Westinghouse Accident Tolerant Fuel Materials

Westinghouse is commercializing two unique accident tolerant fuels (ATFs): silicon carbide (SiC) as produced by General Atomics with uranium silicide (U3Si2) fuel and Cr coated zirconium alloy cladding with U3Si2 fuel. Testing of the cladding alternatives in autoclaves has been performed and samples have begun irradiation at the Massachusetts Institute of Technology Reactor and the Halden Project Reactor. Uranium silicide fuel is undergoing exposure in the Advanced Test Reactor and fuel pins have been removed and are undergoing post irradiation examination (PIE) at the Idaho National Laboratory (INL). This paper provides an update on these activities and a summary of results. INTRODUCTION AND BACKGROUND The Westinghouse Electric Company LLC (Westinghouse) accident tolerant fuel (ATF) program utilizes Cr coated zirconium alloy (CZA) cladding with U3Si2 high density/high thermal conductivity fuel for its lead test rod (LTR) program with irradiation beginning in 2019. The lead test assembly (LTA) program will use both SiC/SiC composites from General Atomics and Cr coated zirconium alloy claddings with the high density/high thermal conductivity U3Si2 pellet which will begin in 2022. Over the past several years, Westinghouse has tested the Cr coated zirconium and SiC claddings in autoclaves and in the Massachusetts Institute of Technology (MIT) reactor and U3Si2 pellets in the Advanced Test Reactor (ATR). High temperature tests at the state-of-the-art facilities in Churchill, PA have been carried out to determine the time and temperature limits for the SiC and Cr coated zirconium claddings. WESTINGHOUSE ATF ACTIVITIES Autoclave Corrosion Testing Westinghouse has performed corrosion testing using the autoclave facility at the Churchill, PA site to screen various coatings and SiC preparation methods for corrosion resistance. As part of a multi-year program, over 12 types of coatings on zirconium alloys and approximately 10 versions of SiC have been tested in autoclaves. As a result of this testing, two coatings (Table I) were identified for testing in the MIT reactor. Testing in the MIT reactor further narrowed the options to the Cr coating. Based on the positive test results, Westinghouse is now exploring methods for production of full length rods for LTRs to be constructed in 2018 for inclusion in a commercial reactor in early 2019. 3 CO PY RI GH TE D M AT ER IA L Table I – Top Zirconium Alloy Coatings Autoclave Corrosion Performance At 360oC Water Material Proces s Vendor Maximu m Days Average Corrosion rate (mg/dm2/day ) Average Zr Corrosion (mg/dm2/day ) Corrosion Rate ( /year) TiN/TiAlN PVD Pennsylvani a State University 169 1.07 2.22 7.67 Cr Cold spray University of Wisconsin, Madison 20 0.03 3.27 0.14 Initial autoclave and reactor testing indicated relatively high levels of SiC corrosion. Autoclave testing with hydrogen peroxide was used to simulate more aggressive oxidation conditions of the reactor and to explore coolant conditions that would minimize SiC corrosion rates. The full battery of testing has been used to refine the manufacturing parameters of the SiC composites such that along with hydrogen addition to the primary coolant above 40 cc/kg [1], the current corrosion rates for SiC meet or exceed the target 7 microns/year recession rate. For a full core of SiC cladding, this would result in a maximum of 150 kg of SiO2 or about 300 ppm over an 18 month cycle. This is well below the solubility limit of ~700 ppm SiO2 at the coldest steam generator conditions. Note also, that resins are commercially available that could be added to the current resins used to maintain water chemistry to remove SiO2 on a continuous basis. High Temperature Testing The goal of the ATF program is to develop fuels that can withstand post-accident temperatures greater than 1200oC without the cladding igniting in steam or air. Therefore a crucial part of the testing carried out by Westinghouse over the previous year was aimed at quantifying the maximum temperature at which the ATF claddings could operate without excessive corrosion. The test apparatus first used current applied directly to the coated zirconium tubes. However, it was found that as the temperatures increased, issues with the connection of the test piece to the current source caused excessive resistance resulting in excessive heating and then burnout of the samples at the connection point. This direct heating method was then replaced with a graphite rod which was inserted into insulation and then into the test piece. This resulted in very stable heating of the test pieces. CZAs have now been run at up to 1400oC. This is above the Cr-Zr low melting eutectic point of 1333oC. At 1400oC, there was noticeable reaction between the Cr and the Zr. However, there was not the rapid oxidation that uncoated Zr experiences at 1200oC, so that there is likely some reasonable residence time that the cladding could survive at temperatures above 1400oC.