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2a Cladding Elongation Data at Beginning-of-Life 2b Comparison of Cladding Elongation Data to Code Calculations 3 Flow Diagram for Program BM2DNS 4a Crack Pattern with Circumferential Crack. 4b Radial Cracks Only • 5 Crack Pattern of Pellet 39 from Rod 6 of IFA-431 6 Thermal Conductivity Factor Versus Hydrostatic Stress 7 The Pellet-Cladding Gap Under Hydrostatic Conditions 8 Radial Elastic Modulus as a Function of Fuel Available Void 9 Axial Elastic Modulus as a Function of Total Available Void 10 Relationship Between Radial and Axial Elastic Moduli for Cracked Pellets .

The authors wish to thank the Core Performance Branch of the Nuclear Regulatory Commission, Division of Systems Safety, for their support and guidance of GAPCON-THERMAL-3 development. We would also like to acknowledge the review comments by W. J. Bailey, K. Granzow and C. R. Hann.

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A simulated loss-of-coolant accident was performed with a full-length test bundle of pressurized water reactor fuel rods. This third experiment of the program produced fuel cladding temperatures exceeding 1033 K (1400°F) for 155 s and resulted in eight ruptured fuel rods. Experiment data and initial results are presented in the form of photographs and graphical summaries. v SUMMARYThe loss-of-coolant accident (LOCA) simulation program in the National Research Universal (NRU) reactor is being conducted to evaluate the thermal hydraulic and mechanical deformation behavior of a full-length pressurized water reactor fuel bundle under LOCA conditions. The test conditions are designed to simulate the heatup, reflood, and quench phases of a large-break LOCA and are performed in situ using nuclear power fission heat to simulate low-leve1 decay power typical of these conditions. This document reports the data and initial results from tt ~ third experiment in the program, Materials Test Two (MT-2). This test series had four major objectives:• to maintain Zircaloy high alpha phase temperatures for greater than 100 s to obtain maximum fuel cladding ballooning and rupture conditions• demonstrate the Disassembly Reassembly Machine (DERM) in reconstituting a test bundle underwater• demonstrate and develop bundle gauging and profilometry• evaluate variable reflood rate characteristics and control to maximize the time in the Zircaloy high alpha-phase temperature ballooning and rupture window.The results of the tests showed that, by using variable reflood conditions, it is possible to alter the peak cladding temperature during the reflood phase. Temperatures in excess of 1033 K (1400°F) were maintained for 155 s. The thermal-hydraulic results were based on using manual selection of three levels of reflood rates of 1.27 cm/s (0.5 in/s), 2.5 cm/s (1.0 in/s), and 4.06 cm/s (1.6 in/s). Ballooning and rupture of the test fuel rod cladding occurred during the adiabatic heatup phase of the experiment. The OERM operation was successful in both performing bundle examination and profilometry and in reconstituting the test assembly under water.Postirradiation examination using the DERM revealed coplaner blockage and rod-to-rod contact at the maximum blockage areas. Significant decreases vii in cladding strain at self-powered neutron detectors (SPNDs) were also noted, which indicates the sensitivity of the deformation of Zircaloy to small changes in cladding temperature.This report presents the preliminary graphical data and photographs of fuel rod temperatures, test conditions, and cladding mechanical deformation axial profiles obtained during this experiment.viii ACKNOWLEDGMENTS