Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
June 26, 1996
NRC GENERIC LETTER 96-04: BORAFLEX DEGRADATION IN SPENT FUEL POOL STORAGE
RACKS
Addressees
All holders of operating licenses for nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to
inform all addressees of issues concerning the use of Boraflex in spent fuel
storage racks. Although this generic letter is being issued to all licensees
of operating nuclear power reactors, only those licensees that use Boraflex
are required to respond. Each addressee that uses Boraflex as a neutron
absorber in its spent fuel storage racks is requested to (1) assess the
capability of the Boraflex to maintain a 5-percent subcriticality margin and
(2) submit to the NRC a plan describing its proposed actions if this
subcriticality margin cannot be maintained by Boraflex material because of
current or projected future Boraflex degradation.
Background
Degradation of Boraflex has been previously addressed by the NRC in
Information Notice (IN) 87-43, "Gaps in Neutron-Absorbing Material in High-
Density Spent Fuel Storage Racks," September 8, 1987; IN 93-70, "Degradation
of Boraflex Neutron Absorber Coupons," September 10, 1993; and IN 95-38,
"Degradation of Boraflex Neutron Absorber in Spent Fuel Storage Racks,"
September 8, 1995. The Electric Power Research Institute (EPRI) has been
studying the phenomenon of Boraflex degradation for several years and has
identified two issues with respect to using Boraflex in spent fuel storage
racks. The first issue related to gamma radiation-induced shrinkage of
Boraflex and the potential to develop tears or gaps in the material. This
phenomenon is typically accounted for in criticality analyses of spent fuel
storage racks. The second issue concerned long-term Boraflex performance
throughout the intended service life of the racks as a result of gamma
irradiation and exposure to the wet pool environment.
Description of Circumstances
Palisades Nuclear Power Station
During the removal of several Boraflex surveillance coupons from the Palisades
spent fuel pool in August 1993, a loss of as much as 90 percent of the
Boraflex was observed and has been attributed to exposure to high-level gamma
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June 26, 1996
Page 2 of 7
radiation in conjunction with interaction with the pool water. The Boraflex
in these coupons was sandwiched and bolted between two stainless steel strips,
allowing a relatively large area of Boraflex to be exposed to the pool water
environment and flow. Neutron attenuation testing (blackness tests) of the
actual Palisades storage racks indicated that because of the relatively
watertight Boraflex panel enclosures, there was no similar degradation.
South Texas Project
The results of blackness tests performed in August 1994 at South Texas
indicated that the Boraflex was degraded, as evidenced by gaps and/or
localized washout of the boron content in 20 of the 37 storage cells tested.
Of the eight cells that had been designated to receive an accelerated gamma
dose, five cells exhibited substantial degradation (0.91 to 1.37 m
[3 to 4.5 ft]). The licensee postulated that the degradation mechanism was
washout-accelerated dissolution of the Boraflex caused by pool water flow
through the panel enclosures. As a justification for continued operation, the
licensee has placed restrictions on the use of the degraded storage cells to
ensure compliance with the required subcriticality margin. In addition, a
long-term neutron absorption panel management plan is being developed, as well
as a dose-to-degradation correlation that will aid in establishing
restrictions for the use of the spent fuel racks.
Fort Calhoun Station
As part of the Fort Calhoun Station rerack project, the old spent fuel storage
racks containing Boraflex were removed and disassembled in December 1994 to
determine the condition of the Boraflex. The new storage racks do not contain
Boraflex. As part of the overall EPRI research program, the licensee
inspected two cells from the removed Boraflex racks which had experienced the
highest gamma flux since 1983. Only 40 percent of the Boraflex remained in
one of the panels from these cells while another panel in the same cell
exhibited no loss of Boraflex. An adjacent cell had a panel which had some
Boraflex loss but subsequent attenuation and density tests confirmed that the
average boron-10 areal density still exceeded the material minimum
certifications. Visual observations made during the course of the rack
disposal process indicated that the vast majority of cells had not undergone a
significant loss of Boraflex. The licensee has determined that there was
sufficient Boraflex in the walls of each cell to meet the minimum requirements
in the design-basis criticality analysis.
Discussion
Experimental data from test programs, including blackness tests performed at
various boiling-water reactor (BWR) and pressurized-water reactor (PWR) spent
fuel storage pools, confirmed that when Boraflex is exposed to gamma
radiation, the material may shrink by as much as 3 to 4 percent. Shrinkage
saturates at an integrated gamma exposure of about 1 to 2x1010 cGy
(1 to 2x1010 rad). The application of realistic assumptions based on these
tests has demonstrated that the reactivity effects of Boraflex shrinkage and
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June 26, 1996
Page 3 of 7
gaps are very small and can generally be accommodated within the existing
design basis of most storage racks.
Data from laboratory tests and spent fuel pool silica measurements have
identified a second factor that could affect storage rack service life, i.e.,
the potential gradual release of silica from Boraflex following gamma
irradiation and long-term exposure to the wet pool environment. When Boraflex
is subjected to gamma radiation in the pool aqueous environment, the silicon
polymer matrix becomes degraded and silica filler and boron carbide are
released. Since irradiated Boraflex typically contains 46 percent of silica,
4 percent of polydimethyl siloxane polymer and 50 percent of boron carbide by
weight, the presence of silica in the pool indicates the likely depletion of
boron carbide from Boraflex. The loss of boron carbide from Boraflex is
characterized by slow dissolution of the Boraflex matrix from the surface of
the Boraflex and a gradual thinning of the material. In a typical spent fuel
pool, the irradiated Boraflex represents a significant potential source of
silica (several thousand kilograms) and is the most likely source of pool
silica contamination. The boron carbide loss, of course, can result in a
significant increase in the reactivity of the storage racks. An additional
consideration is the potential for silica transfer through the fuel transfer
canal into the reactor core during refueling operations and its effect on the
fuel clad heat transfer capability.
