Fuel R & D to Improve Fuel Reliability

Transcription

1 Journal of Nuclear Science and Technology ISSN: (Print) (Online) Journal homepage: Fuel R & D to Improve Fuel Reliability Rosa YANG, Bo CHENG, Jeff DESHON, Kurt EDSINGER & Odelli OZER To cite this article: Rosa YANG, Bo CHENG, Jeff DESHON, Kurt EDSINGER & Odelli OZER (2006) Fuel R & D to Improve Fuel Reliability, Journal of Nuclear Science and Technology, 43:9, To link to this article: Published online: 05 Jan Submit your article to this journal Article views: 501 Citing articles: 12 View citing articles Full Terms & Conditions of access and use can be found at

2 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 43, No. 9, p (2006) REVIEW Fuel R & D to Improve Fuel Reliability Rosa YANG, Bo CHENG, Jeff DESHON, Kurt EDSINGER and Odelli OZER EPRI, 3412 Hillview Avenue, Palo Alto, CA 94304, USA (Received January 16, 2006 and accepted in revised form April 28, 2006) Light water reactor fuel is operating in an increasingly challenging environment. Fuel burnup extension and cycle length increase both can increase the local duty. Reactor water chemistry modifications for the purpose of protection the plant system materials have the potential of increasing fuel surface deposition and cladding corrosion and hydriding. The status of fuel performance in US reactors is summarized and an update of the Fuel Reliability Program established by the utility industry to ensure reliability is provided. KEYWORDS: fuel reliability, fuel performance, axial offset anomalies, fuel cladding corrosion, crud control, hydrogen water chemistry, zinc injection I. Introduction Challenges to Fuel Reliability Pressures to improve fuel cycle economics and plant performance continue to result in light water reactor (LWR) fuel being subjected to new and increasingly challenging operating environments. Challenges to fuel performance have resulted from a combination of plant uprates, increasing discharge burnups (Fig. 1), longer fuel cycles (Fig. 2), higher enrichments and more aggressive water chemistry conditions introduced to mitigate plant material degradation and reduce shutdown dose rate. Although current fuel failure rates in the US remain at considerably lower levels than in the late 1970s and 80s, there has been a noticeable increase in failures, particularly in BWRs, during the last few years (Fig. 3). The increase in BWR failures has been due primarily to corrosion-related and fuel-duty related mechanisms (Fig. 4). The root cause of some of the failures has not been established yet. The analysis is complicated because of coolant chemistry changes introduced for mitigating material degradation and dose control. Debris fretting also remains a problem even after the introduction of debris filters. Continued effort by utilities, through improved foreign material exclusion programs and improved debris filter designs by the vendors, should reduce this type of failure. The primary contributor to PWR failure rates remains grid-to-rod fretting; however, experience with new grid designs appears promising. On the other hand, there has been an increase in the number of failures primarily from optimized fuel designs with a thinner rod diameter (Fig. 5). The root cause of the failures is unknown. In addition, some PCI-suspect failures have been experienced at three B&Wdesigned PWR plants following the movement of axial power shaping rods (APSRs). It is acknowledged that fuel failures can be quite costly. Although typical costs are difficult to quantify, even a single failed BWR rod can cost more than $1,000,000 in outage time, fuel and power replacement costs. Failures affecting Corresponding author, ryang@epri.com ÓAtomic Energy Society of Japan a larger fraction of a reload, e.g., crud/corrosion failures, can easily run in the tens-of-millions of dollars. II. The Fuel Reliability Program (FRP) Highlights The Electric Power Research Institute under the sponsorship of the utility industry launched the Fuel Reliability Program in 1998 (initially under the name of Robust Fuel Program) in recognition of the fact that continuing pressures to improve fuel cycle economics were resulting in LWR fuel being subjected to new and increasingly challenging operating environments. As a result of a gap analysis and prioritization effort, 1) the program activities have been re-grouped into the following four working group areas:. PWR Fuel Corrosion and Crud Control. BWR Fuel Corrosion and Crud Control. Fuel Performance and Reliability. Regulatory Issues A key feature of the FRP is that it is a utility-driven program with international participation. The program collaborates closely with fuel vendors and industry organizations such as INPO and NEI. Its aims are to complement vendor R/D efforts by determining margins in fuel products that are either currently licensed or are clearly licensable to ensure their safety and reliability under the anticipated operating environments. FRP was formulated as a multi-year program with the understanding that resolution of many of the key issues and achievement of program objectives will require a sustained, multi-phase effort. This paper provides an update in the first three areas (not including the regualtory issues) since the publication of reference 1 in the areas related to fuel reliability. 1. PWR Fuel Corrosion and Crud Control The effort in this area continues to be directed into two topics: (a) Understanding and controlling PWR crud and (b) determining the effects of water chemistry changes on fuel in PWRs. In the area of understanding and controlling crud, significant progress has been made. Extensive research over the 951

