Abstract
High Temperature Reactors (HTRs) are among the candidates of the possiblenext generation nuclear plants. HTRs are expected to offer attractive features such asinherent safety, low cost of electricity generation, and short construction period. Thesafety performance of high temperature gas cooled reactors mainly relies on the qualityand integrity of the coated fuel particles.In this study, mechanisms that may lead to pressure vessel failure of a coatedfuel particle are analyzed. In addition to internal pressure rise due to increasing concentrationof fission product gases with burnup, mechanisms that stand out are migrationof the kernel of the fuel particle and failure due to reactivity insertion accident.The analysis is composed of mechanical and thermal parts. Mechanical analysisis performed to obtain stress distribution inside the load bearing layers. Particlefailure fraction can be obtained from those stresses. Thermal analysis predicts the temperaturedistribution inside the coated fuel particle, from which internal pressure loadon the structural layers is calculated.This study consists of three major parts, which are the IAEA (InternationalAtomic Energy Agency) Benchmark Study CRP-6 (Coordinated Research Project 6),kernel migration analysis and analysis of fuel failure due to reactivity insertion accident.CRP-6 study of the IAEA is used to benchmark the coated fuel particle modeland its analysis methodology. The results obtained for the CRP-6 study are consistentwith those of the other participants.Kernel migration is one of the failure mechanisms of a coated fuel particle.The amount of migration of the kernel is calculated and the response of the load bearinglayers are analyzed for different operating temperatures. Results of the analysisshowed that the effect of kernel migration itself is not so critical on fuel failure as ofpressure vessel failure.Reactivity insertion accident is one of the design basis accidents of HTRs.The scenario analyzed in this study assumes withdrawal of one of the control rods andconsequent reactivity insertion. The power level reaches a significant value and thendecreases slightly during the accident. The analysis shows that reactor can stabilizeits power level at a constant value in the absence of reactivity control systems dueto temperature feedbacks. Another result of the analysis is the significant increase intemperature as a result of positive reactivity insertion. Temperature increase as high as200 C may cause tensile stress on the SiC layer, which increases the failure probabilityof the coated fuel particle.Keywords: nuclear reactor, nuclear fuel, high temperature reactor, coated fuel particle,TRISO, kernel migration