Due to extreme working environment of nuclear reactors, such as high temperature and high levels of radiation, nuclear fuel microstructure experiences substantial changes with fuel burnup. Radiation-induced pellet swelling closes the original gap between pellet rims and cladding tube inner surfaces, leading to pellet-cladding mechanical interaction (PCMI). On the other hand, due to high fission density at the fuel rim section, microstructure will experience irradiation induced “grain subdivision” featured by 100-300 nm fresh defect free UO2 grains, forming the so-called high burnup structure (HBS). UO2 fuel microstructure evolution and pellet-cladding mechanical interaction profoundly impact the fuel performance of the reactor systems. A better understanding on how PCMI and the formation of HBS affect the mechanical properties and fuel performance is of significance for enhanced safety margin of the fuel systems, and also enhanced accident tolerance. Additionally, US DOE NEAMS program is spending tremendous efforts in developing advanced fuel modeling in predicating fuel performance based upon MARMOT-BISON-MOOSE framework. Multi-scale and multi-physics models were developed including fracture MARMOT modeling as functions of microstructures. However, no experimental data are available in the mechanical properties and fracture mechanisms as functions of microstructure in order to validate the multi-physics fracture modeling. Key challenges exist in synthesizing and fabricating oxide fuel matrix with well controlled microstructure and also different length scales (e.g., from conventional micron-sized fuel matrix to nano-scale crystalline matrix as observed in HBS) for separate effects.

Upon the support of a DOE NEAMS program (DE – NE 00084) in validating thermal transport and fracture behavior of the MARMOT models, oxide fuels have been sintered by spark plasma sintering with controlled microstructures, enabling the possibility of performing separate effects to obtain critical experimental data for model validation. In this Rapid turn-around proposal, we propose to utilize the capability of the UC Berkeley Nuclear Materials Laboratory, as a part of NSUF, to investigate mechanical properties and fracture mechanisms of the sintered UO2 fuels. The key components of the NSUF RTE proposal include: (1) Nano-indentation experiments to obtain hardness and fracture toughness of sintered UO2 as functions of grain size and temperatures; and (2) FIB sample preparation of lamellas underneath dents from micromechanical test techniques and microstructure characterization by TEM to investigate crack propagation and tip morphologies.

The designed micromechanical property tests of dense UO2 fuel pellets with various grain size, including nanoscale and microscale, will allow us to gain fundamental knowledge of the UO2 fuel performance especially when High burnup structure (HBS) is formed during in-pile burning. Nanoindenation tests at intermediate temperature similar to the fuel rim temperature will provide a true reflection of mechanical behavior of UO2 fuel pellet, possibly the transition of fracture mechanism from brittle to ductile. Localized microstructure deformation from nanoindentation will be studied using TEM focusing on crack morphology and propagating path to provide microstructure features that governing the contact deformation behavior. Generated data will be served as the experimental validation for the currently active NEUP project for RPI team entitled by “Thermal Transport and Fracture Behavior of Sintered Fuel Pellets: Experimental Validation of NEAMS tool MARMOT” (DE – NE 00084).