Constitutive Modeling and Stress Evolution Analysis for Advanced TRISO-Type Nuclear Fuel

Abstract

Driven by the development of High-Temperature Gas-cooled Reactors (HTGRs) and advanced microreactors, TRISO particles have emerged as a critical structural component, whose mechanical reliability directly affects the safety limits of these advanced energy systems. This study develops a mechanical model that incorporates anisotropic irradiation-induced dimensional change (IIDC), irradiation creep, and a fluence-dependent Maxwell viscoelastic constitutive law to analyze the stress evolution in multilayer TRISO particles under irradiation. A mechanism-decoupling approach is employed on a double-layer shell structure to investigate the dynamic competition between irradiationinduced anisotropic shrinkage and creep relaxation. Following validation of the single-layer shell solution against analytical and finite-element results, an additional transient test is compared with OFFBEAT predictions to assess the model’s capability to reproduce time-dependent irradiation creep and IIDC. Subsequently, Latin Hypercube Sampling and sensitivity analysis are applied to the TRISO to quantify the material properties governing failure-related stresses. Results demonstrate that earlystage responses are dominated by irradiation shrinkage, with the IPyC tangential IIDC and creep Poisson's ratio exert the strongest influence on IPyC tensile stress. Under high neutron fluence, PyC shrinkage reduces SiC tensile stress through structural shielding, whereas PyC creep promotes load transfer to the SiC layer. The transient test and sensitivity results demonstrate that the model captures both time-dependent irradiation responses and the dominant mechanisms governing multilayer stress evolution. The model therefore provides a mechanism-resolving framework for analyzing the transient and long-term stress evolution of multilayer TRISO particles under irradiation.