Shutdown-induced tensile stress in monolithic miniplates as a possible cause of plate pillowing at very high burnup

Highlights: • Tensile stress in the fuel increased from 2 MPa to 100 MPa at shutdown. • This may explain pillowing of several test fuel plates irradiated to very high burnup. • Implications for safe use of monolithic fuel in research reactors are discussed. • Effect of burnup and power on shutdown-induced tensile stress explored. - Abstract: Post-irradiation examination of RERTR-12 miniplates showed that in-reactor pillowing occurred in at least 4 plates, rendering performance of these plates unacceptable. To address in-reactor failures, efforts are underway to define the mechanisms responsible for in-reactor pillowing, and to suggest improvements to the fuel plate design and operational conditions. To achieve these objectives, the mechanical response of monolithic fuel to fission and thermally induced stresses was modeled using a commercial finite element analysis code. Calculations of stresses and deformations in monolithic miniplates during irradiation and after the shutdown revealed that the tensile stress generated in the fuel increased from 2 MPa to 100 MPa at shutdown. The increase in tensile stress at shutdown possibly explains in-reactor pillowing of several RERTR-12 miniplates irradiated to the peak local burnup of up to 1.11 × 10²² fissions/cm³. This paper presents the modeling approach and calculation results, and compares results with post-irradiation examinations and mechanical testing of irradiated fuel. The implications for the safe use of the monolithic fuel in research reactors are discussed, including the influence of fuel burnup and power on the magnitude of the shutdown-induced tensile stress.