"Effects of Accelerated Neutron Irradiation on the Microstructure of U-10 wt.% Zr and U-10 wt.% Mo with and without Zr liners" 

Nicole Rodriguez Perez, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

The new generation of nuclear reactor technology requires the development of new fuels capable of complying with modern safety, reliability, and fuel economy requirements. Metallic fuels, such as uranium-10 weight.% zirconium (U-10 wt.% Zr), are promising candidates due to the irradiation performance observed in experimental fast reactors, which include operation at moderate burn-ups, up to 18 at.%, without failure. More recently, U-10 wt.% molybdenum (U-10 wt.% Mo) is being considered for various nuclear fuel applications, since U-Mo has the possibility of achieving a higher U235 density when compared to ceramic UO2, which can be advantageous for small modular reactors and/or longer fuel cycles. The challenge is to validate the new fuel chemistry and designs in a timely manner, since irradiation testing is expensive, time-consuming, and requires high levels of safety management. To reduce the irradiation time, a fission-accelerated steady-state testing (FAST) method was developed using geometrically scaled fuel specimens that allow target burn-ups to be reached in shorter irradiation times. With the change in geometry, and irradiation and temperature profiles, the resulting FAST microstructure might not yield comparable results to full-length fuel assemblies. Hence, the current work focused on analyzing U-10 wt.% Zr and U-10 wt.% Mo fuels irradiated using FAST to understand the effects on the fuel microstructure. The fuel microstructure impacts irradiation performance phenomena such as swelling, constituent redistribution, and fuel-cladding chemical interactions (FCCI). The chemistry, crystal structure, phase distribution, porosity, and FCCI of the specimens were analyzed and compared with historical data from prototypical fuels to determine the capability of FAST to duplicate historical irradiation effects. It was determined that FAST can provide microstructures consistent with historical observations but in a limited manner, only providing similar microstructural features as certain areas of the prototypical fuel where the same irradiation conditions are met. Nevertheless, FAST maintains global tendencies, such as porosity and phase distributions across the fuel. As a result, FAST reproduces the global distribution tendencies for porosity and phases in a full-length pin, but exhibits differences in the localized microstructure. Additionally, FAST evaluated the use of Zr liners as barriers to prevent FCCI, a phenomenon that causes cladding failure and limits fuel burn-up. Thus, Zr liners can prevent FCCI and allow for higher burn-ups. This study determined that Zr liners can prevent FCCI, but the fabrication process and thickness of the liners are crucial parameters to prevent fission product migration through the Zr liner into the cladding.