Investigation of the Cause of Low Blister Threshold Temperatures in the RERTR-12 and AFIP-4 Experiments

Abstract

SUMMARYBlister-threshold testing of fuel plates is a standard method through which the safety margin for operation of plate-type in research and test reactors is assessed. The blister-threshold temperature is indicative of the ability of fuel to operate at high temperatures for short periods of time (transient conditions) without failure. This method of testing was applied to the newly developed U-Mo monolithic fuel system. Blister annealing studies on the U-Mo monolithic fuel plates began in 2007, with the Reduced Enrichment for Research and Test Reactors (RERTR)-6 experiment, and they have continued as the U-Mo fuel system has evolved through the research and development process.Blister anneal threshold temperatures from early irradiation experiments (RERTR-6 through RERTR-10) ranged from 400 to 500°C. These temperatures were projected to be acceptable for NRC-licensed research reactors and the high-power Advanced Test Reactor (ATR) and the High Flux Isotope Reactor (HFIR) based on current safety analysis reports (SARs). Initial blister testing results from the RERTR-12 experiment capsules X1 and X2 showed a decrease in the blister-threshold temperatures. Blister threshold temperatures from this experiment ranged from 300 to 400°C. Selected plates from the AFIP-4 experiment, which was fabricated using a process similar to that used to fabricate the RERTR-12 experiment, also underwent blister testing to determine whether results would be similar. The measured blister-threshold temperatures from the AFIP-4 plates fell within the same blister-threshold temperature range measured in the RERTR-12 plates. The most prominent theory, supported by the data, is that low blister-threshold temperature is the result of mechanical energy imparted on the samples during the fabrication process (hot and cold rolling) without adequate post processing (annealing). The mechanisms are not clearly understood and require further investigation, but can be divided into two categories:x Residual Stress x Undesirable interaction boundary and/or U-Mo microstructure change vi A secondary theory that cannot be dismissed with the information that is currently available is that a change in the test conditions has resulted in a statistically significant downward shift of measured blister temperature.This report outlines the results of the forensic investigations conducted to date. The data and conclusions presented in this report are preliminary. An additional report will be generated when the entirety of the work in PLN-4155 is completed. Definitive cause and effect relationships will be established by future experimental programs.vii

Prediction and Deformation Mechanism Analysis of High Porosity in U–10Mo Monolithic Fuels at High Burnup

“…The fuel plate L1P7A0, consisting of the U-Mo fuel foil, Al alloy cladding and Zr alloy diffusion barrier in the RERTE-12 campaign, was chosen to be simulated [24]. The U-10Mo fuel foil was prepared using hot rolling followed by cold rolling [25]. The Zr diffusion barrier layers were co-rolled on both surfaces of the fuel foil using a hot corolling process [25].…”

“…The U-10Mo fuel foil was prepared using hot rolling followed by cold rolling [25]. The Zr diffusion barrier layers were co-rolled on both surfaces of the fuel foil using a hot corolling process [25]. Then, the Al cladding was subsequently bonded to the fuel foil by hot isostatic pressing (HIP) [9,11,25].…”

“…The Zr diffusion barrier layers were co-rolled on both surfaces of the fuel foil using a hot corolling process [25]. Then, the Al cladding was subsequently bonded to the fuel foil by hot isostatic pressing (HIP) [9,11,25]. Hence, the interfaces of the fuel plate were well-bonded and stable [26].…”