ML19353A030

Aluminum NAM Tlr v1.2 - Review Copy

ML19353A030
Person / Time
Issue date:	05/31/2020
From:	Eric Focht Office of Nuclear Regulatory Research
To:
Eric Focht
Shared Package
ML19353A029	List: ML19353A030 ML19353A032
References
Download: ML19353A030 (48)

Text

Assessment of Aluminum-Based Neutron Absorber Materials Qualification Testing and Surveillance Monitoring Programs Eric M. Focht U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research May 2020

i Executive Summary This report evaluates available materials qualification testing and surveillance program information to assess the adequacy of surveillance monitoring programs for managing aging and degradation of aluminum-based neutron absorber materials (NAMs). BORAL, Metamic, and Boralcan are aluminum-based NAMs used extensively in domestic nuclear power plant spent fuel pools (SFPs). BORAL, an aluminum-cladded aluminum/boron carbide cermet composite, is used in about 60 percent of SFPs, while Metamic and Boralcan are used in about 20 percent of the SFPs combined. This report reviews and assesses the materials qualification programs conducted to demonstrate the adequacy of these materials for potential degradation mechanisms operating in SFPs. Additionally, this report reviews and assesses surveillance monitoring programs, including operating experience, to determine whether the degradation mechanisms to which the neutron absorber may be susceptible are managed effectively.

The materials qualification testing programs for the aluminum-based NAMs consisted of mechanical properties testing, elevated temperature testing, corrosion testing, and radiation exposure testing. Such tests address the potential degradation mechanisms and exacerbating service conditions. Corrosion is the most likely form of degradation in the SFPs and can be accelerated by elevated temperature and altered material properties that may occur from radiation exposure. The corrosion testing conducted in the qualification programs did not, however, explore the performance of the materials in off-normal water chemistry conditions (i.e., elevated levels of contaminants such as chloride). Such testing was conducted in recent testing of BORAL from the Zion Nuclear Power Station decommissioned SFP, discussed in Section 4.5 of this report and documented in a technical letter report [24]. In general, the materials qualification testing, and subsequent testing, addressed the potential degradation mechanisms, and the aluminum-based NAMs exhibited the properties necessary to perform the intended safety function in the SFPs for pressurized-water reactors and boiling-water reactors.

Most of the SFPs with aluminum-based NAMs have surveillance monitoring programs consisting of coupon exposure and testing or in situ (i.e., in-pool) testing to directly measure the boron-10 (B-10) areal density. Coupon-testing programs vary among the licensees, but all the programs reviewed (1) use coupons made from the materials used in the SFP racks, (2) subscribe to an established surveillance interval, and (3) perform a condition assessment of the coupons (most coupon surveillance programs include B-10 areal density measurements).

The differences among the individual programs include coupon sizes and configurations, surveillance intervals, and the measurements made as part of the condition assessments. The assessment of the surveillance monitoring programs for the aluminum-based NAMs was based on available information obtained from license amendments, license renewal applications, U.S. Nuclear Regulatory Commission (NRC) information notices, and information submitted in response to NRC Generic Letter 2016-001, Monitoring of Neutron-Absorbing Materials in Spent Fuel Pools, dated April 7, 2016. The NRC concluded that, overall, the surveillance programs are designed such that degradation of the aluminum-based NAMs will be monitored and

ii managed effectively for the intended service life, but some enhancements could be made, such as adding B-10 areal density measurements, in accordance with Nuclear Energy Institute (NEI) topical report NEI 16-03-A, Guidance for Monitoring of Fixed Neutron Absorbers in Spent Fuel Pools, Revision 0, issued May 2017, that are not included in some programs. Some licensees do not have surveillance programs for BORAL but monitor the performance of BORAL in other SFPs through various industry users groups and research activities.

Operating experience and additional testing results indicate that BORAL, Metamic, and Boralcan should perform their safety function for the service life of the SFPs, provided that SFP water chemistry and temperature are maintained within the Electric Power Research Institute guidelines. Additionally, the review of operating experience did not reveal new degradation or aging mechanisms that might challenge the current regulatory position of the NRC on the use of aluminum-based NAMs in SFPs.

iii Contents Executive Summary i

1. Introduction............................................................................................................................ 1

2. Spent Fuel Pools.................................................................................................................... 2

3. Neutron Absorber Materials................................................................................................... 5 3.1 Polymer-Based Neutron Absorber Materials..................................................................... 5 3.2 Metallic-Based Neutron Absorber Materials...................................................................... 5

4. Qualification Testing............................................................................................................... 7 4.1 Metamic......................................................................................................................... 7 4.1.1 Summary of EPRI Report on Metamic..................................................................... 7 4.1.2 Summary of Holtec Report........................................................................................13 4.2 Boralcan........................................................................................................................13 4.3 BORAL.........................................................................................................................14 4.3.1 Density and Composition..........................................................................................14 4.3.2 Areal Density.............................................................................................................14 4.3.3 Mechanical Properties...............................................................................................15 4.3.4 Radiation Testing......................................................................................................16 4.3.5 Corrosion Testing......................................................................................................16 4.3.6 Elevated Temperature Testing..................................................................................17 4.4 Qualification Testing Versus Potential Degradation Mechanisms....................................17 4.4.1 Corrosion..................................................................................................................18 4.5 Supporting Test Data Since Material Qualification...........................................................20

5. NRC Safety Evaluations........................................................................................................23

6. Operating Experience...........................................................................................................24

7. Surveillance..........................................................................................................................25 7.1 Surveillance Programs.....................................................................................................28 7.1.1 Program Design........................................................................................................28 7.1.2 Testing Intervals........................................................................................................28 7.1.3 Testing......................................................................................................................31 7.1.4 Surveillance Coupon Performance............................................................................31 7.1.5 Coupon Reinsertion..................................................................................................32

iv 7.2 In Situ B-10 Areal Density Measurement Programs.........................................................33 7.2.1 In Situ B-10 Areal Density Measurement Program Performance...............................33 7.2.2 Assessment of the BADGER System for In Situ B-10 Areal Density Measurements.34

8. Discussion............................................................................................................................35 8.1 Qualification Testing and Operating Experience..............................................................35 8.2 Surveillance Methods Effectiveness................................................................................36

9. Conclusions..........................................................................................................................38

10. References..........................................................................................................................39

v List of Figures Figure 1 Representative SFP rack configurations: flux trap design (top) and egg-crate design (bottom)....................................................................................................................... 4 Figure 2 Weight loss results for galvanic corrosion tests performed on Metamic coupled with 304L stainless steel, Inconel 718, and Zircaloy 2: (a) BWR solution, dried weight and (b) PWR solution, dried weight (used with permission from EPRI)..............................10 Figure 3 Crevice corrosion results for Metamic mill-finished and anodized specimens:

(a) BWR solution and (b) PWR solution (used with permission from EPRI)................11 Figure 4 Yield strength and ultimate tensile strength of Metamic versus gamma radiation exposure for materials with 15-percent and 31-percent B4C content...........................12 Figure 5 Elongation of Metamic versus gamma radiation exposure for materials with 15-percent and 31-percent B4C content......................................................................12 Figure 6 Mechanical properties of Metamic with 40 wt.% B4C...............................................13 Figure 7 Neutron attenuation characteristics of BORAL compared to an ideal absorber........15 Figure 8 Linear polarization resistance EC testing results for samples of BORAL with 600-grit freshly ground cladding surfaces in solutions with various concentrations of chloride at various temperatures..................................................................................................22 Figure 9 Corrosion rates obtained for BORAL samples starting with freshly ground surfaces in the nominal water chemistry for (a) PWRs and (b) BWRs........................................23 List of Tables Table 1 EPRI SFP Water Chemistry Recommended Guidelines for PWR and BWR SFPs....... 3 Table 2 Average Mechanical Properties of Metamic for Various B4C Loadings...................... 6 Table 3 Room Temperature Mechanical Properties of BORAL..............................................15 Table 4 List of Plants that Use Aluminum-Based NAMs and the Type of Surveillance Program in Place......................................................................................................................29

vi List of Acronyms ADAMS Agencywide Documents Access and Management System AMP aging management program AOR analysis of record B-10 boron-10 B4C boron carbide BADGER Boral Areal Density Gage for Evaluating Racks BWR boiling-water reactor C

Celsius CFR Code of Federal Regulations EC electrochemical corrosion Entergy Entergy Nuclear Generating Company EPRI Electric Power Research Institute F

Fahrenheit GALL Generic Aging Lessons Learned GDC general design criterion GL generic letter IN information notice NAM neutron absorber material NCS nuclear criticality safety NEI Nuclear Energy Institute NETCO Northeast Technologies Company NRC U.S. Nuclear Regulatory Commission ORNL Oak Ridge National Laboratory PWR pressurized-water reactor SFP spent fuel pool SRNL Savannah River National Laboratory TLR technical letter report

1

1. Introduction Spent nuclear fuel is stored on site at domestic nuclear power plants in spent fuel pools (SFPs) designed to cool the fuel and maintain subcriticality. Compliance with technical specifications and U.S. Nuclear Regulatory Commission (NRC) regulations ensures the achievement of subcriticality by neutron absorption through boron additions to the SFP water (for pressurized-water reactors (PWRs)), fuel-bundle spacing, neutron absorber materials (NAMs) containing boron-10 (B-10), or combinations of the three. These strategies are employed separately or in combination.

NAMs are made from polymers or metals containing boron carbide (B4C), manufactured into long, narrow panels with widths and lengths about the size of one side of a fuel bundle. The metal or polymer matrix distributes the B4C and protects it from the service environment.

Licensee nuclear criticality safety (NCS) analyses of record (AORs) that take credit for the use of NAMs assume a certain B-10 areal density. Therefore, measures must be taken to confirm that the B-10 areal density of the NAM panels in the SFP meet or exceed the assumed B-10 areal density in the NCS AOR. This is accomplished through licensee NAM surveillance monitoring programs or other methods NRC deems acceptable.

NAM surveillance programs are employed to monitor the effects of material aging and degradation over the expected SFP service life. In most cases, licensees employ NAM surveillance monitoring programs to manage NAM degradation. However, before the issuance of NRC Generic Letter (GL) 78-11, OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications, [2F1] dated April 14, 1978, the NRC did not require surveillance and monitoring programs. Thus, some licensees do not currently employ NAM surveillance programs. GL 78-11 is not prescriptive on the specific aspects of NAM surveillance programs, such as coupon sizes, inspection intervals, and acceptance criteria, so licensees designed their own programs to provide the information required.

NRC operating experience and regulatory actions, such as inspections and reviews of license renewal applications and license amendments, revealed gaps in the NRC regulatory knowledge base of NAMs. The NRC also determined that current regulatory guidance did not adequately address the management of aging effects associated with NAMs. The NRC issued GL 2016-01, Monitoring of Neutron-Absorbing Materials in Spent Fuel Pools, [3F2] dated April 7, 2016, to obtain specific information on licensees NAM surveillance programs so that the NRC could determine if the degradation of the neutron-absorbing materials in the SFP for nuclear power plants and the reactor pool, reactor tank, or fuel storage pool for non-power reactors is being managed to maintain reasonable assurance that the materials are capable of performing their safety function, and to verify that the addressees are in compliance with the regulations.

NAM surveillance programs provide leading indicators of potential degradation in the NAM panels installed in the fuel storage racks. Coupons made from the same NAMs as in the racks are placed in the SFP at locations estimated to provide the most severe exposure conditions

2 (i.e., temperature, gamma dose) and are removed periodically for testing and examination. The testing focuses primarily on dimensional changes and B-10 areal density changes, specifically, a reduction in B-10 areal density.

Manufacturers or vendors performed NAM qualification testing programs to demonstrate that the materials will perform their intended safety function. The testing typically addressed the potential degradation mechanisms and conditions that may exacerbate degradation, such as temperature and gamma dose.

