ML092440545 

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

WASHINGTON, DC 20555-0001 

October 28, 2009 

NRC INFORMATION NOTICE 2009-26: DEGRADATION OF NEUTRON-ABSORBING 

MATERIALS IN THE SPENT FUEL POOL 

ADDRESSEES 

All holders of operating licenses or construction permits for nuclear power reactors under the 

provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic Licensing 

of Production and Utilization Facilities.” 

PURPOSE 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform 

addressees on the issue of degradation of the Carborundum neutron-absorbing materials in the 

spent fuel pools (SFP) and the deformation of Boral® panels in SFPs. The NRC expects that 

recipients will review the information for applicability to their facilities and consider actions, as 

appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC 

requirements; therefore, no specific action or written response is required. 

DESCRIPTION OF CIRCUMSTANCES 

Carborundum 

In July 2008, the licensee at the Palisades Nuclear Plant discovered that the region 1 SFP 

storage racks contained less neutron-absorbing material than assumed in the SFP criticality 

analysis of record and that the licensee was in noncompliance with the SFP criticality 

requirements in Technical Specification (TS) 4.3.1.1.b. 

In 1988, the licensee encountered difficulty inserting a fuel assembly into one of the region 1 

SFP storage rack cells because the north wall of the cell had experienced swelling. Since 1988, 

several other locations have been determined to be swollen. Most of these locations were 

identified because their fuel assemblies could not be removed. To date, 14 cells have 

experienced swelling, 11 with fuel stored in them and 3 empty. When a stuck fuel assembly 

was identified in 2007, the licensee performed an apparent cause evaluation that determined 

that the swelling of the region 1 rack may be due to degradation of the Carborundum B4C 

neutron absorber plates. In response to this concern, on July 15, 2008, the licensee performed 

“blackness testing” of the region 1 SFP racks to validate that the racks continue to perform 

neutron attenuation as credited in the licensee’s criticality analysis of record and to satisfy 

regulatory requirements and renewed license commitments. 
IN 2009-26 

Page 2 of 4 

The licensee performed in situ Boron-10 Areal Density Gauge for Evaluating Racks (BADGER) 

testing of approximately 2 percent of the storage locations, which revealed that the Boron-10 

areal density of the SFP racks was, at a minimum, approximately one-third of its original design 

value. The neutron-absorbing material, Carborundum, which is relied on to maintain 

subcriticality in the SFP, was much less effective than assumed in the criticality analysis. 

Therefore, region 1 of the SFP no longer met 10 CFR 50.68, “Criticality Accident 

Requirements,” or TS 4.3.1.1.b, which require that K-effective (Keff) for region 1 fuel racks be 

less than or equal to 0.95 if fully flooded with unborated water. 

The licensee determined that the apparent cause of the degradation of the Carborundum B4C 

plates was the environment of the SFP. The exact degradation mechanism or mechanisms are 

not clearly understood but likely involve changes in the physical properties of the Carborundum 

B4C plates that occur during prolonged exposure to the SFP environment. The swelling of the 

racks, which prevents fuel assemblies from being inserted or removed, indicates a potential 

problem with neutron-absorbing capacity. The swelling in the racks could result from 

dimensional changes of the Carborundum, which may be replaced by a gas-filled space, and 

could challenge the assumptions of the criticality analysis. This unknown impact on the 

criticality analysis led the licensee to perform BADGER testing of the racks and discover the 

Carborundum degradation. This degradation may have been occurring as early as 1988 when 

the first impedance to inserting a fuel assembly was documented at Palisades. Since there was 

no surveillance of the neutron-absorbing capacity of the material, the start of the degradation 

and the degradation rate are unknown. 

The noncompliant criticality analysis was resolved when the licensee submitted and the NRC 

approved a license amendment to change the criticality analysis to remove any credit for the 

Carborundum material as a neutron absorber in the SFP. The amendment also restricted the 

pattern of the fuel assemblies in the pool. 

