Evaluation of missing pellet surface geometry on cladding stress distribution and magnitude
Abstract Missing pellet surface (MPS) defects are local geometric defects in nuclear fuel pellets that result from pellet mishandling or manufacturing. The presence of MPS defects can cause significant clad stress concentrations that can lead to through-wall cladding failure for certain combinations of fuel burnup, and reactor power level or power change. Consequently, the impact of MPS defects has limited the rate of power increase, or ramp rate, in both pressurized and boiling water reactors (PWRs and BWRs, respectively). Improved three-dimensional (3-D) fuel performance models of MPS defect geometry can provide better understanding of the probability for pellet clad mechanical interaction (PCMI), and correspondingly the available margin against cladding failure by stress corrosion cracking (SCC). The Consortium of Advanced Simulations of Light Water Reactors (CASL) has been developing the Bison-CASL fuel performance code to consider the inherently multi-physics and multi-dimensional mechanisms that control fuel behavior, including cladding stress concentrations resulting from MPS defects. This paper evaluates the cladding hoop stress distributions as a function of MPS defect geometry with discrete pellet radial cracks for a set of typical operating conditions in a PWR fuel rod. The results provide a first step toward a probabilistic approach to assess cladding failure during power maneuvers. This analysis provides insight into how varying pellet defect geometries affect the distribution of the cladding stress, as well as the temperature distributions within the fuel and clad; and are used to develop stress concentration factors for comparing 2-D and 3-D models.
A Theoretical Analysis of Anderson Acceleration and Its Application in Multiphysics Simulation for Light-Water Reactors.
TOTH, ALEXANDER RAYMOND. A Theoretical Analysis of Anderson Acceleration and Its Application in Multiphysics Simulation for Light-Water Reactors. (Under the direction of Carl Kelley.) In this work, we are concerned with both contributing to the theoretical foundation for Anderson acceleration, a method for accelerating the convergence rate of Picard iteration, and evaluating its performance in the context of coupled multiphysics problems in nuclear reactor simulation. Anderson acceleration proceeds by maintaining a depth of previous iterate information in order to compute a new iterate as a linear combination of previous evaluations of the fixed-point map, where the linear combination coefficients are obtained by solving a linear leastsquares problem. Prior to this work, theory for this method was fairly sparse, dealing mainly with showing its relation to quasi-Newton multisecant updating and, when applied to linear problems, GMRES iteration. The analysis presented in this work significantly expands upon the theory for this method. As this method is intended as an acceleration method for Picard iteration, our analysis concerns problems for which Picard iteration is convergent, namely when the fixed-point mapping is contractive. We present analysis which represent the first convergence results for limited-memory variations of Anderson acceleration and for nonlinear problems. Additionally, we present analysis for several variations on the standard Anderson acceleration method. In particular, we consider a variation which adjusts the memory utilization in order to maintain good conditioning of the least-squares problem, and we present local improvement results for the case in which the fixed-point map can only be evaluated approximately. With respect to coupled multiphysics problems, we examine Anderson acceleration as an alternative to Picard iteration in the context of black-box code coupling in nuclear reactor simulation. Picard iteration comes with several drawbacks in this context, namely relatively slow convergence and poor robustness. To test the potential for Anderson acceleration to improve upon the weaknesses of Picard iteration, we first consider a one-dimensional model problem which recreates several phenomena observed in higher fidelity couplings. We then consider the Tiamat code coupling being developed as part of the Consortium for Advanced Simulation of LWRs (CASL). Tiamat couples the Bison fuel performance code with the MPACT neutronics and COBRA-TF thermal hydraulics codes to provide a tool for pellet-cladding interaction analysis. Prior to this work, this code utilized exclusively Picard iteration to couple these single-physics codes. We overview Tiamat and describe how Anderson acceleration has been integrated into this coupling by posing the coupled system as a fixed-point problem in terms of coupling parameters. We then examine the performance gains obtained from utilizing Anderson acceleration for this coupling by considering parameter studies at various problem sizes. © Copyright 2016 by Alexander Raymond Toth
Deformation of UO2 at High Temperatures
The effects of temperature, strain rate, and grain size on the mechanical properties of UO2 were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, Tc, the material fractured in a brittle manner, with no macroscopic plastic deformation. Between Tc and a second transition at a higher temperature, Tt, a small amount of plastic deformation was measured before fracture. Beyond Tt, the strength of UO2 decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above Tt: 
 
 
 
and 
 
 
 
where σp and σ88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.