The NRC has evaluated the surveillance techniques employed to monitor the aging of Boraflex

[4F3] through the use of the in situ B-10 areal density measurement system BADGER (Boron Areal Density Gauge for Evaluating Racks) and the computational tool RACKLIFE. In addition, the NRC published technical letter reports that evaluated the materials qualification testing, potential degradation mechanisms, and adequacy of surveillance programs for phenolic resin-based NAMs Carborundum and Tetrabor [5F4, 6F5].

In addition to polymer-based NAMs, metals-based NAMs are used extensively in SFPs. The most prevalent metals-based NAMs are aluminum based, which are used in approximately 80 percent of the domestic SFPs. The purpose of this report is to assess the materials testing performed to qualify aluminum-based NAMs for use in SFPs and the effectiveness of surveillance monitoring programs employed to manage material aging. The NRC staff evaluated information from the original qualification programs that formed the basis of the NRCs approvals to use the aluminum-based NAMs in combination with subsequent testing, operating experience, and information obtained from licensee submittals in response to GL 2016-01. This report ends with an assessment of the extent to which the qualification testing addressed the potential degradation mechanisms and the effectiveness of the surveillance programs for monitoring and managing the aging of aluminum-based NAMs.

2. Spent Fuel Pools To prevent the occurrence of inadvertent criticality events, SFP subcriticality must be maintained in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.68, Criticality accident requirements, and 10 CFR Part 50, Domestic licensing of production and utilization facilities, Appendix A, General Design Criteria for Nuclear Power Plants, General Design Criterion (GDC) 62, Prevention of Criticality in Fuel Storage and Handling. As mentioned above, compliance with the technical specifications and NRC regulations is achieved by neutron absorption through boron additions to the SFP water (for PWRs), fuel-bundle spacing, NAMs, or combinations of the three. Delays in the movement of spent fuel into a long-term repository prompted many licensees to reconfigure (or re-rack) their SFPs by storing fuel closer together to increase storage capacity. In many cases, re-racking required additional measures to meet subcriticality requirements when fuel-bundle spacing or boron additions to the SFP water were insufficient. In such cases, NAMs were integrated into SFP racks in the form of thin panels.

3 SFPs are about 12 meters (40 feet) deep with varying design capacities (i.e., water volume and fuel). SFP racks are about 4 meters (14 feet) tall and are covered by about 8 meters (26 feet) of water to provide sufficient cooling and radiation shielding. The SFPs are filled with high-purity demineralized water, maintained at approximately 27-40 degrees Celsius (C) (80-100 degrees Fahrenheit (F)). PWR SFPs contain about 2,000 parts per million (ppm) of added boron as boric acid to maintain subcriticality in the SFP and to reduce dilution of the reactor pressure vessel coolant during refueling. The Electric Power Research Institute (EPRI) publishes SFP water chemistry guidelines for both PWRs and boiling-water reactors (BWRs) [7F6, 8F7], which are shown in Table 1. Licensees typically maintain impurity levels in their SFPs lower than the recommended guidelines.

SFP storage modules consist of rectangular stainless-steel or aluminum channels or tubes that encase individual fuel assemblies. Typically, modules that store freshly offloaded fuel are designed with water-filled spaces between the fuel assemblies that act as flux traps to provide additional neutron shielding, whereas less active fuel is stored in modules with an egg-crate design. Figure 1 illustrates these configurations.

Table 1 EPRI SFP Water Chemistry Recommended Guidelines for PWR and BWR SFPs

[6, 7]

Monitored Parameter PWR BWR Boron Per technical specification n/a Conductivity (µS/cm) n/a 1.3 Chloride (ppb)

< 150 100 Sulfate (ppb)

(see note) 100 Total organic carbon (ppb)

(see note) 400 Gamma isotopes (see note) n/a Turbidity (see note) n/a Notes: The source documents do not provide specific recommended limits. The levels are monitored periodically and action taken if the levels exceed the normal range.

ppb = parts per billion

µS/cm = microsiemens per centimeter

4 Figure 1 Representative SFP rack configurations: flux trap design (top) and egg-crate design (bottom)

5

3. Neutron Absorber Materials Licensees use NAMs in the form of thin panels attached to the spent fuel storage racks to help meet SFP subcriticality margin requirements. SFPs use two major classes of NAMs: polymer based and metallic based. Each class of NAM uses boron or B4C to absorb neutrons (specifically, the B-10 isotope). The NRC has approved several NAMs for use in SFPs where polymer-based Boraflex and Carborundum and metallic-based BORAL and Metamic are the most widely used (BORAL, alone, is used in about 60 percent of SFPs). This section provides a brief introduction to the various NAMs currently used in SFPs.

3.1 Polymer-Based Neutron Absorber Materials Polymer-based NAMs use a polymer substrate to contain and distribute B4C to provide an effective areal density of B-10 throughout the panels. The three most commonly used polymer-based NAMs are Boraflex, Carborundum, and Tetrabor. Boraflex uses polydimethyl siloxane, a silicone polymer, while Carborundum and Tetrabor use phenolic resins (popularly known as Bakelite) to contain the B4C.

Service experience has shown that NAMs are subject to degradation in the SFP environment.

The most notable case of NAM degradation is that of Boraflex degradation involving the shrinkage and dissolution of the polydimethyl siloxane and subsequent loss of B4C, resulting in gaps and slumping. Tools such as the RACKLIFE computer code and the BADGER in-pool areal density measurement system were developed and are currently used to evaluate the condition of Boraflex panels and provide estimates of the level of degradation based on factors such as pool chemistry, temperature, and radiation exposure. In some cases, licensees have replaced Boraflex with metallic-based NAMs or compensated for the loss of Boraflex by spacing fuel to maintain subcriticality margins. The NRC documented its evaluations of polymer-based NAMs, including assessments of BADGER and RACKLIFE, in several reports

[3, 4, 5].

3.2 Metallic-Based Neutron Absorber Materials Metallic-based NAMs are produced using either aluminum or stainless-steel matrices that incorporate boron in a cermet composite core structure, B4C reinforcement in metal matrix composites, or in solid solution as an alloy addition. The most commonly used metallic-based NAMs are aluminum-based NAMs, such as BORAL, Metamic, and Boralcan.

BORAL is an aluminum-based NAM composed of an aluminum-B4C cermet core, sandwiched between thin 1100 aluminum cladding. A cermet is a mixture of a metal and a nonmetal sintered using thermomechanical processing, but it is distinct from metal matrix composites, in that cermets do not typically reach theoretical density. BORAL is generally produced by filling a container formed from an 1100 aluminum ingot with 1100 aluminum powder and B4C powder such that the container envelops the cermet. The container is thermomechanically processed (e.g., hot rolled) into its final form, such as sheets or plates. The percentage of B4C in the

6 cermet core ranges from 35 to 65 percent, where 65 percent represents the upper limit because higher B4C loadings result in poor sintering due to an insufficient amount of aluminum powder.

To ensure adequate bonding between the cermet and the cladding, one BORAL manufacturer limits the B4C loading to 50-percent maximum.

For SFP storage applications, BORAL is produced in the form of panels, which, for PWR fuel, are typically 190-203 millimeters (7.5-8 inches) wide by 3.6 meters (12 feet) long, and for BWR fuel are typically 140-152 millimeters (5.5-6 inches) wide by 3.6 meters (12 feet) long.0F1 The thickness of the panels varies depending on where they are used in the SFP. Thicker panels (i.e., thicker cermet) are placed in areas of the SFP that store freshly off-loaded fuel, whereas thinner panels are used in areas of the SFP that store older fuel. The BORAL panels are secured to the stainless-steel spent fuel storage racks by stainless-steel cover panels welded to the storage rack or by bands welded onto the storage rack positioned intermittently along the length of the panel. Figure 1 above illustrates the panel usage for the flux trap spent fuel rack and egg-crate configurations.

Metamic is a discontinuous metal matrix composite composed of an aluminum 6061 matrix and American Society for Testing and Materials C-750 Type I B4C particles. Metamic ingots are produced by the powder metallurgy process, extruded into preforms, and then hot-rolled to the final thickness. Metamic is essentially free of porosity (i.e., close to theoretical density),

and the mechanical properties vary with the B4C content. Table 2 shows the mechanical properties of Metamic at 15-percent, 31-percent, and 40-percent B4C loadings.

Boralcan is also a discontinuous metal matrix composite composed of an 1100 aluminum matrix and B4C particles produced by mixing the B4C particles into molten aluminum and direct chilled into billets of 152 millimeters by 152 millimeters (6 inches by 6 inches). The billets can be rolled into the final product form.

Table 2 Average Mechanical Properties of Metamic for Various B4C Loadings [9F8]

B4C Loading (wt. %)

Mechanical Property 15%

31%

40%

Yield Strength, MPa 157 227 207 Ultimate Strength, MPa 232 272 277 Elongations, %

10.6 2.4 1.0 Note: MPa = megapascals 1

These panel dimensions are also typical of other aluminum-based NAM panels such as Metamic and Boralcan.

7

4. Qualification Testing Materials qualification testing is performed to demonstrate that the materials selected for service possess the properties needed to meet the requirements for the intended application, including maintaining the ability to perform their safety function for the service life of the SFP. Various vendors and manufacturers have performed qualification testing on aluminum-based NAMs to demonstrate the adequacy of the materials for service in SFPs. Sections 4.1 through 4.3 review the qualification testing performed for Metamic, Boralcan, and BORAL, respectively.

Section 4.4 shows how the qualification testing addressed the potential degradation mechanisms experienced in the SFP environment. Section 4.5 presents recent testing on BORAL panels removed from the Zion Nuclear Power Station (Zion) SFP (now decommissioned).

4.1 Metamic Two reports document qualification testing of Metamic: (1) an EPRI report, Qualification of Metamic for Spent-Fuel Storage Application, issued 2001 [10F9], and (2) a topical report submitted to the NRC by Entergy Nuclear Generating Company and prepared by Holtec, Use of Metamic in Fuel Pool Applications [11F10]. Based on the NRCs safety evaluation of the Holtec topical report, as reported in Safety Evaluation of the Topical Report to Support the Use of Metamic in Fuel Pool Applications at Arkansas Nuclear One, [12F11] dated May 15, 2003, the NRC approved the use of Metamic as a neutron absorber in SFPs with certain conditions and limitations. (Section 5 of this report discusses the related NRC safety evaluations.)

4.1.1 Summary of EPRI Report on Metamic 4.1.1.1 Materials The materials tested included three compositions of Metamic containing 15 weight percent (wt.%), 31 wt.%, and 40 wt.% B4C. EPRI tested both mill finished and anodized samples, as well as mill-finished 6061 aluminum control specimens. Weld coupons made from the 15 wt.% material were tested in accelerated corrosion tests and as part of radiation exposure testing.

4.1.1.2 Short-Term Elevated Temperature Testing Rectangular specimens and tensile specimens from the 15 wt.% and 31 wt.% alloys were exposed to 482 degrees C (900 degrees F) in an inert atmosphere for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />, simulating storage cask vacuum drying operations. Before and following exposure, a variety of measurements were performed on the rectangular specimens, including visual inspection, high-resolution photography, dimensions, dry weight, density, and neutron attenuation. Except for neutron attenuation, these same measurements were performed on the tensile specimens, along with mechanical testing and radiography.

8 Following exposure at 482 degrees C (900 degrees F) in an inert atmosphere for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />, the mill-finished specimens darkened, indicating that some oxidation occurred despite the oven being purged with helium. The anodized specimens darkened very slightly. Dimensional changes measured on the rectangular and tensile specimens were either within the measurement error or, if measurable, were less than 0.06 percent for length and width and less than 0.5 percent for thickness. Furthermore, weight, density, hardness, and B-10 areal density did not change.

