atw 2018-09v3

More documents

Recommendations

Info

atw Vol. 63 (2018) | Issue 8/9 ı August/September 442 FUEL Innovations for the Future Westinghouse EnCore® Accident Tolerant Fuel Gilda Bocock, Robert Oelrich, and Sumit Ray EnCore® and ADOPTTM are trademarks and registered trademarks of Westinghouse Electric Company LLC, its affiliates and/or its subsidiaries in the United States of America and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited. Other names may be trademarks of their respective owners The development and implementation of accident tolerant fuel (ATF) products, such as Westinghouse’s EnCore® Fuel, can support the long-term viability of nuclear energy by enhancing operational safety and decreasing energy costs. The first introduction of Westinghouse EnCore Fuel into a commercial reactor is planned for 2019 as segmented lead test rods (LTRs) utilizing chromium-coated zirconium cladding with uranium silicide (U 3 Si 2 ) pellets. The EnCore Fuel lead test assembly (LTA) program, with LTAs planned for 2022 insertion, will introduce silicon carbide/silicon carbide composite cladding with U 3 Si 2 pellets. Over the past several years, the Westinghouse EnCore Fuel features have been tested in autoclaves, in research reactors, at national laboratories and in the Westinghouse Ultrahigh Temperature Test Facility to confirm and fully understand the science behind ATF materials. Based on the positive results to date, fuel rod and assembly design in preparation for the LTR and LTA programs is underway, as well as licensing efforts with the U.S. Nuclear Regulatory Commission (NRC). Accident analyses, coupled with economic evaluations, have been continuing to define the value of ATF to utilities. These new designs will offer design- basis-altering safety, greater uranium efficiency and significant economic benefits. Adoption of the Westinghouse ATF, in conjunction with a transition to 24-month cycle operation, is the recommended path forward for implementation of the Westinghouse EnCore Fuel. 1 Introduction Nuclear energy remains a fundamental component of many industrialized nations’ energy supply mixes due to its demonstrated reliability in baseload electrical supply, as well as inherent carbon-free energy production. Two factors are critical to maintaining this capability: (a) enhancing safety to help safeguard the plant and public from highly impacting events such as that which occurred at the Fukushima Daiichi Nuclear Power Plant and (b) decreasing operating costs to compete with other sources of energy. The development and implementation of Accident Tolerant Fuel (ATF) products, such as Westinghouse’s EnCore® Fuel features, can support both of these critical factors for long-term operation. Development of nuclear fuels with enhanced accident tolerance is being accelerated to support implementation into commercial reactors as soon as possible. The major objectives for ATF designs include: 1) improved cladding reaction to high-temperature steam; 2) reduced hydrogen generation; and 3) reduced beyond design basis accident source term. In addition to improving safety margins for light water reactors (LWRs), fuel designs using advanced, ATF materials can improve fuel efficiency, enhance debris resistance and extend fuel management capability. Encore Fuel, being developed by Westinghouse Electric Company LLC (Westinghouse), includes two unique accident tolerant or fault tolerant fuel designs: chromium (Cr)-coated zirconium (Zr) alloy cladding with uranium silicide (U 3 Si 2 ) fuel pellets, and silicon carbide (SiC) cladding with U 3 Si 2 fuel pellets. The first introduction of Westinghouse EnCore Fuel into a commercial reactor is planned for 2019 as segmented lead test rods (LTRs). The LTRs will utilize chromium-coated zirconium cladding with U 3 Si 2 highdensity, high-thermal conductivity pellets. The EnCore Fuel lead test assembly (LTA) program, planned for 2022 insertion, will introduce SiC/SiC composite cladding along with chromium- coated zirconium cladding and the high-density, /highthermal conductivity U 3 Si 2 pellets modified to achieve higher oxidation resistance. Over the past several years, Westinghouse’s ATF test program has tested the chromium-coated zirconium and SiC claddings in autoclaves and in the Massachusetts Institute of Technology’s (MIT) reactor and U 3 Si 2 pellets in Idaho National Laboratory’s (INL) Advanced Test Reactor (ATR). Tests in the Ultrahigh Temperature Test Facility at Westinghouse’s U.S. Materials Center of Excellence Hot Cell Facility in Churchill, Pennsylvania, have been carried out to confirm the time and temperature limits for the SiC and chromiumcoated zirconium claddings. Additionally, an extensive research program to fully understand the science behind ATF materials continues with the Westinghouse-led International Collaboration for Advanced Research on Accident Tolerant Fuel (CARAT) group and at United States (US) and United Kingdom (UK) national laboratories. Based on the positive results to date, fuel rod and assembly design in preparation for the LTR and LTA programs is underway, as well as licensing efforts with the U.S. Nuclear Regulatory Commission (NRC), and accident analyses coupled with economic evaluations for both operating savings and fuel savings have been continuing to define the value of ATF to utilities. 2 Lead test rod program LTR programs are an essential step in the introduction of new nuclear fuel technologies into commercial energyproducing reactors. In the EnCore LTR program, two Westinghouse 17x17 optimized fuel assemblies (OFA) will contain up to 20 ATF rods with Cr-coated Zirconium alloy cladding, and U 3 Si 2 and enhanced ADOPT fuel pellets in Exelon’s Byron Unit 2 in Cycle 22. Coated tubes and U 3 Si 2 and ADOPT pellets will be delivered to the Westinghouse Columbia Fuel Fabrication Facility for manufacturing of the assemblies. The shipping date for the assemblies containing the LTRs is February, 2019. Westinghouse is continuing development work with the University of Wisconsin-Madison to continue the optimization of coating performance, and also working with commercial vendors and the U.S. Army Research Lab (ARL) to scale-up production to full-length tubes. The U 3 Si 2 fuel Fuel Westinghouse EnCore® Accident Tolerant Fuel ı Gilda Bocock, Robert Oelrich, and Sumit Ray
