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Fuel R & D to Improve Fuel ReliabilityRosa YANG a , Bo CHENG a , Jeff DESHON a , Kurt EDSINGER a & Odelli OZER aa EPRI , 3412 Hillview Avenue, Palo Alto , CA , 94304 , USAPublished online: 05 Jan 2012.

To cite this article: Rosa YANG , Bo CHENG , Jeff DESHON , Kurt EDSINGER & Odelli OZER (2006) Fuel R & D to ImproveFuel Reliability, Journal of Nuclear Science and Technology, 43:9, 951-959

To link to this article: http://dx.doi.org/10.1080/18811248.2006.9711181

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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 cyclelength increase both can increase the local duty. Reactor water chemistry modifications for the purpose of protectionthe plant system materials have the potential of increasing fuel surface deposition and cladding corrosion and hydrid-ing. 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 per-formance continue to result in light water reactor (LWR)fuel being subjected to new and increasingly challenging op-erating environments. Challenges to fuel performance haveresulted from a combination of plant uprates, increasing dis-charge burnups (Fig. 1), longer fuel cycles (Fig. 2), higherenrichments and more aggressive water chemistry conditionsintroduced to mitigate plant material degradation and reduceshutdown dose rate.

Although current fuel failure rates in the US remain atconsiderably lower levels than in the late 1970s and 80s,there has been a noticeable increase in failures, particularlyin BWRs, during the last few years (Fig. 3).

The increase in BWR failures has been due primarilyto corrosion-related and fuel-duty related mechanisms(Fig. 4). The root cause of some of the failures has not beenestablished yet. The analysis is complicated because of cool-ant chemistry changes introduced for mitigating materialdegradation and dose control. Debris fretting also remainsa problem even after the introduction of debris filters. Con-tinued effort by utilities, through improved foreign materialexclusion programs and improved debris filter designs by thevendors, should reduce this type of failure.

The primary contributor to PWR failure rates remainsgrid-to-rod fretting; however, experience with new grid de-signs appears promising. On the other hand, there has beenan increase in the number of failures primarily from opti-mized fuel designs with a thinner rod diameter (Fig. 5).The root cause of the failures is unknown. In addition, somePCI-suspect failures have been experienced at three B&W-designed PWR plants following the movement of axial pow-er shaping rods (APSRs).

It is acknowledged that fuel failures can be quite costly.Although typical costs are difficult to quantify, even a singlefailed BWR rod can cost more than $1,000,000 in outagetime, fuel and power replacement costs. Failures affecting

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)—High-lights

The Electric Power Research Institute under the sponsor-ship of the utility industry launched the Fuel ReliabilityProgram in 1998 (initially under the name of Robust FuelProgram) in recognition of the fact that continuing pressuresto improve fuel cycle economics were resulting in LWR fuelbeing subjected to new and increasingly challenging operat-ing environments. As a result of a gap analysis and prioriti-zation effort,1) the program activities have been re-groupedinto 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 pro-gram with international participation. The program collabo-rates closely with fuel vendors and industry organizationssuch as INPO and NEI. Its aims are to complement vendorR/D efforts by determining margins in fuel products thatare either currently licensed or are clearly licensable to en-sure their safety and reliability under the anticipated operat-ing environments. FRP was formulated as a multi-year pro-gram with the understanding that resolution of many of thekey issues and achievement of program objectives will re-quire a sustained, multi-phase effort. This paper providesan update in the first three areas (not including the regualtoryissues) since the publication of reference 1 in the areas relat-ed to fuel reliability.