Tensile testing of the nonwelded 15 wt.% specimens following exposure exhibited yield strengths, ultimate strengths, and elongations to failure statistically similar to those measured for specimens that were not exposed to elevated temperatures. The 31 wt.% nonwelded specimens exhibited a slight but detectable decrease in yield and ultimate strength and an increase in elongation. These slight changes were attributed to annealing. Tensile testing of the welded 15 wt.% coupons exhibited considerable variability in properties and lower strength than the base metal specimens.

4.1.1.3 Long-Term Elevated Temperature Testing The performance of Metamic was evaluated following exposure to 398 degrees C (750 degrees F) in an air atmosphere for 2,133 hours0.00154 days <br />0.0369 hours <br />2.199074e-4 weeks <br />5.06065e-5 months <br />, 4,124 hours0.00144 days <br />0.0344 hours <br />2.050265e-4 weeks <br />4.7182e-5 months <br />, and 6,139 hours0.00161 days <br />0.0386 hours <br />2.29828e-4 weeks <br />5.28895e-5 months <br />. The test temperature is representative of the maximum allowable fuel-cladding temperature for dry cask storage of 400 degrees C (752 degrees F) [13F12]. The coupons were subjected to nondestructive testing consisting of visual inspections, dimensional measurements, and physical property measurements. Further exposure of the 6,139-hour coupons was conducted for an additional 2,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />, after which the nondestructive tests were repeated and additional destructive tests performed.

The results showed that within the precision of the measurement instruments, no detectable changes in dimensions, density, dry weight or areal density occurred. Tensile test results for the 15 wt.% coupons showed that the tensile properties for the postexposure specimens were within one standard deviation of the preexposure coupons. The 15 wt.% welded coupons exhibited lower strength and elongation following elevated temperature exposure than the nonwelded coupons. The 30 wt.% postexposure coupons exhibited a slight decrease in strength and a slight increase in elongation attributed to annealing of residual cold work.

4.1.1.4 Accelerated Corrosion Testing at 90.5 degrees C (195 degrees F)

Accelerated corrosion testing at 90.5 degrees C (195 degrees F) was performed on Metamic coupons in simulated BWR and PWR water. The typical temperature for SFPs is 27-40 degrees C (80-100 degrees F). The test solutions consisted of deionized water for the BWR water and deionized water with 2,500 ppm boron as boric acid for the PWR water. Tests were conducted to characterize the general (uniform) and localized (pitting) corrosion behaviors after approximately 2,000, 4,000, 6,000, and 8,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of exposure for coupons in the mill-finished and anodized surface conditions.

9 Specific coupon types were used to evaluate the various corrosion mechanisms. For both coupon surface conditions, coupons for general corrosion, crevice corrosion, weldment corrosion and galvanic corrosion were tested. Additionally, coupons were encapsulated in stainless-steel capsules to simulate the semi-stagnant conditions of panels in fuel racks. A set of the anodized coupons were tested with intentional scratches on the surface.

4.1.1.5 Corrosion Testing Results The general corrosion tests performed on the mill-finished coupons resulted mostly in localized pitting in both BWR and PWR simulated water. The 15 wt.% BWR water coupons showed a 3-percent weight loss after 9,020 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> in addition to pitting, while the PWR coupons did not exhibit detectable weight loss. The 31 wt.% coupons showed only limited pitting near the coupons holders, which was likely from crevice effects. These coupons, unlike the 15 wt.%

coupons, were cleaned by glass beading after extrusion. The anodized coupons were chemically cleaned before being anodized. Thus, most of the anodized coupons did not exhibit pitting. However, after 6,139 hours0.00161 days <br />0.0386 hours <br />2.29828e-4 weeks <br />5.28895e-5 months <br /> and 9,020 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />, four specimens in total exhibited limited pitting but no detectable weight loss. Cleaning the coupons before anodizing removes impurities that can potentially lead to pitting, and anodizing protects against corrosion.

Anodized specimens that were scratched to remove the anodized layer and subjected to general corrosion testing did not exhibit any indications of accelerated corrosion in the scratched areas. Based on optical microscopy observations, it appeared that the oxide was reforming in the scratched areas.

The results of the tests performed on encapsulated coupons showed that neither the mill-finished nor anodized coupons tested in BWR and PWR water exhibited weight loss after 9,020 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> of testing.

Galvanic corrosion testing coupons consisted of bimetallic coupons clamped together, where the Metamic was clamped to Inconel 718, 304L stainless steel, and Zircaloy 2. The coupons were inspected after 4,124 hours0.00144 days <br />0.0344 hours <br />2.050265e-4 weeks <br />4.7182e-5 months <br /> and 9,030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> of testing. The mill-finished coupons exhibited varying degrees of localized pitting attributed to surface contaminants from the extrusion process. The anodized coupons did not exhibit pitting except for one 31 wt.%

Metamic-to-304L stainless-steel coupon after 9,030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> of testing. The pitting was attributed to surface contamination. Figure 2 shows the results of the weight change measurements for the BWR and PWR test solutions; the BWR solution appears to be more aggressive than the PWR solution based on higher weight losses.

10 (a)

(b)

Figure 2 Weight loss results for galvanic corrosion tests performed on Metamic coupled with 304L stainless steel, Inconel 718, and Zircaloy 2: (a) BWR solution, dried weight and (b) PWR solution, dried weight (used with permission from EPRI)

Crevice corrosion testing results after 9,030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> of exposure showed that in both the BWR and PWR solutions, the mill-finished coupons exhibited some crevice corrosion, as shown in Figure 3. The oxide formation in the test location resulted in a thickness increase of about 20-30 percent, compared to the area of the specimens remote from the test location. The anodized specimens showed no thickness changes.

11 (a)

(b)

Figure 3 Crevice corrosion results for Metamic mill-finished and anodized specimens:

(a) BWR solution and (b) PWR solution (used with permission from EPRI)

The weld coupons for the mill-finished material exhibited substantial corrosion localized at the weld metal. The weight loss measured for the mill-finished specimens tested in the BWR solution was approximately 2 percent, while the weight loss in the PWR solution was not measurable after 9,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. The anodized coupons did not exhibit any corrosion.

4.1.1.6 Accelerated Radiation Testing Radiation exposure testing was performed on 15 wt.% and 31 wt.% Metamic specimens irradiated at six increasing gamma doses (in rads) of 4.5x109, 9.0x109, 3.6x1010, 5.6x1010, 7.5x1010, and 1.5x1011. The maximum dose of 1.5x1011 rads is equivalent to the exposure of a fuel rack for approximately 40 years of exposure. The corresponding fast neutron fluence was 5.8x1019 neutrons per square centimeter (n/cm2) which is orders of magnitude in excess of the fast neutron fluence expected in wet or dry storage applications [9]. Visual inspection of the coupons showed that all of the coupons turned a dark gray color, and the mill-finished coupons developed an oxide film. Some coupons showed indications that boiling occurred. Localized

12 pitting was observed on the surfaces of both mill-finished and anodized coupons for gamma doses of 3.6x1010 and greater. No measurable changes in dimension, dry weight, density, B-10 areal density, or hardness were observed following irradiation to 1.5x1011 rads. Figure 4 and Figure 5 show the mechanical properties testing performed on specimens subjected to increasing gamma doses. The general trends indicate that the ultimate tensile strength decreased, and the yield strength increased over the range of the gamma doses that were tested. The elongation also decreased. These material-property changes are not significant because NAMs are not structural materials.

Figure 4 Yield strength and ultimate tensile strength of Metamic versus gamma radiation exposure for materials with 15-percent and 31-percent B4C content Figure 5 Elongation of Metamic versus gamma radiation exposure for materials with 15-percent and 31-percent B4C content 15000 20000 25000 30000 35000 40000 0.00E+00 2.00E+10 4.00E+10 6.00E+10 8.00E+10 1.00E+11 1.20E+11 1.40E+11 1.60E+11 Yield and Ultimate Strengths [psi]

Dose [rads]

YS (15% B4C)

YS 31%

UTS 15%

UTS 31%

0 2

4 6

8 10 12 14 16 0.00E+00 2.00E+10 4.00E+10 6.00E+10 8.00E+10 1.00E+11 1.20E+11 1.40E+11 1.60E+11 Elongation [%]

Dose [rads]

%el 15%

%el 31%

13 4.1.2 Summary of Holtec Report Entergy submitted the Holtec report, Use of Metamic in Fuel Pool Applications, [10] to the NRC as a topical report to support the use of Metamic in SFP applications. The report summarized the EPRI report, Qualification of Metamic for Spent-Fuel Storage Application, described above. The Holtec report included additional tensile test data on 40 wt.% B4C Metamic at increasing test temperatures without prior high-temperature exposure. Figure 6 shows the average yield strength, ultimate tensile strength, and percent elongation to failure at each test temperature (three specimens per test). The trends in the properties are consistent with those of aluminum-based metal matrix composites [14F13].

Figure 6 Mechanical properties of Metamic with 40 wt.% B4C [10]

4.2 Boralcan Boralcan is the trade name for the B4C reinforced 1100 aluminum metal matrix composite produced by Rio Tinto Alcan for neutron-absorber applications [15F14]. Northeast Technologies Company (NETCO), a subsidiary of Curtiss-Wright, uses Boralcan for its NAM SNAP-IN inserts for SFP racks and published a report [14] detailing a qualification program in support of obtaining NRC approval to use Boralcan in U.S. nuclear plant SFP racks.

NETCO performed accelerated corrosion testing on Boralcan to characterize the general corrosion and galvanic corrosion of rolled sheet and bent specimens. The bent specimens 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 0

5000 10000 15000 20000 25000 30000 35000 40000 45000 0

200 400 600 800 1000 Average % El. to Failure Average YS and UTS [psi]

Test Temperature [°F]

YS psi UTS psi

%el

14 simulated the stresses and strains associated with the angled configuration of the SNAP-IN panels and were tested to characterize the effect of applied strain on the corrosion behavior.

The accelerated corrosion tests were conducted in purified water to simulate BWR SFP water and in purified water with 2,500 ppm boron added as boric acid to simulated PWR SFP water for a maximum duration of 8,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />. The tests were performed at 90.5 degrees C (195 degrees F) to accelerate the testing compared to typical SFP temperatures. Galvanic corrosion test specimens consisted of bimetallic specimens of Boralcan attached separately to 304L stainless steel, Zircaloy, and Inconel 718.

The accelerated corrosion testing in the BWR test solution resulted in similar general and galvanic corrosion rates of 0.01-0.02 mils per year (mpy), including the bent samples. Similarly, testing in the PWR test solution resulted in general corrosion rates of 0.01-0.03 mpy; for galvanic corrosion, 0.01-0.04 mpy, including the bent samples. Neutron attenuation testing did not indicate B4C losses greater than the measurement uncertainty of the neutron attenuation test method.

4.3 BORAL The qualification testing of BORAL was performed mostly by Brooks & Perkins [16F15, 17F16, 18F17, 19F18, 20F19, 21F20, 22F21] and summarized in Handbook of Neutron Absorber Materials for Spent Nuclear Fuel Transportation and Storage Applications2006 Edition, published by EPRI [8].

4.3.1 Density and Composition As a mixture of a metal and a ceramic compressed under heat, cermets may not reach theoretical density, as is the case for BORAL. For example, the Brooks & Perkins testing showed that a 50/50 mixture of aluminum and B4C may contain approximately 5-percent porosity by volume (or 95-percent core compaction density). Brooks & Perkins surveyed 40 panels, and the compaction density ranged from 89.5 to 96.1 percent, with an average of 93 percent, and indicated that the variability in the compaction density can be decreased through processing controls; however, the EPRI NAM Handbook provided no examples [8].

4.3.2 Areal Density Figure 7 shows the neutron attenuation characteristics for BORAL versus an ideal neutron absorber as a function of the B-10 areal density. For equal areal densities, the neutron attenuation of BORAL is less than the ideal absorber due to self-shielding effects arising from the B4C particle-size distribution. The average B4C particle size in BORAL is 85 microns, with individual particles reaching 250 microns. Such particle sizes may lead to self-shielding and reduced neutron absorption.