3 952 R. YANG et al PWR BWR GWD/MTU Year Fig. 1 Average Discharge Burn-up US Fig. 2 Average Cycle Length US past several years in crud characterization and deposition mechanisms have increased our knowledge in the crud deposition and growth processes. This research resulted in a report with extensive data on PWR feul crud in Crud was scraped from nine US PWR cores and analyzed using specialized sample preparation procedures and analytical techniques. Also included in the report were analysis results from high temperature samplers installed at two PWR units. The advantage of these samples is that a high fraction of the particulate-born circulating material remains in its crystalline form and does not dissolve from being cooled under normal sample collection procedures. The data from the samplers were used to offer insight into what is circulating in the coolant during the operating cycle versus what is meas- JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

4 Fuel R & D to Improve Fuel Reliability 953 # of Failed Assemblies / GWe PWR BWR Year Fig. 3 Trend in US fuel failure rates Crud/Corrosion PCI-SCC Debris Fabrication Unknown/Uninspected Number of Failures Year Fig. 4 Trend in US BWR failure root causes ured from crud scrapes following shutdowns. The steaming rate at the fuel cladding surface had a profound effect on core deposits and axial offset anomaly (AOA). High assembly/rod steaming rates produced the following effects:. Crud was concentrated in areas with high steaming rates. VOL. 43, NO. 9, SEPTEMBER 2006

5 954 R. YANG et al. Number of Failures Grid Fretting Crud/Corrosion PCI-SCC Debris Handling Damage Fabrication Unknown/Uninspected Year Fig. 5 Trend in US PWR failure root causes. Thickness of deposits generally increased at the boiling duty.. New crud forms were produced at locations with subcooled boiling (bonaccordite, needles with high nickel content).. Layered structures developed, including zones rich in zirconium oxide. The layered deposits were shown to resist dissolution, and, thus, create the potential to present a self-perpetuating problem. Of the coolant chemistry parameters that were considered, coolant zinc concentration appeared to have the strongest influence on crud structure, making it thinner, less crystalline, more mobile, and richer in chromium. Carbon was found in core deposits where zinc was added, and while the acetate form in which the zinc is added is likely responsible for the carbon presence, this isn t certain. The high-temperature coolant analyses established that sub-micron metallic nickel particles are a common component of circulating crud during operation. Such particles are probably responsible for a large fraction of nickel released at shutdown. This knowledge should be useful in optimizing shutdown chemistry. While a common component to fuel crud, no NiO was observed in the high temperature samples, reflecting the fact that under normal coolant conditions (e.g. hydrogen ranging from cc/kg), NiO is not thermodynamically stable over most system surfaces. This suggests it s presence in fuel crud is a result of the local conditions present at the clad surface and within the crud. Results from the crud studies helped lead to the publication of the PWR Axial Offset Anomaly (AOA) Guidelines in This document provides guidance to utilities on how changes in various plant parameters, core design and chemistry can impact crud deposition and AOA. The crud data will also be used to improve EPRI s Boron-Induced Offset Anomaly (BOA) Code. The BOA code is an integrated thermal hydraulic chemistry software package that predicts where crud will deposit, its thickness and the susceptibility of a particular core design to AOA. Additionally, more utilities are using the FRP-developed ultrasonic fuel cleaning technology to remove crud from reload fuel. During spring 2005, this new technology was applied at seven US reactors and one in Spain. By the end of 2005, a total of 13 reactors worldwide will have used this technology. Through years of R & D effort, several tools (Fig. 6) are available for plants to manage and mitigate the impact of crud deposition and AOA on plant operation. Mindful of the interrelationship between fuel duty and chemistry, FRP has established active plant demonstrations at two PWRs operating outside the general fuel-duty/chemistry experience base. The specific programs under FRP sponsorship are designed to evaluate reactor coolant zinc chemistry under aggressive core design conditions, and elevated and constant ph (ph t =7.4) with lithium levels as high as 6 ppm at beginning-of-cycle. As the demonstrations mature and assuming no adverse effects are realized, these programs will pave the way for many more PWRs to apply similar chemistry regimes without necessarily incurring the high cost of fuel examinations for every plant at the end of each operating cycle. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