1. PWR Fuel Corrosion and Crud ControlThe 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 onfuel in PWRs.

In the area of understanding and controlling crud, signifi-cant progress has been made. Extensive research over the

�Atomic Energy Society of Japan

�Corresponding author, E-mail: ryang@epri.com

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past several years in crud characterization and depositionmechanisms have increased our knowledge in the crud dep-osition and growth processes. This research resulted in a re-port with extensive data on PWR feul crud in 2004. Crudwas scraped from nine US PWR cores and analyzed usingspecialized sample preparation procedures and analyticaltechniques. 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 theparticulate-born circulating material remains in its crystal-line form and does not dissolve from being cooled under nor-mal sample collection procedures. The data from the sam-plers were used to offer insight into what is circulating inthe coolant during the operating cycle versus what is meas-

Fig. 2 Average Cycle Length–US

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ured from crud scrapes following shutdowns. The steamingrate at the fuel cladding surface had a profound effect oncore deposits and axial offset anomaly (AOA).

High assembly/rod steaming rates produced the followingeffects:. Crud was concentrated in areas with high steaming rates.

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. Thickness of deposits generally increased at the boilingduty.

. New crud forms were produced at locations with sub-cooled boiling (bonaccordite, needles with high nickelcontent).

. Layered structures developed, including zones rich in zir-conium oxide.The layered deposits were shown to resist dissolution,

and, thus, create the potential to present a self-perpetuatingproblem. Of the coolant chemistry parameters that were con-sidered, coolant zinc concentration appeared to have thestrongest influence on crud structure, making it thinner, lesscrystalline, more mobile, and richer in chromium. Carbonwas found in core deposits where zinc was added, and whilethe acetate form in which the zinc is added is likely respon-sible for the carbon presence, this isn’t certain.

The high-temperature coolant analyses established thatsub-micron metallic nickel particles are a common compo-nent of circulating crud during operation. Such particlesare probably responsible for a large fraction of nickel re-leased at shutdown. This knowledge should be useful in op-timizing shutdown chemistry.

While a common component to fuel crud, no NiO was ob-served in the high temperature samples, reflecting the factthat under normal coolant conditions (e.g. hydrogen rangingfrom 25–50 cc/kg), NiO is not thermodynamically stableover most system surfaces. This suggests it’s presence in fuelcrud is a result of the local conditions present at the clad sur-face and within the crud.

Results from the crud studies helped lead to the publica-

tion of the PWR Axial Offset Anomaly (AOA) Guidelinesin 2004. This document provides guidance to utilities onhow changes in various plant parameters, core design andchemistry can impact crud deposition and AOA. The cruddata will also be used to improve EPRI’s Boron-InducedOffset Anomaly (BOA) Code. The BOA code is an integrat-ed thermal hydraulic–chemistry software package that pre-dicts where crud will deposit, its thickness and the suscepti-bility of a particular core design to AOA.

Additionally, more utilities are using the FRP-developedultrasonic fuel cleaning technology to remove crud from re-load fuel. During spring 2005, this new technology was ap-plied at seven US reactors and one in Spain. By the end of2005, a total of 13 reactors worldwide will have used thistechnology. Through years of R & D effort, several tools(Fig. 6) are available for plants to manage and mitigate theimpact of crud deposition and AOA on plant operation.

Mindful of the interrelationship between fuel duty andchemistry, FRP has established active plant demonstrationsat two PWRs operating outside the general fuel-duty/chem-istry experience base. The specific programs under FRPsponsorship are designed to evaluate reactor coolant zincchemistry under aggressive core design conditions, and ele-vated and constant pH (pHt=7.4) with lithium levels as highas 6 ppm at beginning-of-cycle. As the demonstrations ma-ture and assuming no adverse effects are realized, these pro-grams will pave the way for many more PWRs to apply sim-ilar chemistry regimes without necessarily incurring the highcost of fuel examinations for every plant at the end of eachoperating cycle.