15 Figure 7 Neutron attenuation characteristics of BORAL compared to an ideal absorber

[20]

4.3.3 Mechanical Properties 1100 aluminum is commercially pure aluminum and is not typically used for structural applications. It is used in BORAL in powder form to help bind the cermet and bond the cermet to the cladding. The aluminum 1100 cladding provides support for the panel and corrosion resistance. Table 3 shows the reported room temperature mechanical properties. Mechanical properties testing of BORAL at elevated temperatures up to 260 degrees C (500 degrees F) showed that the tensile strength can decrease by 30-50 percent compared to room temperature values.

Table 3 Room Temperature Mechanical Properties of BORAL Modulus of elasticity 9 msi Tensile strength 10 ksi Elongation 0.1%

16 4.3.4 Radiation Testing BORAL samples were exposed to gamma, thermal neutron, and fast neutron radiation in water, with gamma exposures up to 7x1011 rads [21] in preparation for the use of BORAL at the Hope Creek Generating Station. Nine years of exposure resulted in fast neutron fluence of 3.6x1018 n/cm2 and thermal neutron fluence of 2.7x1019 n/cm2 [8], representing more than 40 years of exposure of the Hope Creek Generating Station spent fuel racks at the time of the publication [21]. The samples were subjected to visual inspection, neutron radiography, neutron attenuation, tensile testing, and chemical analysis for B-10 content. Following testing, the surfaces of the specimens were severely oxidized, but no loss of B4C was detected by neutron attenuation, neutron radiography, or chemical analysis. The severity of the corrosion was attributed to elevated temperatures due to the high energy levels of the gamma exposure, which may have resulted in localized boiling at the sample surfaces. Testing was also performed [19]

to determine if irradiation-induced outgassing occurred with BORAL due to the possible formation of helium gas; the testing showed that it did not occur.

4.3.5 Corrosion Testing 4.3.5.1 General Corrosion Brooks & Perkins conducted corrosion testing of BORAL during the development stages of individual spent fuel storage modules. Testing performed in simulated BWR water (i.e., demineralized water) at 20-26 degrees C (68-78 degrees F) for 1 year resulted in a reported two-sided corrosion rate of 1.91 milligrams per square centimeter per year (mg/cm2/yr), or 0.28 mils per year [15]. The water was not refreshed during the test. Brooks & Perkins also performed corrosion testing of preirradiated BORAL samples sealed in a stainless-steel enclosure to simulate the potential PWR environment present in the spent fuel storage module. Borated water was added into the stainless-steel enclosure between the BORAL and the stainless steel prior to suspending it in the pool of the University of Michigans Ford Reactor for 1 year at 100 degrees C (212 degrees F). The results indicated that no pitting, corrosion damage or other physical damage was observed [19].

4.3.5.2 Galvanic and Crevice Corrosion Brooks & Perkins [16] performed galvanic and crevice corrosion testing on 304 stainless steel-to-aluminum 1100 couples in high-purity, demineralized water with pH between 5 and 6 and oxygen concentrations between 4 and 5 ppm. The testing was conducted at 90-180 degrees C (194-356 degrees F). After 2,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of exposure, the maximum pit depth measured in the aluminum was less than 0.127 millimeter (0.005 inch), and no pitting was observed on the stainless steel.

17 4.3.6 Elevated Temperature Testing The results of tests performed on BORAL after exposure to simulated dry cask storage container loading and vacuum drying were reported by AAR Advanced Structures and summarized in the EPRI NAM Handbook. The simulation involved exposing the BORAL to the following conditions [8]:

soaked in distilled water at 66 degrees C (150 degrees F) for 1 week removed and weighed samples heated to 315 degrees C (600 degrees F)at 3 torr for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> sealed and weighed samples heated to 315 degrees C (600 degrees F) for 168 hours0.00194 days <br />0.0467 hours <br />2.777778e-4 weeks <br />6.3924e-5 months <br /> weighed and subjected samples to mechanical tests The samples exhibited slight increases in weight and thickness attributed to the formation and growth of an oxide layer. The EPRI NAM Handbook mentions that the mechanical properties were not affected by the simulated processing conditions, but it did not present the results.

Section 4.3.3 reports other elevated-temperature testing results for BORAL.

4.4 Qualification Testing Versus Potential Degradation Mechanisms The SFPs are filled with high-purity demineralized water maintained at 27-40 degrees C (80-100 F). PWR SFPs contain about 2,000 ppm of added boron as boric acid to reduce dilution of the RPV coolant during refueling. Table 1 in Section 2 of this report shows the EPRI guidelines for SFP water chemistry for PWR and BWR SFPs. SFPs typically maintain lower impurity levels than the recommended guidelines.

Proper materials selection considers not only structural loading factors (e.g., strength, fatigue, modulus) but also service environment factors (e.g., temperature, water chemistry and corrosion) because environmental degradation can alter a components structural performance, or other performance characteristics, needed throughout the components service life. Metallic NAMs used in SFPs generally do not serve a structural function, although they do need to resist buckling under their own weight. Metallic NAMs are typically held in place by a thin stainless-steel wrapper welded to the storage rack cell wall or by stainless-steel straps at intermittent locations along the length of the panels. Thus, aside from neutron attenuation properties, the most important properties to consider for NAMs in SFPs are service environment factors.

This section summarizes the data obtained for BORAL, Boralcan, and Metamic during qualification programs to address service environment factors such as corrosion, temperature, and radiation. Specifically, it addresses available vendor data generated to demonstrate the suitability of these materials for use in SFPs.

18 4.4.1 Corrosion Corrosion can affect metallic components by causing widespread material loss (e.g., general corrosion), localized penetrations (e.g., pitting corrosion), or localized gross material loss (e.g., galvanic and crevice corrosion), all of which can limit the service life of components. If NAMs experience material losses, B4C might also be lost, resulting in a decrease in the B-10 areal density and reducing the effectiveness of the panel to absorb neutrons. Thus, it must be demonstrated that the corrosion resistance of NAMs is such that potential material losses will not result in B4C losses that challenge the assumptions in the NCS AOR.

4.4.1.1 General Corrosion General corrosion is the corrosion of the specimen or component on all surfaces. It is typically characterized by base metal weight loss and thinning and can be accompanied by oxide layer growth. Oxide layer growth may result in weight gain and an increase in the total thickness, but the section thickness of the NAM would be reduced. BORAL, Boralcan, and Metamic are aluminum-based materials that typically exhibit low general corrosion rates in aqueous solutions with low concentrations of contaminants, such as chloride or sulfate.

BORAL, Metamic, and Boralcan underwent general corrosion testing as part of the qualification testing. Testing performed by Brooks & Perkins on BORAL showed that the two-sided corrosion rate in BWR water (demineralized water) was 1.91 mg/cm2/yr (0.28 mil per year). Corrosion rates in BWR water are expected to be somewhat higher than in PWR water due to the higher pH in the absence of boron. As demonstrated in the qualification reports by EPRI and Holtec, mill-finished (unanodized) Metamic exhibited a 3-percent weight loss in BWR water and no measurable weight loss in PWR water during accelerated corrosion testing at 90.5 degrees C (195 degrees F). Anodized Metamic exhibited no measurable weight loss in either the BWR or PWR water test environments. NETCO performed accelerated corrosion testing on Boralcan and measured corrosion rates of 0.03 mpy or less in BWR and PWR test environments.

4.4.1.2 Pitting Corrosion-resistant materials, such as aluminum and stainless steel, develop oxide films that protect the surface from general corrosion. Localized corrosion of these metals can occur in the form of pits if the oxide layer is disturbed, destabilized, or prevented from forming locally. For aluminum, chloride ions may cause pitting by destabilizing the oxide locally. Additionally, localized galvanic corrosion, such as near iron deposits from the manufacturing process, may lead to pitting.

Corrosion testing of BORAL showed that minor pitting can occur. In later sections of this report, additional data for BORAL pitting show that pits can penetrate the cladding and reach the cermet core with no observed loss of B4C. For Metamic, testing of mill-finished (unanodized) samples showed that minor pitting can occur and was often due to localized

19 galvanic corrosion from contaminants on the surface, such as iron, left behind in the manufacturing process. Most of the anodized Metamic specimens did not exhibit pitting corrosion, but four did, and the pitting was attributed to surface contaminants that were not removed during the cleaning process. The analysts did not observe pitting corrosion of Boralcan.

4.4.1.3 Crevice Corrosion Crevice corrosion is a form of localized corrosion that occurs at the interface between two adjoined components. Crevice corrosion takes the form of pitting and may occur in environments in which the material is otherwise resistant to corrosion. Changes to the local solution chemistry in a crevice result in localized corrosion.

Brooks & Perkins conducted a 2,000-hour crevice corrosion and galvanic corrosion test on 1100 aluminum (BORAL cladding), which resulted in pitting up to 0.127 millimeter (0.005 inch) deep when coupled to stainless steel [16]. Crevice corrosion testing on mill-finished Metamic for 9,030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> exhibited a 20-30-percent increase in total thickness from oxide growth, while anodized specimens exhibited no crevice corrosion [9]. Crevice corrosion testing was not performed on Boralcan. However, since Boralcan is made of aluminum 1100, its crevice corrosion properties are likely similar to BORAL, which is clad with aluminum 1100.

4.4.1.4 Galvanic Corrosion When two metals with different, freely corroding potentials are electrically connected in a conductive solution, the material with the lowest corrosion potential (the anode) will corrode at a faster rate sacrificially to the material with the higher corrosion potential (cathode). This is called galvanic corrosion. The potential for galvanic corrosion in the SFP NAM rack system is the contact between the aluminum NAM and the stainless-steel rack. Galvanic corrosion rates depend on physical factors such as the cathode-to-anode surface area ratio and the corrosivity of the environment. For the SFP NAM rack systems, the cathode-to-anode surface area ratio is close to unity in the vicinity of the NAM, which may help reduce to galvanic corrosion.

Additionally, SFP purity levels result in low conductivities, which further deceases the potential for galvanic corrosion.

Brooks & Perkins [16] evaluated the galvanic corrosion performance of BORAL by testing BORAL (or aluminum) coupled with 304 stainless steel in tests designed to evaluate both crevice and galvanic corrosion. Minor pitting of the aluminum was observed. Galvanic corrosion of Metamic using three types of bimetallic specimens consisting of Metamic attached separately to 304L stainless steel, Zircaloy 2 and Inconel 718 showed that the mill-finished Metamic exhibited pitting associated with surface contamination, while the anodized specimens did not exhibit galvanic attack. Boralcan galvanic testing of bimetallic specimens consisted of Boralcan attached separately to 304L stainless steel, Zircaloy, and Inconel 718, and the results indicated a corrosion rate of less than 0.05 mpy for both simulated PWR and BWR water environments.

20 4.4.1.5 Radiation Effects Radiation testing at doses equivalent to 40 years of exposure for representative SFP racks showed that both Metamic and BORAL maintain their mechanical properties, physical properties, and neutron attenuation properties after irradiation (see Sections 4.1.1.6 and 4.3.4, respectively). The testing did not address radiation exposures for longer service lives.

4.4.1.6 Blistering and Swelling BORAL has experienced blistering while in service. The blisters were usually concentrated near the exposed edges of the panels where the semiporous cermet core is exposed to the SFP water. The postulated mechanism involved water intrusion through the pore network of the cermet followed by corrosion of the aluminum in the cermet, resulting in the production of hydrogen gas. After continued corrosion, corrosion products block the pores and trap the hydrogen, resulting in pressures high enough to force the cladding and the cermet to separate at the interface [8, 19]. Neutron attenuation testing of blistered BORAL panels has demonstrated that the B-10 areal density is not affected because blistering does not result in the loss of B4C [8]. However, blistering might affect criticality assumptions due to potential moderator (i.e., water) displacement. Also, fuel binding in the SFP may occur as a result of blistering.