6 Fuel R & D to Improve Fuel Reliability 955 Fig. 6 Tools to Mitigating PWR Crud and AOA Fig. 7 Example of tenacious crud and surface spallation 2. BWR Fuel Corrosion and Crud Control This is a new Working Group established in 2004 to focus on addressing emerging BWR fuel reliability issues associated with fuel cladding corrosion, crud deposition and spallation, and evolving water chemistry conditions. During , three US BWRs experienced fuel failures due to accelerated cladding corrosion. Understanding the root causes of those high impact fuel failures is needed to avoid future recurrence of similar failures. Another fuel reliability concern relates to observations of thick tenacious crud and surface spallation on some fuel rods at plants with noble metal chemical application (NMCA) and zinc injection, Fig. 7. While no fuel failure has been attributed to the NMCA, 2) avoidance of thick tenacious crud and surface spallation is essential to long-term fuel reliability. This working group coordinates with other EPRI programs to ensure that water chemistry changes for plant material protection and doserate reduction will not compromise the integrity of fuel rods. A new approach to characterize crud has been developed and first applied at River Bend. The new technique utilizes a metal blade to scrape the fuel surface to obtain flakes of the tenacious crud. The crud flakes are analyzed in a hot laboratory for morphology and distribution of elements in the flakes. The results have enhanced our understanding of how tenacious crud may become harmful to fuel heat transfer and causing fuel failures. This technique has now been employed to study crud flakes from several other BWRs. Results to date indicate that zinc ferrite is the major constituent in tenacious crud at plants with zinc injection. Co-deposition of zinc silicate and/or copper oxide in the zinc ferrite crud may occur in some cases, Fig. 8. Deposition of multiple compounds on the fuel surface may result in degradation of the local heat transfer of the fuel rod. An extensive study on the solubility and stability of various zinc compounds has VOL. 43, NO. 9, SEPTEMBER 2006

7 956 R. YANG et al. Fig. 8 Tenacious crud deposit rich in zinc ferrite and zinc silicate Fig. 9 Water chemistry changes in US BWRs been initiated to study the pathways that would lead to deposition of the various elements in the tenacious crud. The result of the study is expected to provide the technical basis for future chemistry control of the crud-forming elements. The FRP is also planning to study the effect of tenacious crud on heat transfer under different chemistry conditions in a test reactor. The recent cladding corrosion failures have shown localized symptoms, including a preferential concentration of crud on peripheral rods and enhanced corrosion at the upper rod elevations. The localized phenomena suggest a role of the local thermal-hydraulic duty, which has become more complicated in fuel designs with zoned enrichment and burnable poison, as well as part length rods. The FRP has initiated a project with Argonne National Laboratory using an advanced 3D code, 3) named Numeric Nuclear Reactor or NNR, coupled with computational fluid dynamics (CFD) to analyze the local fuel duty and its correlation with the crud and corrosion behaviors. BWR water chemistry was first modified by adding hydrogen (0.4 to 1.8 ppm) to the feedwater (HWC) to control oxygen in the reactor water in Zinc was first added to a BWR for shutdown dose rate control in In 1996, NMCA, which treats the reactor core surfaces with a monolayer of Pt and Rh using a chemical process, was introduced to enhance the efficiency of hydrogen, hence reducing the need of feedwater hydrogen concentration to 0.15 to 0.4 ppm (Fig. 9). Both HWC and NMCA make system stainless steel surfaces electrochemically more reducing. The purpose is to minimize propagation of stress corrosion cracks in system components. The various EPRI programs have worked with utilities and vendors to ensure that the technologies JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

