Abstract
As part of Westinghouse flawless fuel initiatives, Westinghouse has initiated companywide efforts to enhance overall quality of UO_2 pellets by enhancing pellet surface quality, pressing processes, examining pellet handling during various stages of pellet fabrication process and enhancing inspection process with primary focus of reducing pellets with missing surfaces. Benchmarking/best practices were examined among all Westinghouse and some of the Westinghouse licensee sites to minimize pellet chipping, enhance overall pellet quality and improve inspection processes. Numbers of 3 - Dimensional (3-D) finite element models were developed to analyze the impact of cladding and pellet defects on the cladding temperature, stress and strain. The Finite Element Analysis (FEA) results demonstrated that: 1. Large Missing Pellet Surface (MPS) influences the fuel rod performance characteristics such as temperature, stress and strain distributions and 2. MPS dimensions should be controlled to improve the fuel rod performance characteristic impact. Based on the FEA conclusion, review of the in-reactor fuel performance of pellets with chamfers, characterization of pellets with side chips and correlating all of the above, a new set of criteria for side chips was identified and implemented. By analyzing each process step in pellet manufacturing and performing studies to determine forces needed to induce chipping in pellets, a knowledge based handling system can be established. Critical steps were identified and analyzed for handling of pellets between the processes steps of pellet pressing and storage. Extensive process evaluations and tests have been completed in manufacturing to show that manual inspection is effective. As deemed necessary, manual inspection process was modified to further enhance current manual inspection process. Westinghouse has also initiated a development effort to replace manual inspection with an automatic inspection system for pellets. Different sites use different conversion and pelleting processes and therefore site specific improvements are being made where it would have most impact.