4.5 Supporting Test Data Since Material Qualification The NRC and EPRI conducted cooperative research under a memorandum of understanding to evaluate the performance of BORAL panels removed from the decommissioned Zion SFP.

During decommissioning, when fuel was being moved into storage containers, NETCO, under contract to EPRI, performed B-10 areal density measurements on select panels using the BADGER neutron attenuation measurement system. Alaron Nuclear Services, under contract to EPRI, removed the panels from the SFP and sectioned each panel into 12 equal lengths. For each panel, half of the sections were sent to EPRIs contractor and the other half were sent to the Savannah River National Laboratory (SRNL) (the NRCs contractor) for evaluation. In addition to the BORAL panels, NETCO evaluated the Zion SFP surveillance coupons and compared the results to previously evaluated coupons to monitor material performance.

EPRIs evaluations included visual exams, dimension measurements, pitting evaluations, and B-10 areal density measurements through neutron attenuation (at Pennsylvania State University), and EPRI reports provide the results [23F22, 24F23]. The NRC evaluations included visual exams, corrosion testing, and chemical analysis, with the results reported in SRNL-TR_2018-00244, Characterization and Analysis of Boral from the Zion Nuclear Power Plant Spent Fuel Pool, Revision 0, issued 2019 [25F24].

The visual examination results by NETCO and SRNL showed that the panels were generally in good condition after more than 20 years in the SFP environment. The panels exhibited

21 discoloration and some pitting but no indications of significant corrosion. NETCOs pitting analyses for both the surveillance coupons and the panels indicated that some pits penetrated the cladding through to the cermet core, but there was no loss of B4C. The B-10 areal density measurement results confirmed this and showed that the B-10 areal density for the panels was higher than the minimum certified value and, for the coupons, there was no change from the pre-insertion values.

SRNL performed electrochemical corrosion (EC) testing and immersion testing on samples at a variety of temperatures, water chemistries, and sample conditions to assess the performance of the BORAL panels. The corrosion rates measured in the SRNL testing were consistent with the results obtained in the qualification testing under similar conditions. Corrosion rates increased at elevated temperatures and elevated water contamination levels. Some of the EC test results obtained are provided below. SRNL performed immersion corrosion testing on the fully exposed cermet core under elevated conditions of temperature and contamination and observed no loss of B4C.

The EC testing solutions consisted of distilled water containing three levels of contaminants (i.e., chloride, sulfate, and fluoride) that represented (1) nominal levels based on Zion historical data, (2) base conditions based on the EPRI SFP water chemistry guidelines, and (3) off-normal levels representing elevated contamination levels. Boron, as 2,500 ppm boric acid, was added to represent PWR water chemistry. Tests were performed at 25 degrees C, 40 degrees C (select tests only), 75 degrees C, and 98 degrees C. Figure 8 shows the testing results for samples with 600-grit freshly ground surfaces. The results show that elevated temperatures and the off-normal water chemistry resulted in increased corrosion rates.

22 Figure 8 Linear polarization resistance EC testing results for samples of BORAL with 600-grit freshly ground cladding surfaces in solutions with various concentrations of chloride at various temperatures Recognizing that freshly ground surfaces are not representative of oxidized materials in service, experiments were performed on freshly ground samples placed in the nominal test solution, and linear polarization resistance measurements were performed once per week for 4 weeks to determine the effect of oxide formation on the corrosion rate. The results, given in Figure 9, show that the corrosion rates decreased from the respective initial values by a factor of about 6 in the PWR solution and by a factor of 3 in the BWR solution. The final corrosion rates are consistent with the corrosion rates measured for BORAL, as reported by Brooks & Perkins during the qualifications testing for BORAL [15, 19].

23 (a)

(b)

Figure 9 Corrosion rates obtained for BORAL samples starting with freshly ground surfaces in the nominal water chemistry for (a) PWR water and (b) BWR water

5. NRC Safety Evaluations The NRC staff issued a safety evaluation [26F25] approving the use of Metamic in SPFs, based on the Holtec topical report [10] submitted by Entergy and summarized above. The staff approved its use upon the condition of implementation of a coupon surveillance program, a limitation of a maximum B4C content of 31 wt.% in the Metamic, and a description of the anodizing and cleaning methods, if used.

The NRC approved a license amendment request for the use of Boralcan in the LaSalle County Station Unit 1 SFP [27F26] based on a NETCO topical report [14] for the use of Boralcan SNAP-IN inserts. In addition, the NRC approved the licensees proposed coupon surveillance program, which consisted of monitoring changes to the physical properties and neutron attenuation characteristics of the material.

24 BORAL has the longest service history of the aluminum-based NAMs. Some of the earliest uses were at Salem Nuclear Generating Station, Monticello Nuclear Generating Plant, and Duane Arnold Energy Center (Duane Arnold) in the 1976-to-1978 timeframe and were approved on a case-by-case basis through safety evaluations of license amendments issued, typically, to approve the installation of high-density storage racks. Unlike that for Metamic and Boralcan, the approval for the use of BORAL in SFPs was not based on a specific topical report but, rather, on the reported results of BORAL qualification programs. Some of the earliest uses of BORAL predated NRC guidance on NAM performance monitoring (e.g., GL 78-11 [1]); thus, their use did not always include monitoring programs. Reracking NAM rack systems in SFPs after the issuance of GL 78-11 involving the use of BORAL typically included either a coupon surveillance program or a panel monitoring schedule (i.e., periodic in situ B-10 areal density testing).

6. Operating Experience BORAL is the most used aluminum-based NAM in domestic SFPs, used in about 58 of the 97 SFPs. Operating experience for BORAL includes instances of blistering, as documented in NRC Information Notice (IN) 1983-29, Fuel Binding Caused by Fuel Rack Deformation, dated May 6, 1983 [28F27], and IN 2009-26, Degradation of Neutron-Absorbing Materials in the Spent Fuel Pool,: dated October 28, 2009 [29F28]. In 1982, Main Yankee Atomic Power Company reported instances of spent fuel rack bulging during a refueling operation. Fuel rack bulging was identified in 15 empty cells and in 6 occupied cells, causing fuel binding. The licensee drilled holes into the BORAL through the rack cover plate, releasing gas (postulated to be hydrogen) and decreasing deformation. The source of the gas was attributed to the reaction between water and aluminum within the cermet core. IN 1983-29 cites two other instances of BORAL bulging: Connecticut Yankee Nuclear Power Plant in 1978 and Kewaunee Power Station in 1980 (Licensee Event Reports 50-213/78-004 and 50-305/80-039). Somewhat more recent examples of BORAL blistering were reported by Seabrook Station in 2003 and by Beaver Valley Power Station in 2007. Seabrook Station performed neutron attenuation testing on the affected coupons and determined that the B-10 areal density was within specification and no loss of B4C occurred.

BORAL has also experienced pitting corrosion in service, as observed on surveillance coupons and reported in responses to GL 2016-01. Pitting was also observed on panels removed from service during SFP decommissioning. EPRI analyzed the pitting experienced by BORAL surveillance coupons and panels removed from the Zion SFP and documented its results in two reports [22] and [23]. EPRI measured the number and dimensions (surface size and depth) and found that most pits had not penetrated the aluminum cladding, but some pits penetrated through the cladding to expose the cermet core. Close examination of the pits that penetrated through the cladding did not reveal indications of B4C loss.

A search of the NRC Agencywide Documents Access and Management System (ADAMS) for operating experience related to Metamic and Boralcan degradation did not find any instances for either material. A similar search performed in the NRC Licensee Event Report

25 database and the Institute of Nuclear Power Operations Industry Reporting and Information System also did not identify any reports.

7. Surveillance Surveillance monitoring programs for NAMs are in place in SFPs to help identify changes in the material that could challenge the NCS AOR, where the most important possible material change is the loss or displacement of the NAM (i.e., B4C). Before the issuance of GL 78-11, the NRC did not require surveillance and monitoring programs. GL 78-11 provides guidance to licensees on the information that reviewers need to prepare safety evaluations, which includes nuclear, thermal hydraulics, mechanical, material, structural, and environmental aspects. Information requested in GL 78-11 relates specifically to neutron absorber materials and includes the following:

Section III, Nuclear and Thermal-Hydraulic Considerations, Section 1.4, Rack Modifications, states, in part, the following:

(d) For lattices which use boron or other strong neutron absorbers, provide:

(1) The effective areal density of the boron-ten atoms (i.e., B10 atoms/cm2 or the equivalent number of boron-ten atoms for other neutron absorbers) between fuel assemblies.

Section III, Section 1.5, Acceptance Criteria for Criticality, states, in part, the following (emphasis added):

(1) For those facilities which employ a strong neutron absorbing material to reduce the neutron multiplication factor for the storage pool, the Licensee shall provide the description of onsite tests which will be performed to confirm the presence and retention of the strong absorber in the racks. The results of an initial, onsite verification test shall show within 95 percent confidence limits that there is a sufficient amount of neutron absorber in the racks to maintain the neutron multiplication factor at or below 0.95. In addition, coupon or other type of surveillance testing shall be performed on a statistically acceptable sample size on a periodic basis throughout the life of the racks to verify the continued presence of a sufficient amount of neutron absorber in the racks to maintain the neutron multiplication factor at or below 0.95.

Section IV, Mechanical, Material and Structural Considerations, Section 7, Material, Quality Control and Special Construction Techniques, states, in part, the following:

26 Acceptance criteria for special materials such as poison materials should be based upon the results of the qualification program supported by test data and/or analytical procedures.

Section IV, Subsection (8), Testing and Inservice Surveillance, states the following:

Methods for verification of long-term material stability and mechanical integrity of special poison material utilized for neutron absorption should include actual tests. Inservice surveillance requirements for the fuel racks and the poison material, if applicable, are dependent on specific design features. These features will be reviewed on a case by case basis to determine the type and the extent of inservice surveillance necessary to assure long-term safety and integrity of the pool and the fuel rack system.

In the years since the issuance of GL 78-11, licensees that have installed high-density SFP storage rack systems using NAMs have either instituted coupon surveillance monitoring programs or periodic NAM panel monitoring. The monitoring programs were reviewed on a case-by-case basis through the license amendment process. As such, the specific aspects of monitoring programs vary among licensees, but the programs are consistent with the guidance in GL 78-11. The aspects that vary among monitoring programs include the number and size of coupons, the testing periodicity, the types of tests performed, and the acceptance criteria. The Nuclear Energy Institute (NEI) recently established further guidance for monitoring NAMs in SFPs.

The NEI published guidance for designing NAM surveillance programs in the NRC-approved topical report NEI 16-03-A, Guidance for Monitoring of Fixed Neutron Absorbers in Spent Fuel Pools, Revision 0, issued May 2017 [30F29]. NEI 16-03-A, Revision 0, establishes practices consistent with NRC guidance for NAM-related staff reviews in NUREG-1800, Standard Review Plan for Review of License Renewal Applications for Nuclear Power Plants, Revision 2, issued December 2010 [31F30], NUREG-1801, Generic Aging Lessons Learned (GALL) Report, Revision 2, issued December 2010 [32F31], and NUREG-2191, Generic Aging Lessons Learned for Subsequent License Renewal (GALL) Report, issued May 2017 [33F32]. Aging management programs (AMPs) in NUREG-1801, Revision 2, provide guidance for NRC reviews of NAM surveillance programs for Boraflex in AMP XI.M22 and for all other NAMs separately in AMP XI.M40. NAM surveillance programs are intended to ensure compliance with applicable regulations in 10 CFR 50.68, GDC 61, Fuel Storage and Handling and Radioactivity Control, and GDC 62. NEI 16-03-A and the NRC guidance documents are consistent with each other about the design of surveillance programs and acceptance criteria.