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Fig. 5 Trend in US PWR failure root causes

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2. BWR Fuel Corrosion and Crud ControlThis is a new Working Group established in 2004 to focus

on addressing emerging BWR fuel reliability issues associat-ed with fuel cladding corrosion, crud deposition and spalla-tion, and evolving water chemistry conditions. During 2001–03, three US BWRs experienced fuel failures due to acceler-ated cladding corrosion. Understanding the root causes ofthose high impact fuel failures is needed to avoid future re-currence of similar failures. Another fuel reliability concernrelates to observations of thick tenacious crud and surfacespallation on some fuel rods at plants with noble metalchemical 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 isessential to long-term fuel reliability. This working groupcoordinates with other EPRI programs to ensure that waterchemistry 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 ametal blade to scrape the fuel surface to obtain flakes of thetenacious crud. The crud flakes are analyzed in a hot labora-tory for morphology and distribution of elements in theflakes. The results have enhanced our understanding ofhow tenacious crud may become harmful to fuel heat trans-fer and causing fuel failures. This technique has now beenemployed to study crud flakes from several other BWRs. Re-sults to date indicate that zinc ferrite is the major constituentin tenacious crud at plants with zinc injection. Co-depositionof zinc silicate and/or copper oxide in the zinc ferrite crudmay occur in some cases, Fig. 8. Deposition of multiplecompounds on the fuel surface may result in degradationof the local heat transfer of the fuel rod. An extensive studyon the solubility and stability of various zinc compounds has

Fig. 6 Tools to Mitigating PWR Crud and AOA

Fig. 7 Example of tenacious crud and surface spallation

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been initiated to study the pathways that would lead todeposition of the various elements in the tenacious crud.The result of the study is expected to provide the technicalbasis for future chemistry control of the crud-forming ele-ments. The FRP is also planning to study the effect of tena-cious crud on heat transfer under different chemistry condi-tions in a test reactor.

The recent cladding corrosion failures have shown local-ized symptoms, including a preferential concentration ofcrud on peripheral rods and enhanced corrosion at the upperrod elevations. The localized phenomena suggest a role ofthe local thermal-hydraulic duty, which has become morecomplicated in fuel designs with zoned enrichment and burn-able poison, as well as part length rods. The FRP has initiat-ed a project with Argonne National Laboratory using an ad-vanced 3D code,3) named Numeric Nuclear Reactor or NNR,

coupled with computational fluid dynamics (CFD) to ana-lyze the local fuel duty and its correlation with the crudand corrosion behaviors.

BWR water chemistry was first modified by adding hydro-gen (0.4 to 1.8 ppm) to the feedwater (HWC) to controloxygen in the reactor water in 1983. Zinc was first addedto a BWR for shutdown dose rate control in 1987. In1996, NMCA, which treats the reactor core surfaces with amonolayer of Pt and Rh using a chemical process, was intro-duced to enhance the efficiency of hydrogen, hence reducingthe need of feedwater hydrogen concentration to 0.15 to 0.4ppm (Fig. 9). Both HWC and NMCA make system stainlesssteel surfaces electrochemically more reducing. The purposeis to minimize propagation of stress corrosion cracks in sys-tem components. The various EPRI programs have workedwith utilities and vendors to ensure that the technologies

Fig. 8 Tenacious crud deposit rich in zinc ferrite and zinc silicate

Fig. 9 Water chemistry changes in US BWRs

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are implemented without adverse impact on fuel reliability.Multi-cycle fuel surveillances, including hot cell post-irradi-ation examinations, were implemented to quantify the ef-fects of HWC, Zn and NMCA on fuel integrity. While nofuel impact was found with HWC alone,4) the combinationof NMCA and Zn injection has been found to lead to thicktenacious crud in some cases.2) FRP and EPRI BWRVIP(BWR Vessel Internal Project) surveillance programs haveled 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 NMCAand some have reapplied. As the BWR community continuesto search for optimum solutions to protect system materialsfrom cracking, new water chemistry approaches will likelybe introduced. One such program is Online-NMCA, whichwas first demonstrated at an international BWR4 in June2005. FRP has committed to a fuel surveillance program atthis plant to assess fuel performance issues. The On-lineNMCA technology is also under review by some US BWRsfor potential implementation in the near term. FRP will con-tinue to follow the evolution of NMCA, Online-NMCA, andother new water chemistry additives to ensure sound fuel re-liability.