Additional information is available in (1) Palisades Licensee Event Report 05000255/2008-004, 

“Noncompliance with Technical Specification 4.3.1.1.b,” dated September 15, 2008, and 

viewable on the NRC’s public Web site in the Agencywide Documents Access and Management 

System (ADAMS) under Accession No. ML082660584, and (2) a letter from Entergy Nuclear 

Operations, Inc. (Palisades), to the NRC, “Commitments to Address Degraded Spent Fuel Pool 

Storage Rack Neutron Absorber,” dated August 27, 2008, which can be found in ADAMS under 

Accession No. ML082410132. 

Boral 

Beaver Valley Power Station. In a license renewal application supplement dated 

January 19, 2009 (ADAMS Accession No. ML090220216), the licensee at Beaver Valley stated 

that licensee inspections in 2007 of the Boral neutron absorber material coupons identified 

numerous blisters of the aluminum cladding, while only a few small blisters were identified in 

2002. In region 1 fuel storage racks, blisters can displace water from the flux traps between 

storage cells and challenge dimensional assumptions used in the criticality analysis. Based on 

these inspections, the licensee determined that the Boral aluminum cladding blistering was an 

aging effect and that it would credit the existing Boral Surveillance Program with management of 

this aging. 
IN 2009-26 

Page 3 of 4 

Susquehanna Steam Electric Station. In a license renewal application letter dated 

May 13, 2009 (ADAMS Accession No. ML091520031), the licensee at Susquehanna stated that 

it had identified a significant bulge in a poison can wall. Although the licensee has not 

definitively determined the cause of the bulge, the licensee’s letter states that it may be the 

result of hydrogen gas generation from either moisture contained in the Boral at the time of 

manufacture or a leaking seal weld in the poison can. This bulge prevented the placement of a 

blade guide into the deformed cell. 

BACKGROUND 

NRC Generic Letter 78-11, “Guidance for Spent Fuel Pool Modifications,” and enclosure, “OT 

Position for Review and Acceptance of Spent Fuel Storage and Handling Applications,” dated 

April 14, 1978 (ADAMS Accession No. ML031280383), provide NRC guidance on testing of the 

SFP neutron-absorbing materials to ensure the long-term safety and integrity of the pool and 

fuel rack system. Generic Letter 78-11 states, “Methods for verification of long-term material 

stability and mechanical integrity of special poison material utilized for neutron absorption 

should include actual tests...to assure long-term safety and integrity of the pool and fuel rack 

system.” 

Electric Power Research Institute Report TR1013721, “Handbook of Neutron Absorber 

Materials for Spent Fuel Transportation and Storage Applications,” issued 2006, states the 

following concerning Boral: 

Similarly, in-pool blistering of Boral has, to date, proved to be primarily an 

esthetic effect; however, the potential effects on fuel assembly clearance and the 

reactivity state of region 1 racks have been noted. In addition, it has been noted 

that, in a few instances, rack cell wall deformation has occurred making it difficult 

to remove fuel. With plant life extension now the norm at most [light water 

reactors] LWRs in the US, some Boral, which originally had a design service life 

of 40 years, will be in service more than 60 years. This suggests a prudent 

course is continued vigilance and surveillance so that onset of any degradation 

can be detected early and appropriate mitigation measures applied. 

DISCUSSION 

The regulations in 10 CFR 50.68(b)(4) state that if no credit for soluble boron is taken, the Keff of 

the spent fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must not 

exceed 0.95, if flooded with unborated water. If credit is taken for soluble boron, the Keff of the 

spent fuel storage racks loaded with fuel of the maximum fuel assembly reactivity must not 

exceed 0.95, if flooded with borated water, and the Keff must remain below 1.0 (subcritical), if 

flooded with unborated water. In addition, standard TS 4.3.1.1.b requires that the spent fuel 

storage racks be designed and maintained with Keff less than or equal to 0.95 if fully flooded with 

unborated water. This IN provides examples of degradation of neutron-absorbing materials that 

resulted in, or had the potential to result in, noncompliance with 10 CFR 50.68(b)(4) and 

standard TS 4.3.1.1.b. 
IN 2009-26 

Page 4 of 4 

At Palisades, degradation of the neutron-absorbing material Carborundum, which is relied on to 

maintain subcriticality in the SFP, was much less effective than assumed in the criticality 

analysis. This degradation also may have resulted in the physical deformation (swelling) of the 

Carborundum plates. 