Surveillance programs consist of a combination of three elements: (1) periodic removal of NAM coupons installed in the SFP for testing, (2) direct measurement of the neutron absorbing capacity of the NAM panels installed in the storage racks (i.e., in situ measurement), and (3) monitoring the SFP water chemistry for signs of NAM corrosion products. Surveillance programs first establish the initial conditions of the monitored materials and then monitor

27 changes using inspections, testing, and analysis to ensure compliance with NRC regulations.

AMP XI.M40 states the following:

The parameters monitored include the physical condition of the neutron-absorbing materials, such as in situ gap formation, geometric changes in the material (formation of blisters, pits, and bulges) as observed from coupons or in situ, and decreased boron areal density, etc. The parameters monitored are directly related to determination of the loss of material or loss of neutron absorption capability of the material(s).

Coupon-testing programs are the most prevalent monitoring program type and use samples of the NAM in use in the monitored SFP, configured to simulate the service conditions of the NAMs in the storage racks. Thus, coupons can be encased in stainless steel or left exposed directly to the SFP water. To meet testing and trending guidance, the program includes several coupons, typically installed in the SPF using an apparatus that allows them to be attached to the storage rack structure and also readily removed for testing. This apparatus, called a coupon tree, consists of several coupons linked together to form a chain-like configuration. It is usually placed near rack locations designated for freshly discharged fuel assemblies to generate an accelerated rate of accumulated exposure to those parameters that may impact aging/degradation mechanisms. [29]

Coupon surveillance programs consist of the following five elements:

(1) a sufficient number of coupons to provide data at appropriate intervals for the service life of the NAM (i.e., SFP service life, including license renewal and decommissioning)

(2) sampling intervals sufficient to detect changes in the materials (3) basic coupon testing that identifies physical changes in the material such as dimensions, weight, and appearance, and full testing, which adds evaluation of the neutron absorption capabilities of the material (e.g., B-10 areal density measurements); basic testing is conducted every 10 years, while full testing is conducted at least every 10 years and every 5 years for materials with known degradation mechanisms (4) placed in SFP locations that either represent in-service NAMs or exposure parameters (e.g., gamma fluence, temperature) considered conservative (5) acceptance criteria for B-10 areal density that ensure compliance with NRC regulations Measuring the B-10 areal density in situ typically involves employing a neutron attenuation technique designed and calibrated for the specific NAM application (i.e., SFP rack design, NAM B-10 areal density range). In situ measurement programs must sample a certain minimum number of panels based upon either the method prescribed in NUREG/CR-6698, Guide for Validation of Nuclear Criticality Safety Calculational Methodology, issued January 2001 [34F33], or by selecting panels believed to be exposed to the most severe conditions that are known to

28 influence degradation. For the former approach, at least 59 panels must be evaluated; for the latter approach, at least 1 percent of the total number of panels must be evaluated. The sampling interval for in situ measurements is 5 years initially for materials with little operating history, which can be extended to 10 years. For materials with sufficient operating history in the SFP environment and that have demonstrated stability in the material condition [29], the sampling interval may be up to every 10 years. For materials with known degradation or degradation mechanisms, the sampling interval is 5 years.

7.1 Surveillance Programs 7.1.1 Program Design At the time of this writing, there are 97 SFPs in service: 62 PWR pools and 35 BWR pools. Of the 97 SFPs, 68 use aluminum-based NAMs: BORAL in 56 pools, Metamic in 14 pools, and Boralcan in 4 pools. (Five SFPs use more than one type of aluminum-based NAM.) Of the 68 SFPs that use aluminum-based NAMs, 54 have coupon surveillance programs or a combination of coupons and in situ measurements, 41F2 SFPs rely or relied solely on in situ condition monitoring and areal density measurement techniques, and 5 have no surveillance program. Based on a review of the NRC NAM surveillance program database, most existing surveillance programs are consistent with the guidance outlined in NEI 16-03-A for the design of the programs (materials, configuration, intervals, and testing), but some do not currently include B-10 areal density measurements. Coupon surveillance programs consist of coupons made from the same materials used in the SFP racks. The coupons are either contained in stainless-steel sheathing, unsheathed, or a mixture of both. The coupons typically are the same width as the panels installed in the racks but vary in length depending on testing needs. The most common method used to install the coupons in the SFPs is using coupon trees suspended in an empty spent fuel rack cell near other cells designated to receive freshly discharged fuel assemblies. Table 4 shows the plants that use aluminum-based NAMs and the type of surveillance program in place.

7.1.2 Testing Intervals Surveillance coupon testing intervals vary among the licensees but, in general, follow the guidance in NEI 16-03-A with intervals of 2, 5, or 10 years. This is accomplished by either testing according to a prescribed number of years between testing or based on refueling cycles.

Several programs use an expanding interval approach with shorter intervals for the first three to five coupons, then increase the interval for subsequent coupons. In every case, the licensee initially includes enough coupons to cover the expected service life of the SFP. For programs that use only in situ testing, the surveillance interval is typically every 10 years after the baseline measurements are made.

2 Two of the four plants relied on in situ B-10 areal density measurements but recently have entered decommissioning and no longer have monitoring programs. They are included to help assess in situ monitoring programs.

29 Table 4 List of Plants that Use Aluminum-Based NAMs and the Type of Surveillance Program in Place Plant Absorber Material Surveillance Program Type Plant Type BORAL Metamic Boralcan Coupons or Coupons & In Situ In Situ only None Arkansas Nuclear One, Unit 1 X

X PWR Arkansas Nuclear One, Unit 2 X

X PWR Beaver Valley Power Station, Unit 1 X

X PWR Beaver Valley Power Station, Unit 2 X

X PWR Braidwood Station, Unit 1 X

X PWR Braidwood Station, Unit 2 X

X PWR Browns Ferry Nuclear Plant, Unit 1 X

X BWR Browns Ferry Nuclear Plant, Unit 2 X

X BWR Browns Ferry Nuclear Plant, Unit 3 X

X BWR Brunswick Steam Electric Plant, Unit 1 X

X BWR Brunswick Steam Electric Plant, Unit 2 X

X BWR Byron Station, Unit 1 X

X PWR Byron Station, Unit 2 X

X PWR Callaway Plant, Unit 1 X

X1 PWR Clinton Power Station, Unit 1 X

X X

BWR Comanche Peak Nuclear Power Plant, Unit 1 X

X PWR Comanche Peak Nuclear Power Plant, Unit 2 X

X PWR Cooper Nuclear Station X

X X

BWR Crystal River Unit 3 X

X2 PWR D. C. Cook Nuclear Plant, Unit 1, 2 (shared)

X X

PWR Davis-Besse Nuclear Power Station, Unit 1 X

X PWR Dresden Nuclear Power Station, Unit 2 X

X BWR Dresden Nuclear Power Station, Unit 3 X

X BWR Duane Arnold Energy Center X

X X3 BWR Edwin I. Hatch Nuclear Plant, Unit 1 X

X BWR Edwin I. Hatch Nuclear Plant, Unit 2 X

X BWR Fermi, Unit 2 X

X BWR Fort Calhoun Station X

X PWR Hope Creek Generating Station, Unit 1 X

X BWR Indian Point Nuclear Generating Unit 3 X

X PWR James A. Fitzpatrick Nuclear Power Plant X

X BWR Kewaunee Power Station X

X2 PWR La Salle County Station, Unit 1 X

X BWR La Salle County Station, Unit 2 X

X BWR Limerick Generating Station, Unit 1 X

X BWR Limerick Generating Station, Unit 2 X

X BWR McGuire Nuclear Station, Unit 1 X

X PWR

30 Plant Absorber Material Surveillance Program Type Plant Type BORAL Metamic Boralcan Coupons or Coupons & In Situ In Situ only None McGuire Nuclear Station, Unit 2 X

X PWR Millstone Power Station, Unit 3 X

X PWR Monticello Nuclear Generating Plant, Unit 1 X

X BWR Nine Mile Point Nuclear Station, Unit 1 X

X BWR Nine Mile Point Nuclear Station, Unit 2 X

X BWR Oyster Creek Nuclear Generating Station X

X BWR Palisades Nuclear Plant X

X PWR Peach Bottom Atomic Power Station, Unit 3 X

X BWR Perry Nuclear Power Plant, Unit 1 X

X BWR Pilgrim Nuclear Power Station X

X X

BWR Quad Cities Nuclear Power Station, Unit 1 X

X BWR Quad Cities Nuclear Power Station, Unit 2 X

X BWR Salem Nuclear Generating Station, Unit 1 X

X PWR Salem Nuclear Generating Station, Unit 2 X

X PWR Seabrook Station, Unit 1 X

X PWR Sequoyah Nuclear Plant, Unit 1, 2 (shared)

X X

PWR Shearon Harris Nuclear Power Plant, Unit 1 Pool A X

X PWR Shearon Harris Nuclear Power Plant, Unit 1 Pool B X

X PWR Shearon Harris Nuclear Power Plant, Unit 1 Pool C X

X X

PWR Shearon Harris Nuclear Power Plant, Unit 1 Pool D X

X X

PWR St. Lucie Plant, Unit 1 X

X X4 X5 PWR St. Lucie Plant, Unit 2 X

X X4 X5 PWR Susquehanna Steam Electric Station, Unit 1 X

X BWR Susquehanna Steam Electric Station, Unit 2 X

X BWR Three Mile Island Nuclear Station, Unit 1 X

X X

PWR Turkey Point Nuclear Generating Unit 3 X

X PWR Turkey Point Nuclear Generating Unit 4 X

X PWR Virgil C. Summer Nuclear Station, Unit 1 X

X PWR Vermont Yankee Nuclear Power Station X

X BWR Vogtle Electric Generating Plant, Unit 1 X

X6 PWR Waterford Steam Electric Station, Unit 3 X

X PWR Watts Bar Nuclear Plant, Unit 1 X

X PWR Wolf Creek Generating Station, Unit 1 X

X PWR

31 Plant Absorber Material Surveillance Program Type Plant Type BORAL Metamic Boralcan Coupons or Coupons & In Situ In Situ only None

1.

Callaway Plant has an approved monitoring program as part of its license renewal application that will be implemented before the period of extended operations.

2.

In decommissioning/SAFSTOR. All fuel has been removed from the SFP.

3.

PaR racks use in situ neutron attenuation only. Holtec racks use coupons.

4.

Metamic only.

5.

St. Lucie Plant has no monitoring program for its BORAL panels.

6.

A baseline inspection will be performed before the period of extended operations.

7.1.3 Testing Most coupon surveillance programs consist of visual exams; dimensional measurements; and weight, density, and B-10 areal density measurements using neutron attenuation. Some programs perform only visual exams and dimensional measurements, while others perform only B-10 areal density measurements using neutron attenuation.

7.1.4 Surveillance Coupon Performance The purpose of surveillance programs is to monitor the performance of the NAMs with the purpose of verifying that [they] continue to provide the criticality control relied upon in the criticality analyses [29]. As discussed above, NAM surveillance coupon testing is performed periodically to detect degradation that might result in the loss of the NAM (e.g., B4C). The NRC issued GL 2016-01 to collect information on the use of NAMs in SFPs, and the licensee responses fell into one of the following four categories:

Category 1: Power reactor addressees that do not credit neutron-absorbing materials other than soluble boron in the AOR. In some cases, no neutron-absorbing material is present in the spent fuel storage racks, and in other cases, credit for the neutron-absorbing material has been removed through a regulatory action (e.g., approved license amendment). Those addressees may submit a response letter confirming that no neutron-absorbing materials are currently credited to meet NRC subcriticality requirements in the SFP.

Category 2: Power reactor addressees that have an approved license amendment to remove credit for existing neutron-absorbing materials and that intend to complete full implementation no later than 24 months after the issuance of GL 2016-01. Licensees may request extensions to this implementation timeframe if there are extenuating circumstances. Those addressees may submit a response letter affirming that they will implement the approved license amendment request within the specified time. However, they must still provide information equivalent to Category 3 or Category 4 for any other neutron-absorbing material credited in the SFP criticality AOR after the license amendment has been fully implemented.