8 Fuel R & D to Improve Fuel Reliability 957 (a) (b) Fig. 10 Typical visual examination results (a) before and (b) after fuel cleaning (3 rd spacer from bottom of assembly shown) are implemented without adverse impact on fuel reliability. Multi-cycle fuel surveillances, including hot cell post-irradiation examinations, were implemented to quantify the effects of HWC, Zn and NMCA on fuel integrity. While no fuel impact was found with HWC alone, 4) the combination of NMCA and Zn injection has been found to lead to thick tenacious crud in some cases. 2) FRP and EPRI BWRVIP (BWR Vessel Internal Project) surveillance programs have led to recommendations on limits of noble metal additions (NMCA) and zinc injection to minimize the risks to fuel. Thus far, 24 of the 34 US BWRs have implemented NMCA and some have reapplied. As the BWR community continues to search for optimum solutions to protect system materials from cracking, new water chemistry approaches will likely be introduced. One such program is Online-NMCA, which was first demonstrated at an international BWR4 in June FRP has committed to a fuel surveillance program at this plant to assess fuel performance issues. The On-line NMCA technology is also under review by some US BWRs for potential implementation in the near term. FRP will continue to follow the evolution of NMCA, Online-NMCA, and other new water chemistry additives to ensure sound fuel reliability. Leveraging the success from the PWR ultrasonic cleaning experience, FRP initiated an ultrasonic fuel cleaning qualification for BWRs to address some specific concerns. In particular, one US utility had been routinely performing chemical decontaminations, but experienced short-lived reductions in dose rates. A significant cobalt source existed and a strategy was developed to remove that source, but the source term on the fuel meant the recontamination rate would remain high for several cycles. In order to reduce the fuel source term, ultrasonic cleaning was qualified for BWRs. Many of the tests were similar to those in the PWR qualification process, e.g. vibrational testing of actual fuel assemblies mocked up in a laboratory. However, BWR fuel did present unique considerations, including the necessity to transmit the ultrasonic fields through the fuel channel, increased concern over fuel pellet damage due to PCI concerns, and the potential to create partially delaminated crud layers in plants with thick, tenacious crud. The last two considerations drove the industry to sponsor a hot cell investigation for resolution. The laboratory and hot cell results were both positive. Results from the laboratory and hot cell testing were used to support a full-scale BWR ultrasonic cleaning demonstration at Exelon s Quad Cities plant. Prior to the demonstration outage, four discharged assemblies were cleaned at Quad Cities with detailed characterization before and after cleaning, including visual inspection of all faces, and crud scrapes/analyses to quantify crud removal. No unusual characteristics were noted and the process was found to remove an average of 80% of the activity in the fluffy crud layer (Fig. 10). Sixteen reload assemblies were then cleaned during the outage. These assemblies were loaded in high power positions in the core and are currently operating for a two year cycle ending Spring Although the demonstration is still ongoing, a second utility has since employed the technology to clean fuel. In this case, nearly 100 assemblies were cleaned. 3. Fuel Performance and Reliability Fuel failures are the most visible indication of fuel reliability. Many operational surprises utilities have experienced in recent years were related to higher than expected cladding corrosion and hydriding, new cladding materials and water chemistry changes. These occurrences suggest the margins in current fuel designs, when operating in today s more demanding operating environment, are not always adequate. The Fuel Performance and Reliability area of FRP focuses on determining fuel operating margins under bounding conditions and evaluating failure root causes, both of which rely heavily on hot cell examinations. This area also encourages the development of non-destructive poolside inspection and diagnostic capabilities in order to minimize the time and resource required in the hot cell examinations. The objective of hot cell root cause investigations is to eliminate known causes of failure and minimize the impact of fuel failures on plant operations. The root cause investigations currently underway include corrosion-related failures VOL. 43, NO. 9, SEPTEMBER 2006

