Fully Coupled Material Behavior Analysis of Accident Tolerant Fuels and Claddings

完全耦合的事故容錯燃料及包殼的性能分析

Student thesis: Doctoral Thesis

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Award date29 Aug 2018

Abstract

UO2 has a long history being used as a nuclear fuel. However, its low thermal conductivity has become one of the major concerns limiting nuclear reactor performance and safety. Enhanced thermal conductivity nuclear fuels become one of the most possible solutions of accident-tolerant fuel systems. The current research results on the UO2-BeO composite fuel have indicated BeO can be a good candidate to fabricate composite fuel together with UO2. Due to its excellent thermal conductivity compared to pure UO2 fuel, UO2-BeO has become one of the most promising accident tolerant nuclear fuels. Two different UO2-BeO fabrication methods have demonstrated the capability in fabricating enhanced thermal conductivity UO2-BeO composite fuels and improving fuel performance with a distinguishable difference. To calculate the thermal conductivity of UO2-BeO composite, Hasselman-Johnson model has been applied, which shows good agreement with experimental data. In this model, it includes the influence of thermal conductivity of matrix and particle, volume fraction of particle, radius of particle and the interfacial thermal conductivity between matrix and particle. In this thesis, the thermal conductivity has been intensively studied as well as fuel performance in two different types of UO2-BeO fuels. A novel composite nuclear fuel of UO2-GaN, which has never been reported before, and its fully coupled multiphysics fuel performance has also been investigated using the CAMPUS code in this thesis. Two different fabrication methods have been proposed to obtain the UO2-GaN fuel, which are Green Granule/Slug Bisque and Spark Plasma Sintering, respectively, and resulting in different fuel thermal conductivities, like UO2-BeO composite. The gap width, gap conductance, fission gas release, plenum pressure, deviation of oxygen to metal ratio and displacement of two kinds of UO2-GaN fuel are all studied in this work. The performance of this new fuel is also compared with the traditional UO2 fuel and other types of high thermal conductivity doping UO2 fuel. The enhanced thermal conductivity UO2-GaN composite fuel shows the potential of decreasing the fuel temperature, and improving fuel performance and reactor safety, which makes GaN a good candidate to fabricate composite fuel with UO2 from the thermal standpoint.

The thermophysical performance and solid mechanics behavior of UO2-36.4vol % BeO fuel pellets cladded with Zircaloy, SiC, and FeCrAl, and Zircaloy cladding materials coated with SiC and FeCrAl, are investigated based on simulation results obtained by the CAMPUS code. Besides, the effect of coating thickness (0.5, 1 and 1.5 mm) on fuel performance and mechanical interaction is discussed. The modeling results show that Zircaloy claddings are more effective in decreasing fuel centerline temperature and fission gas release than other kinds of cladding material because of the smaller gap between cladding and fuel at the same burnup. SiC claddings and SiC-coated Zircaloy claddings possess smaller plenum pressure than other kinds of the cladding. SiC claddings contribute more to fuel radial displacement but less to fuel axial displacement. FeCrAl claddings exhibit very different radial and axial displacements in different axial positions. FeCrAl-coated Zircaloy claddings have a lower fuel centerline temperature than Zircaloy claddings at burnup below 850 MWh/kg U, but a higher fuel centerline temperature at higher burnup. The gap between FeCrAl-coated Zircaloy claddings and fuel pellets closes earlier than that of Zircaloy claddings. SiC-coated claddings increase fuel radial and axial displacements, and cladding axial displacements of inner and outer cladding surfaces.