Category 3: Power reactor addressees that have incorporated their neutron-absorbing material monitoring programs into their licensing basis through an NRC-approved technical specification GL 2016-01 change or license condition. Those addressees may

32 submit a response letter referencing their approved technical specification change or license condition and affirming that no change has been made to their neutron-absorbing material monitoring program, as described in the referenced license amendment request. If a change has been made since NRC approval of the reference, the response letter should also describe any such changes. (Licensees with a monitoring program approved as part of a license amendment request or license renewal application that was not incorporated as a technical specification change or license condition are considered to belong in Category 4.)

Category 4: All other power reactor addressees.

The Category 4 responses included results of coupon surveillance testing programs for BORAL, Metamic, and Boralcan. Licensees submitted 39 Category 4 GL responses, covering 59 SFPs using aluminum-based NAMs, which were reviewed to assess the surveillance programs.

The review of the Category 4 responses for aluminum-based NAMs showed that, in general, the degradation mechanisms experienced by the coupons are consistent with the predicted mechanisms, such as pitting and blistering, based on qualification testing and previous experience. The review revealed no new degradation mechanisms. Neutron attenuation testing was performed on some coupons that exhibited blistering, pitting, or both, and the results showed no measured decrease in the B-10 areal density.

7.1.5 Coupon Reinsertion As mentioned above, coupon testing programs consist of coupon trees with enough coupons such that a certain number can be removed and tested at predetermined intervals to cover the anticipated service life of the SFP. Coupons that are not returned to the pool following testing provide information on the degradation mechanisms operating on the coupons, but they do not provide trending data. Reinserted coupons may provide data for trending, such as the size and number of blisters or coupon thickness. Testing on reinserted coupons is nondestructive and typically involves visual exams and dimensional measurements performed poolside or at an offsite facility.

Trending data may prompt the licensee to modify the testing program if needed. For example, one licensee monitored the performance of BORAL initially by inserting sheathed coupons in the SFP. After 2 years, the first inspection revealed swelling of the sheathing and subsequent blistering of the BORAL after the coupon was unsheathed. Areal density measurements showed no decrease in B-10. Additional test coupons were unsheathed and reinserted in the SFP, and the testing interval was decreased from every 2 years to every 6 to 12 months for about 10 years (visual and dimensional testing only). After 7 years, the number of new blisters and growth of existing blisters appeared to stop. After reaching 10 years of monitoring, the testing frequency was adjusted to every 10 years. Thus, under controlled conditions, data from reinserted coupons can be used to make decisions about surveillance program parameters.

33 7.2 In Situ B-10 Areal Density Measurement Programs Up until recently, four plants monitored BORAL neutron absorber panel performance exclusively through in situ neutron attenuation testing to measure the B-10 areal density of the rack panels in lieu of a coupon monitoring program: Crystal River Nuclear Plant (Crystal River),

Unit 3, Davis-Besse, Duane Arnold, and Kewaunee. Crystal River Unit 3 and Kewaunee are no longer operating and have removed all the spent fuel from their respective SFPs; they no longer implement a monitoring program.

7.2.1 In Situ B-10 Areal Density Measurement Program Performance In their respective license renewal applications, Crystal River Unit 3 and Kewaunee committed to implementing a BORAL monitoring program using BADGER, or an equivalent neutron attenuation measurement system, to measure the B-10 areal density every 10 years. The first measurement campaigns were to occur before entering the period of extended operations.

However, before entering the period of extended operations, both plants entered the decommissioning process and removed all fuel from their respective SFPs; Crystal River Unit 3 in January 2018, and Kewaunee in June 2017. As a result, these plants did not perform the B-10 areal density measurements.

Davis-Besse installed BORAL in 2002 and conducted the first B-10 areal density measurement campaign in 2015, with a monitoring program consisting of B-10 areal density measurements every 10 years. The results of the 2015 measurement campaign showed that the average B-10 areal density of all the panels measured was above the nominal value of 0.0324 gram per square centimeter (g/cm2) used in the NCS AOR. The average B-10 areal density of the panel with the lowest measured average value was 0.0308 g/cm2, which was lower than the nominal value used in the NCS AOR but above the minimum certified value 0.03 g/cm2. The nominal value of the B-10 areal density used in the NCS AOR of 0.0324 g/cm2 includes a reactivity penalty in the calculation of Keff that is based on the minimum certified value of 0.03 g/cm2 to account for manufacturing tolerances. Davis-Basse reported that no degradation of the neutron-absorbing capabilities of the NAM has been observed, which was verified by the in situ B-10 areal density measurements.

Duane Arnold installed BORAL in 1978 with a minimum certified B-10 areal density of 0.0232 g/cm2. As part of its license renewal application, Duane Arnold committed to in situ neutron attenuation testing to measure the B-10 areal density measurements, with the first test to be conducted before entering the period of extended operations. Thereafter, the B-10 areal density would be measured at least every 10 years to confirm the assumptions of the NCS AOR. The results of the first tests found that the lowest average B-10 areal density was 0.0208 g/cm2, which was lower than the value used in the NCS AOR. No material degradation of the NAM was recorded; however, the NCS AOR was revised to use a B-10 areal density value of 0.015 g/cm2.

34 7.2.2 Assessment of the BADGER System for In Situ B-10 Areal Density Measurements The BADGER in situ neutron attenuation measurement system, designed and operated by NETCO, is commonly used for measuring the B-10 areal density of installed NAM panels. The NRC published reports on the BADGER system covering its design, operation, and potential accuracy in technical letter reports [3, 4]. The results of Oak Ridge National Laboratorys (ORNLs) initial assessment of the BADGER system were published in the NRC technical letter report, Initial Assessment of Uncertainties Associated with BADGER Methodology, issued dated September 30, 2012 [4]. ORNL did not have access to the BADGER system or any of its supporting documentation when it performed its initial assessment, which made it difficult to determine measurement uncertainty quantitatively. The uncertainty analysis identified more than 40 different factors that influence the measurement uncertainty, many of which could not be assessed, even with expert judgment, due to the lack of information. However, ORNL experts did identify several factors that had the potential for combined uncertainty greater than 40 percent.

Subsequent to the publication of the original ORNL report [4], BADGER received several upgrades to the hardware to help improve its performance. Through an addendum to a memorandum of understanding, the NRC and EPRI conducted a cooperative research project to assess the newly improved BADGER system. At that time, the Zion SFP was being decommissioned, which provided a unique opportunity to perform BADGER measurements on the BORAL neutron absorbing panels and then remove the panels for confirmatory testing.

ORNL performed an uncertainty analysis for certain factors using the data obtained during the Zion campaign and other data from NETCO. ORNL made improved expert opinions about other factors based on further information provided by NETCO and more in-depth discussions with NETCO personnel. For instance, ORNL assessed the uncertainties associated with calibration, model bias, and panel-averaged measurement error. ORNL determined that the errors postulated for several of the factors are lower than originally estimated and that NETCOs algorithms appear to calculate B-10 areal density and propagate the uncertainties associated with BADGER measurement data correctly.

The details of ORNLs most current assessment of BADGER are reported in a technical letter report, Addressing Uncertainties in SuperBADGER Measurements at the ZION Nuclear Power Plant (ZNPP) Spent Fuel Pool on December 5-8, 2014Revision 1, issued September 2019 [35F34]. The report summarizes the uncertainty evaluation as follows:

To summarize the main contributions to measurement uncertainty based on operational experience, the following indicative relative standard deviations were calculated: calibration, 2% (systematic; precision can be reduced by averaging);

calibration bias against chemical analysis, 3%; under reporting due to interpolation, 5% (but dependent on the number of standards); statistical precision for a sector or full panel, 3-6%; reproducibility following disassembly of all equipment and return to the job-site sometime later, 10%. The actual

35 uncertainties are case specific and are generated from the actual data collected by NETCO and reported as per client needs.

Additional testing and evaluations by NETCO are needed to develop a complete model for calculating the total measurement uncertainty for the BADGER version 2 system, but the uncertainty calculations and independent B-10 areal density calculations performed by ORNL should benefit future assessments of the system.

8. Discussion Section 3 of this report documents the Metamic, Boralcan, and BORAL materials qualification testing, and Section 5 covers aluminum-based materials operating experience.

Section 8.1 discusses the degree to which the qualification testing results reflect the materials performance based on the operating experience. Section 8.2 discusses the apparent effectiveness of the surveillance programs for aluminum-based NAMs.

8.1 Qualification Testing and Operating Experience As discussed in Section 4.4, the potential materials degradation mechanisms to which aluminum-based NAMs may be exposed include various corrosion mechanisms (general, galvanic, pitting), radiation effects, and blistering and swelling.

Corrosion Performance: Qualification testing for the aluminum-based NAMs included accelerated corrosion testing of samples from prototypical heats of materials in deionized water with and without added boron to simulate PWR and BWR SFP water chemistries, respectively.

The tests used elevated temperatures to accelerate the corrosion mechanism(s), which are assumed to obey an Arrhenius dependence with temperature. The qualification testing programs did not evaluate the effects of off-normal water chemistry.

Corrosion rates for Metamic were very low, such that they were within the margin of error of the measurement. Boralcan corrosions rates were on the order of 0.01-0.03 mpy, and the corrosion rate for BORAL was 0.14 mpy. Aluminum alloys, in general, are susceptible to pitting under certain conditions due to localized breakdown of the protective oxide. The qualification testing showed that the corrosion that was observed taking place on the Metamic, Boralcan, and BORAL samples was mostly pitting due to surface contaminants originating from the manufacturing process. Galvanic corrosion testing showed that pitting can occur when the aluminum alloys are coupled with more noble materials, such as 304 and 304L stainless steels. Anodized Metamic appeared to be the most corrosion resistant of the aluminum-based NAMs when cleaned of surface contaminants. None of the corrosion observed in the qualification testing resulted in measurable loss of B4C.

Accelerated Elevated Temperature Testing and Radiation Testing: The qualification testing of the aluminum-based NAMs included mechanical properties measurements following accelerated high-temperature testing and exposure to accelerated radiation doses to simulate

36 the service life conditions to which NAMs might be exposed (these were separate tests). The results showed that the testing conditions did not significantly affect the mechanical properties of the NAMs. Additionally, no changes in the B-10 areal density were detected following the elevated temperature testing or the accelerated radiation testing. No service life equivalent was reported for the elevated temperature testing, but the radiation testing was performed to doses equivalent to more than 40 years at the expected service conditions.

Operating Experience: No operating experience, such as reports of degradation, were found for Metamic and Boralcan used in U.S. nuclear plant SFPs. Section 6 of this report summarizes the BORAL operating experience. Based on a review of submittals in response to GL 2016-01 and various NRC INs, pitting corrosion and blistering were the most prevalent forms of degradation for BORAL. Blistering of the BORAL aluminum cladding is believed to result from the accumulation of hydrogen gas released during corrosion of the cermet core due to water intrusion from the exposed edges of the panels. Blistering of BORAL has been observed in surveillance coupons during routine coupon evaluations and in panels discovered during fuel transfers when the fuel bundles experienced binding due to bulging of the storage rack. It should be noted that, to date, the B-10 areal density of BORAL has not been affected by corrosion or blistering.

The qualification testing performed on the aluminum-based NAMs addressed the environmental conditions expected in the SFP under nominal water chemistry conditions and in relevant configurations such as encapsulated samples to simulate flow conditions and coupled to stainless steel to simulate galvanic conditions arising from the NAM panels being in contact with the racks. Qualification tests such as these provide quantitative and qualitative data that are useful for determining the acceptability of the materials for use in SFPs. Operating experience has indicated that the testing performed on Metamic, Boralcan, and BORAL appear to have addressed the degradation mechanisms expected to operate in the SFPs, while considering the combinations and configurations of materials and components (i.e., racks and panels) known to be in use.

