Improving fuel performance modelling of TRISO fuel failure

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Tri-structural isotropic (TRISO) particle fuel will be used in the next generation of very high temperature reactor (VHTR) designs and is already deployed in the Chinese HTR-PM reactor. First conceptualised and tested in the UK, TRISO fuel has proven to be a very robust fuel type able to withstand temperatures up to 1600-1800C with minimal failure. This behaviour allows VHTR reactors to be walk-away safe in the case of a loss of coolant accident. However, there are inevitably areas where there is little understanding or where design improvements could be made. At present the majority of TRISO fuel failures arise from defective fuel particles. Defects can occur in many forms where layers are deposited with different/non-uniform thicknesses or the shape is oval or contains regions that are flat. All of these defects are likely to impact on the likelihood of a particle failing. There are also many different designs of TRISO fuel with different dimensions and indeed fuel kernel materials (UO2, UCO and others). Recent work at Imperial College has developed a finite element model that incorporates the residual stresses from manufacture of TRISO fuel [1]; we have developed Peridynamic 2D models of full TRISO particles under accident conditions [2] and most recently we have further developed these models into 3D [3]. These high fidelity models can be used to understand the causes of fuel failures better and therefore improve the more practical fuel performance codes such as STRESS3 (NNL TRISO fuel performance code) or indeed more widely used codes such as the US code BISON. The project will explore the development and use of our peridynamics models to incorporate more statistical behaviour of materials through Weibull models of fracture behaviour [4-5] and geometric defects. From these models we will look to refine both the parameters, and errors on the parameters, used in the fuel performance codes used by industry.[1] A. Battistini, T. A. Haynes, Daniel Shepherd, M. R. Wenman. A finite element analysis of the residual stresses in as-fabricated TRISO coated particle fuel. J. Nucl. Mater. (2023) in review. [2] T.A. Haynes, A. Battistini, A.J. Leide, D. Liu, L. Jones, D. Shepherd, M.R. Wenman. Peridynamic modelling of cracking in TRISO particles for high temperature reactors. J. Nucl. Mater. 576 (2023).154283[3] A. Battistini, T. A. Haynes, L. D. Jones, M. R. Wenman. 3D Peridynamics models of cracking in SiC and buffer layers of TRISO fuel particles (unpublished work).[4] L.D. Jones, L.J. Vandeperre, T.A. Haynes, M.R. Wenman. Modelling of Weibull Distributions in Brittle Solids Using 2-Dimensional Peridynamics. Procedia Structural Integrity 28, (2020) 1856-1874.[5] L.D. Jones, L.J. Vandeperre, T.A. Haynes, G. Rossiter, M. R. Wenman. The Effect of Weibull Distributions on Modelling Nuclear Fuel Fracture Using Peridynamics. J. Nucl. Mater. 572 (2022) 154087.