A reactor physics framework to detect anomalies in HTGR core

This dissertation extends the application of neutron noise analysis as a tool for detecting anomalies in High Temperature Gas-cooled Reactors (HTGRs). Neutron noise is defined as the fluctuations in the neutron detector signal caused by perturbations in a nuclear reactor. Neutron noise analysis is performed by obtaining the neutron flux signals in the time domain, which is then converted to the frequency domain to perform spectral analysis of the neutron noise. This analysis is possible only for steady-state conditions. Neutron noise analysis has been used in the past in LWRs and has proven effective in several applications. This includes the application for core monitoring and surveillance, model validation, and core diagnostics for anomaly detection. Considering the effectiveness of the analysis, computational methods and models of neutron noise were developed from the neutron transport equation. The neutron noise models can be divided into two categories, namely the absorber of variable strength (AVS) and the vibrating absorber. The AVS is modeled as fluctuations in the macroscopic cross-section at a fixed location r₀. This includes the cases of perturbations traveling with the coolant flow in molten salt reactors (MSRs) and perturbations due to unseated fuel assemblies in boiling water reactors (BWRs). The vibrating absorber is modeled as a displacement of the assembly from its nominal position, with the assumption that neutron noise is induced by the vibrating interface. Several tools have been developed to solve neutron noise equations, using both Monte Carlo and deterministic methods. This dissertation is divided into two contributions. First, we determined if neutron noise analysis and current neutron noise unfolding methods are applicable to prismatic-type HTGRs. To make such determination, a neutron noise equation solver was developed for both hexagonal and rectangular geometries. The developed solver solves the multigroup neutron diffusion equation in the frequency domain, using a finite volume method for spatial discretization. The solver has been verified using several benchmarks. Then, we simulated neutron noise in a prismatic-type HTGR core to obtain its neutron noise characteristics, including the point-kinetic and spatial components of neutron noise, as well as the zero-power reactor transfer functions. The results show that, generally, the neutron noise in HTGRs exhibits a similar behavior to that in LWRs. However, there are a few aspects that distinguish HTGRs. From the zero-power reactor transfer function, HTGRs have a lower plateau, which means that HTGRs are more sensitive to perturbations within a certain range of frequencies. Neutron noise in HTGRs is largely dominated by the point-kinetic component of neutron noise, which means that the neutron noise is largely driven by the reactivity effect of the perturbation. The second contribution is to introduce two new methods to unfold neutron noise. The challenge of neutron noise unfolding is that the neutron noise is not known (measured) at all positions and energy, it is only known at the detector positions. There are three primary methods for neutron noise unfolding: the inversion method, the zoning method, and the scanning method. These methods were applied to unfold the neutron noise source in the 2D HTTR core. The results show that the inversion method lacks accuracy due to the use of linear interpolation. The scanning method shows satisfactory results but is limited to only one AVS-type source. The zoning method incorrectly identifies the location and magnitude of the neutron noise source. This dissertation developed two novel methods for neutron noise unfolding that use Green’s function, namely the brute force method and the greedy method. The brute force method solves the combinations of Green’s function and its coefficient until it finds the correct combination of Green’s functions. The greedy method iterates Green’s function to find the Green’s function matrix and the local minimum of the residual. This results in several valid solutions to the noise sources. The correct solution is the solution that requires the least number of Green’s function combinations to minimize the residual. The results show that both brute force and greedy methods can simultaneously unfold multiple AVS-type noise sources, which is not possible by the currently used methods (inversion, zoning, scanning methods).

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Ardiansyah, Harun

Contributors dc:contributor

• Kozlowski, Tomasz
• Di Fulvio, Angela
• Alam, Syed Bahauddin
• Kirr, Eduard-Wilhelm
• Demazière, Christophe

• Reactor Physics
• Neutron Noise
• HTGR
• Noise Unfolding

Statement dc:rights

• Copyright 2025 Harun Ardiansyah

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132569

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132569