Leveraging the success from the PWR ultrasonic cleaningexperience, FRP initiated an ultrasonic fuel cleaning qualifi-cation for BWRs to address some specific concerns. In par-ticular, one US utility had been routinely performing chemi-cal decontaminations, but experienced short-lived reductionsin dose rates. A significant cobalt source existed and a strat-egy was developed to remove that source, but the sourceterm on the fuel meant the recontamination rate would re-main high for several cycles. In order to reduce the fuelsource term, ultrasonic cleaning was qualified for BWRs.Many of the tests were similar to those in the PWR qualifi-cation process, e.g. vibrational testing of actual fuel assem-blies mocked up in a laboratory. However, BWR fuel didpresent unique considerations, including the necessity totransmit the ultrasonic fields through the fuel channel, in-creased concern over fuel pellet damage due to PCI con-cerns, and the potential to create partially delaminated crud

layers in plants with thick, tenacious crud. The last two con-siderations drove the industry to sponsor a hot cell investiga-tion for resolution. The laboratory and hot cell results wereboth positive.

Results from the laboratory and hot cell testing were usedto support a full-scale BWR ultrasonic cleaning demonstra-tion at Exelon’s Quad Cities plant. Prior to the demonstra-tion outage, four discharged assemblies were cleaned atQuad Cities with detailed characterization before and aftercleaning, including visual inspection of all faces, and crudscrapes/analyses to quantify crud removal. No unusual char-acteristics were noted and the process was found to removean average of �80% of the activity in the fluffy crud layer(Fig. 10). Sixteen reload assemblies were then cleaned dur-ing the outage. These assemblies were loaded in high powerpositions in the core and are currently operating for a twoyear cycle ending Spring 2006. Although the demonstrationis still ongoing, a second utility has since employed the tech-nology to clean fuel. In this case, nearly 100 assemblies werecleaned.

3. Fuel Performance and ReliabilityFuel failures are the most visible indication of fuel relia-

bility. Many operational surprises utilities have experiencedin recent years were related to higher than expected claddingcorrosion and hydriding, new cladding materials and waterchemistry changes. These occurrences suggest the marginsin current fuel designs, when operating in today’s more de-manding operating environment, are not always adequate.The Fuel Performance and Reliability area of FRP focuseson determining fuel operating margins under bounding con-ditions and evaluating failure root causes, both of which relyheavily on hot cell examinations. This area also encouragesthe development of non-destructive poolside inspection anddiagnostic capabilities in order to minimize the time and re-source required in the hot cell examinations.

The objective of hot cell root cause investigations is toeliminate known causes of failure and minimize the impactof fuel failures on plant operations. The root cause investiga-tions currently underway include corrosion-related failures

(a) (b)

Fig. 10 Typical visual examination results (a) before and (b) after fuel cleaning (3rd spacer from bottom of assemblyshown)

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at two BWRs, duty-related failures in barrier fuel from otherreactors, and duty-related failure mechanisms in PWR fuel.Investigations selected for hot cell examination are thosethat can not be resolved through poolside examinationsand those with important fuel failure mechanisms and indus-try-wide implications.

This group also acquires performance data to ensure suf-ficient operating margins for fuel operated under existingand changing water chemistry conditions (Figs. 9 and 11)and basic fuel and cladding data to facilitate evaluationand prediction. The needed data are obtained by conducting(nondestructive) poolside and (destructive) hot cell examina-tions on key fuel designs from Framatome-ANP (M5 fromNorth Anna) and GNF (Process 5, 6/7 from Limerick) atrod average burnups as high as 72GWd/MTU (the examina-tion of Westinghouse ZIRLO fuel is now complete). TheBWR data will be used to evaluate fuel performance marginsof two cladding types with and without NMCA at the highestexposure available in the US. A separate program to charac-terize modern fuel designs from all three major fuel suppliersunder irradiation at the KKL reactor will continue with pool-side investigations at end-of-life. Much of the basic fuel andcladding property data will be obtained through highly lever-aged internationally-sponsored programs such as Halden andNuclear Fuel Industry Research (NFIR).