The other examples in this IN involve degradation of aluminum cladding of the neutronabsorbing material Boral. Blisters and bulges of Boral cladding are material deformations that 

change the dimensions of the material. These blisters and bulges can be either water filled or 

gas filled (from the reaction of the SFP water and aluminum from the Boral), which may not be 

accounted for in the criticality analysis. In the case of Beaver Valley, the blisters could grow to a 

point where the water from the flux trap of the region 1 rack could be displaced with gas. This 

deformation has the potential to challenge dimensional assumptions made in the fuel pool 

criticality analysis. In the case of Susquehanna, the SFP racks do not have flux traps; however, 

if the bulges in the rack are filled with gas, the dimensional assumptions of the combined 

moderation and neutron attenuating geometry assumed in the SFP criticality analysis could be 

challenged. Susquehanna currently stores only non-fuel bearing components in the vicinity of 

the identified bulged cell. 

The degradation mechanisms and deformation rates of any of the neutron-absorbing materials 

in the SFP are not well understood. Therefore, for licensees that credit the use of a 

neutron-absorbing material to maintain subcriticality in their SFP, knowing the condition of the 

neutron-absorbing material in the SFP and monitoring the SFP for any indications that 

degradation of the material may be occurring can prevent noncompliance with SFP criticality 

requirements. 

CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA by TQuay for/ 

Timothy J. McGinty, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contacts: Emma L. Wong, NRR Matthew G. Yoder, NRR 

301-415-1217 301-415-4017 

E-mail: emma.wong@nrc.gov E-mail: matthew.yoder@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 
IN 2009-26 

Page 4 of 4 

At Palisades, degradation of the neutron-absorbing material Carborundum, which is relied on to 

maintain subcriticality in the SFP, was much less effective than assumed in the criticality 

analysis. This degradation also may have resulted in the physical deformation (swelling) of the 

Carborundum plates. 

The other examples in this IN involve degradation of aluminum cladding of the neutronabsorbing material Boral. Blisters and bulges of Boral cladding are material deformations that 

change the dimensions of the material. These blisters and bulges can be either water filled or 

gas filled (from the reaction of the SFP water and aluminum from the Boral), which may not be 

accounted for in the criticality analysis. In the case of Beaver Valley, the blisters could grow to a 

point where the water from the flux trap of the region 1 rack could be displaced with gas. This 

deformation has the potential to challenge dimensional assumptions made in the fuel pool 

criticality analysis. In the case of Susquehanna, the SFP racks do not have flux traps; however, 

if the bulges in the rack are filled with gas, the dimensional assumptions of the combined 

moderation and neutron attenuating geometry assumed in the SFP criticality analysis could be 

challenged. Susquehanna currently stores only non-fuel bearing components in the vicinity of 

the identified bulged cell. 

The degradation mechanisms and deformation rates of any of the neutron-absorbing materials 

in the SFP are not well understood. Therefore, for licensees that credit the use of a 

neutron-absorbing material to maintain subcriticality in their SFP, knowing the condition of the 

neutron-absorbing material in the SFP and monitoring the SFP for any indications that 

degradation of the material may be occurring can prevent noncompliance with SFP criticality 

requirements. 

CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA by TQuay for/ 

Timothy J. McGinty, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contacts: Emma L. Wong, NRR Matthew G. Yoder, NRR 

301-415-1217 301-415-4017 

E-mail: emma.wong@nrc.gov E-mail: matthew.yoder@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 

ADAMS Accession Number: ML092440545 ME1659 

OFFICE DCI:CSGB TECH EDITOR BC:DCI:CSGB D:DCI BC:DSS:SRXB 

NAME EWong KA-Kribbs MGavrilas MEvans GCranston 

DATE 9/30/2009 9/11/2009 email 10/1/2009 10/5/2009 10/7/2009 

OFFICE PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR 

NAME DBeaulieu CHawes MMurphy TQuay for TMcGinty 

DATE 10/7/2009 10/8/2009 10/28/09 10/28/09 

OFFICIAL RECORD COPY