as the ideal candidate due to its excellent accuracy under the extreme scenario of continuous and complete removal of its precursor elements. The performance of the detection methodology was evaluated through several plutonium diversion scenarios, and the methodology was able to accurately predict the 239Pu content in the reactor and determine the amount of diverted 239Pu in each scenario. Using the prediction uncertainty of the 138mCs/134mI pair, the sensitivity of the detection methodology was estimated to be 860 g of plutonium diversion before detection (out of 89.5 kg of plutonium generated after 364 days in a scaled-up 1000 MWth reactor). Meanwhile, using alternative candidate pairs, such as 85mKr/135mXe which require additional considerations of interfering species transfer processes, the predictive performance of the methodology can be improved significantly to 30 g of plutonium diversion before detection. This strengthening of the detection methodology performance is enabled by the accurate accounting of the influence from concurrent species transfer processes, which is addressed by the modeling methods developed in this dissertation. The new and significant contributions made in this dissertation to the body of knowledge are: -the unification of phenomena with disparate time-scales, length-scales, and physical mechanisms under a single framework that is analogous to isotope depletion (i.e., λ_transfer×N_transfer), which is trivial to implement in depletion codes supporting user-defined material transfer, -the derivation of relations to convert the physical reactor parameters and measurable physical quantities into the transfer rate constants λ_transfer, -the demonstration of utility of such approach for bounding the reactor core, material, and component performance metrics, and -the use of the above framework to develop a novel method for detecting plutonium diversion.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129890

Copyright and License Information

© 2025 by Alvin Lee. All rights reserved.

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