Information Notice 2009-26, Spent Fuel Pool Neutron Absorbing Materials Degradation

Spent Fuel Pool Neutron Absorbing Materials Degradation
	ML092440545
Person / Time
Issue date:	10/28/2009
From:	Mcginty T; Division of Policy and Rulemaking
To:	;
	David Beaulieu, 415-3243
References
	IN-09-026
	Download: ML092440545 (5)
	v • d • e

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 licensees criticality analysis of record and to satisfy

regulatory requirements and renewed license commitments.

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