9 958 R. YANG et al. Fig. 11 Water chemistry changes in US PWRs at two BWRs, duty-related failures in barrier fuel from other reactors, and duty-related failure mechanisms in PWR fuel. Investigations selected for hot cell examination are those that can not be resolved through poolside examinations and those with important fuel failure mechanisms and industry-wide implications. This group also acquires performance data to ensure sufficient operating margins for fuel operated under existing and changing water chemistry conditions (Figs. 9 and 11) and basic fuel and cladding data to facilitate evaluation and prediction. The needed data are obtained by conducting (nondestructive) poolside and (destructive) hot cell examinations on key fuel designs from Framatome-ANP (M5 from North Anna) and GNF (Process 5, 6/7 from Limerick) at rod average burnups as high as 72 GWd/MTU (the examination of Westinghouse ZIRLO fuel is now complete). The BWR data will be used to evaluate fuel performance margins of two cladding types with and without NMCA at the highest exposure available in the US. A separate program to characterize modern fuel designs from all three major fuel suppliers under irradiation at the KKL reactor will continue with poolside investigations at end-of-life. Much of the basic fuel and cladding property data will be obtained through highly leveraged internationally-sponsored programs such as Halden and Nuclear Fuel Industry Research (NFIR). Examinations focusing on margins generally include other assembly components, e.g., spacer grids, guide tubes in PWRs and spacers, water rods and channels in BWRs. The group has conducted a number of activities recently in the area of BWR channels, particularly as related to channel bow; investigating the role of fluence gradients, corrosion/ shadow corrosion, and hydrogen pickup. Most recently, examinations have been initiated on assessing margins and service life of PWR control rods and BWR control blades. Both of these were identified as gaps in our knowledge base. The hot cell examinations (Fig. 12) are closely coordinated with the particular fuel supplier in terms of technical scope and cost sharing. This ensures the vendor is able to fully utilize the investigation results and the collaboration offers an ideal balance of leveraged funding and an independent assessment of results. An up-to-date web-based fuel reliability database (FRED) has been implemented to provide utilities with timely industry-wide perspectives on trends in failure root causes, fuel reliability statistics and good operating practices. All US utilities are particpating in the database and FRP is seeking participation internationally. III. Advanced Fuel Design Development Recent experience indicate that existing fuel designs do not have adequate margins to operate reliably under the more demanding operating environment nuclear plants face today and in the future. It would be valuable to the industry to develop advanced fuel designs which have more robust operating margins. In addition, studies 5,6) indicate the fuel cycle economics can be improved significantly beyond 5% U-235 enrichment. Burnup levels of 100 MWd/MTU or higher is economically desirable if performance issues can be resolved. In addition to reducing production costs for LWR and ALWR Advanced Light Water eactor), robust and reliable fuel designs can reduce the spent fuel inventory and reduce storage and disposal costs. Interestingly, some innovative concepts exist, but lack the resources to further develop them. A proposal has been prepared for the US government and the industry to work together to develop, design, test and implement robust advanced fuels for LWRs and ALWRs in the next years. IV. Conclusion Fuel failure rates have increased in recent years despite best efforts by utilities and fuel vendors. Fuel Reliability Program, with active participation of US and international JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

10 Fuel R & D to Improve Fuel Reliability 959 Fig. 12 Hot cell projects to quantify margins utilities and vendors, has been restructured to focus more of its efforts on fuel reliability. While the FRP has been successful in resolving a significant number of operating and reliability issues, many issues remain. Some fuel failure root causes remain unknown. There is growing interest to develop advanced fuel designs which will provide adequate margin in the more demanding operating environments anticipated for future LWR and ALWR. It would be very beneficial to the industry if such fuel designs can be made possible within 15 years through government and private partnership. References 1) R. Yang, O. Ozer, K. Edsinger, B. Cheng, J. Deshon, An integrated approach to improve fuel reliability, 2004 Int. Topical Meeting on Light Water Reactor Fuel Performance, Orlando, Florida, Sept. 2004, (2004). 2) B. Cheng, K. Turnage, G. Potts, D. Lutz, R. Pathania, R. Rohrer, M. Eyre, E. Armstrong, Effects of noble metal chemical application on fuel performance ANS2004, Presented at ANS 2004 Fuel Topical Meeting, Orlando, FL, Sept ) T. Sofu, D. Weber, T. Chun, H. Joo, J. Thomas, Z. Zhong, T. Downar, Development of a comprehensive modeling capability based on rigorous treatment of multi-physics phenomena influencing reactor core design, Proc. ICAPP, Pittsburgh, PA, June 13 17, 2004, (2004). 4) B. Cheng, R. B. Adamson, A. J. Machiels, D. O. Oboyle, Effect of hydrogen water chemistry on fuel performance at Dresden-2, Proc. Int. Topical Meeting on LWR Fuel Performance, ANS-ENS, Avignon, France, April 21, 1991, (1991). 5) J. Secker, B. Johansen, D. Stucker. O. Ozer, K. Ivanov, S. Yilmaz, E. Young, Optimum discharge burnup and cycle length for PWRs, Nucl. Technol., 151, 109 (2005). 6) Optimum Cycle Length and Discharge Burnup for Nuclear Fuel-Phase II: Results Achievable with Enrichments Greater than 5 w/o, Report , Electric Power Research Institute, (2002). VOL. 43, NO. 9, SEPTEMBER 2006