Examinations focusing on margins generally include otherassembly components, e.g., spacer grids, guide tubes inPWRs and spacers, water rods and channels in BWRs. Thegroup has conducted a number of activities recently in thearea of BWR channels, particularly as related to channelbow; investigating the role of fluence gradients, corrosion/shadow corrosion, and hydrogen pickup. Most recently, ex-aminations have been initiated on assessing margins andservice 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 coordinat-ed with the particular fuel supplier in terms of technical

scope and cost sharing. This ensures the vendor is able tofully utilize the investigation results and the collaborationoffers an ideal balance of leveraged funding and an inde-pendent assessment of results.

An up-to-date web-based fuel reliability database (FRED)has been implemented to provide utilities with timely indus-try-wide perspectives on trends in failure root causes, fuelreliability statistics and good operating practices. All USutilities are particpating in the database and FRP is seekingparticipation internationally.

III. Advanced Fuel Design Development

Recent experience indicate that existing fuel designs donot have adequate margins to operate reliably under themore demanding operating environment nuclear plants facetoday and in the future. It would be valuable to the industryto develop advanced fuel designs which have more robustoperating margins. In addition, studies5,6) indicate the fuelcycle economics can be improved significantly beyond 5%U-235 enrichment. Burnup levels of 100MWd/MTU orhigher is economically desirable if performance issues canbe resolved. In addition to reducing production costs forLWR and ALWR Advanced Light Water eactor), robustand reliable fuel designs can reduce the spent fuel inventoryand reduce storage and disposal costs. Interestingly, someinnovative concepts exist, but lack the resources to furtherdevelop them. A proposal has been prepared for the USgovernment and the industry to work together to develop,design, test and implement robust advanced fuels for LWRsand ALWRs in the next 10–20 years.

IV. Conclusion

Fuel failure rates have increased in recent years despitebest efforts by utilities and fuel vendors. Fuel ReliabilityProgram, with active participation of US and international

Fig. 11 Water chemistry changes in US PWRs

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utilities and vendors, has been restructured to focus more ofits efforts on fuel reliability. While the FRP has been suc-cessful in resolving a significant number of operating andreliability issues, many issues remain. Some fuel failureroot causes remain unknown. There is growing interest to de-velop advanced fuel designs which will provide adequatemargin in the more demanding operating environments an-ticipated for future LWR and ALWR. It would be verybeneficial to the industry if such fuel designs can be madepossible within 15 years through government and privatepartnership.

References

1) R. Yang, O. Ozer, K. Edsinger, B. Cheng, J. Deshon, ‘‘An inte-grated approach to improve fuel reliability,’’ 2004 Int. TopicalMeeting 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 chemi-cal application on fuel performance ANS2004,’’ Presented atANS 2004 Fuel Topical Meeting, Orlando, FL, Sept. 2004.

3) T. Sofu, D. Weber, T. Chun, H. Joo, J. Thomas, Z. Zhong,T. Downar, ‘‘Development of a comprehensive modeling capa-bility based on rigorous treatment of multi-physics phenomenainfluencing 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 atDresden-2,’’ Proc. Int. Topical Meeting on LWR Fuel Perform-ance, 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 cyclelength for PWRs,’’ Nucl. Technol., 151, 109 (2005).

6) Optimum Cycle Length and Discharge Burnup for NuclearFuel-Phase II: Results Achievable with Enrichments Greaterthan 5w/o, Report 1003217, Electric Power Research Institute,(2002).

Fig. 12 Hot cell projects to quantify margins

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