Monolithic fuel is a fuel form that is considered for the conversion of high performance research reactors. In order to qualify this new fuel system, the fuel plates should meet the safety standards and perform well in reactor. The fuel system must maintain its mechanical integrity, sustain a geometric stability and should have stable and predictable irradiation behavior. The requirement to maintain mechanical integrity under normal operating conditions is primarily demonstrated by a successful testing of fuel plates up to the limiting conditions defined by the fuel performance envelope, including an adequate margin. Although large number of plates have been tested with satisfactory thermo-mechanical performance, post-irradiation examination of plates from previous RERTR-12 experiments have revealed that pillowing occurred in several plates, rendering performance of these plates unacceptable. To address such failures, efforts are underway to define the mechanisms responsible for the in-reactor pillowing, and suggest improvements to the fuel plate design and operational envelope. For this purpose, selected plates from previous experiments were simulated to understand the thermo-mechanical response of the plates to the fission density and thermally induced stresses. Simulation results were then comparatively evaluated with post-irradiation examinations of selected plates. The simulation results and experimental observations established a possible correlation between failure by plate pillowing, high porosity and a presence of tensile stress state. The study has implied that porosity leading to degradation of material properties, accompanied by a sufficiently large tensile stress state can lead to a pillowing-type failure at reactor shutdown. This paper presents these findings, discusses such failure modes, and the influence of fuel burn-up and power on the magnitude of the shutdown-induced tensile stresses.

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