atw Vol. 63 (2018) | Issue 8/9 ı August/September Material Process Vendor Maximum Days Titanium Nitride/ Titanium Aluminum Nitride Average Corrosion Rate (mg/dm 2 /day) Std. Dev. Corrosion Rate (mg/dm 2 /day) Average Zr Corrosion (mg/dm/day) Corrosion Rate (microns/year) PVD* PSU** 169 1.07 0.80 2.22 7.67 Chromium Cold spray UW*** 20 0.03 0.06 3.27 0.14 *Physical Vapor Deposition **Pennsylvania State University ***University of Wisconsin | | Tab. 1. Autoclave Corrosion Performance for the Top Zirconium Alloy Coatings. FUEL 443 pellets are being fabricated at INL. The fuel rod and fuel assembly designs are progressing and the manufacturing plan is being refined. 3 Recent testing 3.1 Autoclave testing of ATF claddings A primary benefit of ATF coating is to enhance survivability in high-temperature steam or water conditions, as may occur in postulated accident scenarios. To demonstrate this improved survivability, Westinghouse has performed corrosion testing using the autoclave facility at the Churchill, Pennsylvania site to screen various coatings and SiC preparation methods for corrosion resistance. As part of a multi-year program, more than 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, titanium-nitride/titanium-aluminumnitride and chromium (Table 1), were identified for testing in the MIT reactor. Testing in the MIT reactor further narrowed the options to the chromium coating (Figure 1). The chromiumcoated zirconium showed no signs of peeling and had minimal weight gain after taking into account the uncoated inner surface of the tube. The very positive results from these tests helped validate the viability of the Cr coating for use in LTRs being inserted in a commercial pressurized water reactor (PWR). Initial autoclave and reactor testing resulted in relatively high levels of SiC corrosion. Autoclave testing with hydrogen peroxide was used to simulate the more aggressive oxidation conditions of the reactor and to explore coolant conditions that would minimize SiC corrosion rates. This 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 [2], the current corrosion rates for SiC meet or exceed the target 7of microns/year recession rate. For a full core of SiC cladding, this would result in a maximum of 150 kg of silicon dioxide (SiO 2 ) or about 350 ppm over an 18-month cycle. This is well below the solubility limit of ~700 ppm SiO 2 at the coldest steam generator conditions. Note also that commercially available resins to remove SiO 2 could be added to the current resins used to maintain water chemistry on a continuous basis. In addition to corrosion resistance, crud buildup on the outside surface of fuel rod claddings has long been identified as a potential factor in fuel rod operation, especially at higher operating temperatures. Westinghouse continues to assess the potential for crud buildup on advanced ATF claddings. Limits on crud buildup on SiC claddings are likely to be different than for coated claddings because the SiC surface may be corroding underneath any potential crud buildup. Therefore, testing in the high heat transfer rate and crud deposition test loop (WALT loop) at the Westinghouse facility in Churchill, Pennsylvania, has been carried out from mid-2017 and will continue until 2019 to study heat transfer rates and crud buildup on the SiC and chromium-coated cladding surfaces. Preliminary results indicate a somewhat higher crud deposition rate for chromium-coated cladding than for uncoated zirconium cladding. Surface treatments are being explored to reduce the crud deposition rate. 3.2 High-temperature testing of ATF claddings One goal of the ATF program is to develop fuels that can withstand post-accident temperatures greater than 1,200 °C without the cladding igniting in steam or air. Therefore, a crucial part of the testing carried out by Westinghouse during the previous year was aimed at quantifying the maximum temperature at which the ATF claddings could operate without excessive corrosion. The test apparatus (Figure 2) currently uses a graphite rod which is inserted into insulation and then into the test piece. | | Fig. 1. Chromium-coated zirconium alloy tubes before and after testing in the MIT reactor [1]. | | Fig. 2. SiC rodlet undergoing testing in the ultra-high temperature apparatus in steam at 1600°C at Churchill. The sample is mounted inside the shield tube that is glowing white in the photograph. The SiC tube is inside the shield tube with steam injected both above and below the sample. The steam exits through the hole that is visible in the shield tube. This results in a very stable heating of the test pieces. Chromium-coated zirconium has now been tested at up to 1,500 °C. This is above the chromium- zirconium low melting eutectic point of 1,333 °C. At 1,400 °C, there was noticeable reaction between the Cr and the Zr. However, there was not the rapid oxidation that Fuel Westinghouse EnCore® Accident Tolerant Fuel ı Gilda Bocock, Robert Oelrich, and Sumit Ray
Page 1 and 2: nucmag.com 2018 8/9 437 Akademik Lo
Page 3 and 4: atw Vol. 63 (2018) | Issue 8/9 ı A
Page 5 and 6: Kommunikation und Training für Ker
Page 17: atw Vol. 63 (2018) | Issue 8/9 ı A
Page 67 and 68: Kommunikation und Training für Ker

atw 2018-09v3

Create successful ePaper yourself

Delete template?

Save as template?