Five Babcock & Wilcox-designed Mark B (15 x 15) pressurized water reactor (PWR) fuel assemblies are being irradiated to extended burnups in Duke Power Company's Oconee Unit 1 reactor. This effort is part of a Department of• Energy-sponsored program to improve uranium utilization by extending the burnup of light water reactor fuel. These fuel assemblies were nondestructively examined after three cycles of irradiation in the Oconee Unit 1 reactor prior to their reinsertion for a fourth cycle of operation (as Oconee 1 cycle 5). Data obtained included fuel assembly and fuel rod dimensiods, water channel spacings, holddown spring preload forces, and fuel column axial gap and stack lengths. The data were within design limits and generally followed projected behavior. This report, covering the January-June 1979 period, is the second semi-annual progress report for the program. Efforts during this period included reduction and analysis of the post-irradiation examination data, performance tracking of the extended burnup assemblies during their fourth cycle, and analytical work. to identify and quantify those factors limiting fuel assembly lifetime. The Oconee Unit 1 reactor operated at a 94% capacity factor during this report period. At the end of the period,'four of the extended burnup assemblies had average burnups of 37,700 MWd/mtU, and the other one had an average burnup of 33,890 MWd/mtU. Irradiation of the fuel assemblies will continue during the next report period.

Baboock & Wilcox CONTENTS. . Moderator aqd Doppler Reaf.ti.vity Coefficients for Equilibrium Cycles .

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