8.2 Surveillance Methods Effectiveness For a surveillance monitoring program to be effective, the coupon materials, configuration, and exposure must be representative of the material and component being monitored. In addition, the testing and measurements performed on the coupons must be capable of detecting and measuring the effects of the known and potential degradation mechanisms. NAM surveillance programs consist primarily of coupon programs or in situ (i.e., in-pool) B-10 areal density measurements using neutron attenuation. Section 7 of this report provides details on the coupon surveillance programs, which showed that, in general, the programs have many aspects in common, but vary in the details. All of the programs (1) use coupons made from the materials used in the SFP racks, (2) subscribe to an established surveillance interval, and (3) perform a condition assessment of the coupons (most coupon surveillance programs include B-0 areal density measurements). The differences between the individual programs include

37 coupon sizes and configurations, surveillance intervals, and the measurements made as part of the condition assessments.

The effectiveness of the NAM surveillance programs can be assessed by how well they monitor the performance of the panels in the racks, specifically the loss of material and the degradation of neutron absorbing capacity. NAM panels in the storage racks are not easily accessible for direct inspection because there is fuel stored in the rack cells and they are typically covered by stainless steel or aluminum cover plates. Thus, the coupons serve as the primary source of information about the potential condition of the panels in the racks. As such, corrective actions, if any, are based on coupon performance. For programs that rely only on in situ B-10 areal density measurements, loss of the NAM is measured directly for a sample of panels in the pool and, thus, serves as the basis for corrective actions.

The NRC staff assessed information on surveillance program effectiveness based on licensee submittals in response to GL 2016-001 and operating experience reported in various NRC IN (see Section 6) on aluminum-based NAMs. The visual and dimensional inspections required by the surveillance programs appear to be effective at detecting the expected degradation mechanisms. The most common forms of degradation observed on the coupons were blistering, pitting corrosion, and general corrosion; in no cases were there measured losses of the NAM (i.e., loss of B4C), as confirmed in several cases by areal density measurements by laboratory-performed neutron attenuation testing. Licensees implementing only in situ B-10 areal density measurements produced results that, in some cases, met the minimum specified areal density, but in other cases, the areal density results were lower than the minimum specified value. If the minimum specified B-10 areal density was not met, corrective actions included revision of the NCS AOR based on a lower areal density. It should be noted that if the in situ B-10 areal density measurements resulted in nonconforming values, the physical loss of the NAM was not verified.

Surveillance intervals vary among coupon-based programs, and they are based on either years of service or refueling cycles. Coupon-based programs perform testing on 2-5-year intervals, and, in some cases, expand the intervals to 10 years for the latter years of service. Based on the observations reported in various GL 2016-01 submittals, the most common form of degradation was blistering, which appeared to begin during the early years of service. Blister formation and growth rates tended to taper off as the service life increased. Corrosion rates for the aluminum cladding of BORAL and for the matrix of Metamic and Boralcan are low, and pitting tends to be associated with surface impurities. In BORAL, pits tend to terminate at the cermet core. No service-related degradation has been reported for Metamic and Boralcan.

Based on the results of the coupon-based surveillance programs, the testing intervals appear capable of detecting and trending the expected and active degradation mechanisms. In situ B-10 areal density testing only measures losses of the NAM (i.e., B4C) and is typically performed on 10-year intervals. Testing has been insufficient to trend the results of in situ testing, but based on the results of coupon-based programs that measured no losses of B4C, the 10-year intervals are reasonable.

38

9. Conclusions This report assesses the adequacy of aluminum-based NAM qualification testing and surveillance monitoring programs for managing the aging of NAM panels in the SFP. Sections 4 and 6 summarize information on the materials qualification testing and operating experience.

Section 7 reviews the surveillance programs.

The purpose of the NAMs in the SFP is to maintain subcriticality margins in accordance with NRC regulations. Any decrease in margin due to a decrease in the NAM B-10 areal density would be addressed through the licensees corrective action program. Decreases in the B-10 areal density in the aluminum-based NAM may be due to a loss of B4C by some degradation mechanism resulting in gross material losses, such as corrosion. The purpose of the materials qualification testing is to demonstrate that the material is suitable for use in the SFP environment for the intended service life. Qualification testing consists of tests that subject the material to simulated or actual service environments to assess its performance while inducing expected degradation mechanisms. Often, such tests are conducted using accelerated conditions to obtain data faster, to obtain data under bounding conditions, or for both reasons.

The materials qualification testing performed on the aluminum-based NAMs, BORAL, Metamic, and Boralcan by various organizations addressed the expected degradation mechanisms of corrosion, elevated temperatures, and radiation. The testing results were used to gain NRC approval of their use in SFPs; in many cases, the NRC conditioned its approvals on the establishment of surveillance monitoring programs.

The NRC staff assessed the surveillance monitoring programs for the aluminum-based NAMs based on available information from license amendments, license renewal applications, NRC INs, and information submitted in response to NRC GL 2016-001. This assessment concluded that most surveillance programs are designed in accordance with NEI 16-03-A, Revision 0 [29],

such that degradation of the aluminum-based NAMs will be monitored and managed effectively for the intended service life. Some programs require the addition on B-10 areal density measurements to fully meet the guidance in NEI 16-03-A, Revision 0.

Operating experience and additional testing results indicate that BORAL, Metamic, and Boralcan should perform their safety function for the service life of the SFPs, provided that SFP water chemistry and temperature are maintained within EPRI guidelines; excursions for limited periods should not result in significant degradation based on SRNL accelerated testing.

Additionally, this review did not identify any previously unidentified degradation or aging mechanisms that challenge the current regulatory position of the NRC on the use of aluminum-based NAMs in SFPs.

39

10. References 1

NRC Generic Letter 78-11, OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications, April 14, 1978, ADAMS Accession No. ML031280383.

2 NRC Generic Letter 2016-01, Monitoring of Neutron-Absorbing Materials in Spent Fuel Pools, April 7, 2016, ADAMS Accession No. ML16097A169.

3.

Technical Letter Report, Boraflex, RACKLIFE and BADGER: Description and Uncertainties, September 30, 2012, ADAMS Accession No. ML12216A307.

4.

Technical Letter Report, Initial Assessment of Uncertainties Associated with BADGER Methodology, September 30, 2012, ADAMS Accession No. ML12254A064.

5.

Technical Letter Report, Monitoring Degradation of Phenolic Resin-Based Neutron Absorbers in Spent Nuclear Fuel Pools, June 5, 2013, ADAMS Accession No. ML13141A182.

6.

PWR Primary Water Chemistry Guidelines: Volume 1, Revision 4, TR-105714-V1R4, Electric Power Research Institute (EPRI), Palo Alto, CA: 1999.

7.

BWRVIP-130: BWR Vessel and Internals Project, BWR Water Chemistry Guidelines 2004 Revision, EPRI Product No. 1008192, Palo Alto, CA: 2004.

8.

Handbook of Neutron Absorber Materials for Spent Nuclear Fuel Transportation and Storage Applications2006 Edition, EPRI Product No. 1013721, Palo Alto, CA: 2006.

9.

Qualification of Metamic for Spent-Fuel Storage Application, EPRI Product No. 1003137, Palo Alto, CA: 2001.

10.

Use of Metamic in Fuel Pool Applications, Holtec Report No. HI-2022871 (nonproprietary), Holtec International, Marlton, NJ, 2009.

11 Safety Evaluation by the Office of Nuclear Reactor Regulation Related to the Topical Report in Support of the Use of Metamic in Fuel Pool Applications at Arkansas Nuclear One, Units 1 and 2 Docket Nos.: 50-313 and 50-368, May 15, 2003, ADAMS Accession No. ML031360755 (nonpublic).

12.

Interim Staff Guidance11, Revision 3, Cladding Considerations for the Transportation and Storage of Spent Fuel, November 17, 2003.

13.

Khalifa, T.A. and T.S. Mahmoud, Elevated Temperature Mechanical Properties of Al Alloy AA6063/SiCp MMCs, Proceedings of the World Congress on Engineering 2009, Vol. II, WCE 2009, London, UK, July 1-3, 2009.

14.

Material Qualification of Alcan Composite for Spent Fuel Storage: Revision 5, Report No. NET-259-03, Northeast Technology Corporation, Kingston, NY: August 2008.

15.

Corrosion Resistance of BORAL to One Year of Exposure to BWR Storage Pool Water, Report No. 551, Brooks & Perkins, Inc., Livonia, MI: February 1977.

16.

Mollon, L., Spent Fuel Storage Module Corrosion Report, Report No. 554, Brooks &

Perkins, Livonia, MI: June 1977.

17.

Storage Module Corrosion Testing Final Report, Report No. 561, Brooks & Perkins, Livonia, MI.

40

18.

Suitability of Brooks & Perkins Spent Fuel Storage Module for Use in BWR Storage Pool, Report No. 577, Brooks & Perkins, Livonia, MI: July 1978.

19.

Suitability of Brooks & Perkins Spent Fuel Storage Module for Use in PWR Storage Pool, Report No. 578, Brooks & Perkins, Livonia, MI: July 1978.

20.

Mollon, L., Boral Neutron Absorbing/Shielding Material: Product Performance Report, Report No. 624, Brooks & Perkins, Livonia, MI: July 1982.

21.

Brown, T.B., Effect of Gamma Radiation on the Neutron Attenuation Properties of Boral', Report No. 637, Brooks & Perkins, Livonia, MI: February 1985.

22.

Evaluation of BORAL Coupons from Zion Spent Fuel Pool, EPRI Product No. 3002008195, Palo Alto, CA: 2016.

23.

Evaluation of BORAL Panels from Zion Spent Fuel Pool and Comparison to Zion Coupons, EPRI Product No. 3002008196, Palo Alto, CA: 2016.

24.

Characterization and Analysis of Boral from the Zion Nuclear Power Plant Spent Fuel Pool, SRNL-TR-2018-00244, Revision 0, Savannah River National Laboratory, Aiken, SC: 2019.

25.

Safety Evaluation of the Topical Report to Support the Use of Metamic in Fuel Pool Applications at Arkansas Nuclear One, Docket Nos. 50-313, 50-368, May 15, 2003, ADAMS Accession No. ML031360755.

26.

LaSalle County Station, Units 1 and 2, Issuance of Amendments Concerning Spent Fuel Neutron Absorbers (TAC Nos.: ME2376 AND ME2377) (RS-09-133),

January 28, 2011, ADAMS Accession No. ML110250051.

27 NRC Information Notice 1983-29, Fuel Binding Caused by Fuel Rack Deformation, May 6, 1983.

28 NRC Information Notice 2009-26, Degradation of Neutron-Absorbing Materials in the Spent Fuel Pool, October 28, 2009, ADAMS Accession No.: ML092440545.

29.

Guidance for Monitoring of Fixed Neutron Absorbers in Spent Fuel Pools, NEI 16-03-A, Revision 0, Nuclear Energy Institute, Washington, DC: May 2017.

30.

NUREG-1800, Standard Review Plan for Review of License Renewal Applications for Nuclear Power Plants, Revision 2, December 2010.

31.

NUREG-1801, Generic Aging Lessons Learned (GALL) Report, Revision 2, December 2010.

32.

NUREG-2191, Generic Aging Lessons Learned for Subsequent License Renewal (GALL) Report, May 2017.

33.

NUREG/CR-6698, Guide for Validation of Nuclear Criticality Safety Calculational Methodology, Science Applications International Corporation, Oak Ridge, TN:

January 2001, ADAMS Accession No. ML050250061.

34.

Chapman, J. and S. Croft, Addressing Uncertainties in SuperBADGER Measurements at the ZION Nuclear Power Plant (ZNPP) Spent Fuel Pool on December 5-8, 2014 Revision 1, ORNL/SPR-2015/74, Revision 1, Oak Ridge National Laboratory, Oak Ridge, TN: September 2019, ADAMS Accession No. ML19318F214.