EPRI has identified several factors that influence the rate of silica release
from Boraflex. The access of water to and around the Boraflex panels is
perhaps the most significant factor influencing the rate of silica dissolution
from Boraflex. Because of the different rack designs, this water access will
vary from plant to plant. The rate of dissolution also increases with higher
pool temperature and gamma exposure, suggesting that pool temperatures be
maintained as low as practical and that freshly discharged fuel assemblies
should not be placed in the same storage cells at each refueling outage.
Experimental data indicates that once silica reaches an equilibrium value, the
rate of dissolution is dramatically reduced. However, when water purification
systems are used to remove silica from the pool water, the solubility
equilibrium becomes unbalanced and panel dissolution resumes. Thus, although
pool temperatures should be maintained as low as practical, additional cooling
may require an increase in pool water flow and create the negative effect of
forcing more water past the Boraflex panels thereby disturbing any localized
silica equilibria.
Because Boraflex is used in spent fuel storage racks for nonproductive
absorption of neutrons, a reduction in the amount of Boraflex could result in
an increase in the reactivity of the spent fuel pool configuration, which may
approach, or even exceed, the current NRC acceptance criterion of keff no
greater than 0.95. The NRC has established this 5-percent subcriticality
margin to comply with General Design Criterion (GDC) 62 of Appendix A to
Part 50 of Title 10 of the Code of Federal Regulations (10 CFR Part 50), which
addresses the prevention of criticality in fuel storage and handling. Those
plants that have installed storage racks containing Boraflex have the
5-percent subcriticality margin included in the plant technical specifications
and/or a written commitment to meet this subcriticality margin, as reflected . GL 96-04
June 26, 1996
Page 4 of 7
in the plant updated final safety analysis report (FSAR). The technical
specifications for most other operating power reactors also include this
5-percent subcriticality requirement.
Several corrective actions have been used to account for any reactivity
increase due to Boraflex loss. Many licensees have taken credit for the
reactivity decrease associated with fuel depletion or have restricted storage
patterns to a checkerboard-type configuration. Others have inserted neutron
absorber rods into stored assemblies with protective features to prevent
inadvertent removal. The NRC is also presently evaluating a proposed
methodology by which credit could be taken for the soluble boron in PWR pool
water. Although some of these schemes cannot be used for BWR fuel storage
facilities, there have been discussions and demonstrations of specially
designed neutron absorbing inserts as a replacement for deteriorating Boraflex
which would be applicable to both PWR and BWR storage racks.
Safety Assessment
On the basis of test and surveillance information from plants that have
detected areas of Boraflex degradation, no safety concern exists that warrants
immediate action. Boraflex dissolution appears to be a gradual and localized
effect forewarned by relatively high silica levels in the pool water. This
occurrence of increased pool silica is more pronounced in PWRs than BWRs
because of the greater effectiveness of silica removal by the BWR
demineralizers in the non-borated pool water environment. Because of the
safety margin present in spent fuel storage pools, compliance with the
required subcriticality margin (or conformance with the same margin to which
licensees have committed in their updated FSARs) can be expected to be
maintained during the initial stage of Boraflex degradation. This safety
margin is due to the conservatism in treating the reactivity effects of
possible variations in material characteristics and mechanical tolerances and
the generally lower reactivity of stored fuel than that assumed in the safety
analysis. However, to verify compliance with both the regulatory requirements
of GDC 62 and the 5-percent subcriticality margins, either contained in the
technical specifications or committed to in the updated FSARs, and to maintain
an appropriate degree of defense-in-depth measures, the NRC staff has
concluded that it is appropriate for licensees to submit the following
information.
Requested Information
All licensees of power reactors with installed spent fuel pool storage racks
containing the neutron absorber Boraflex are requested to provide an
assessment of the physical condition of the Boraflex, including any
deterioration, on the basis of current accumulated gamma exposure and possible
water ingress to the Boraflex and state whether a subcritical margin of
5 percent can be maintained for the racks in unborated water. Monitoring
programs or calculational models in effect or being developed, or an
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June 26, 1996
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estimation of anticipated concerns based on the specific rack design, are
considered an appropriate basis for this response. All licensees are further
requested to submit to the NRC a description of any proposed actions to
monitor or confirm that this 5-percent subcriticality margin can be maintained
for the lifetime of the storage racks and describe what corrective actions
could be taken in the event it cannot be maintained. Licensees should
describe the results from any previous post operational blackness tests and
state whether blackness testing, or other in-situ tests or measurements, will
be periodically performed. Chronological trends of pool reactive silica
levels, along with the timing of significant events such as refuelings, pool
silica cleanups, etc., should be provided. Implications of how these pool
silica levels relate to Boraflex performance should be described. All
licensees are requested to submit the information to the NRC to ensure that
the onsite storage of spent fuel is in compliance with GDC 62 for the
prevention of criticality in fuel storage and handling and with the 5-percent
subcriticality margin position of the NRC staff to assure compliance with
GDC 62.
Required Response
All addressees that use Boraflex in their spent fuel storage racks are
required to submit a written response to the information requested above
within 120 days of the date of this generic letter. If an addressee chooses
not to respond to specific questions, an explanation of the reason and a
description of any proposed alternative course of action should be provided,
as well as the schedule for completing the alternative course of action (if
applicable), and the safety basis for determining the acceptability of the
planned alternative course of action.
Address the required written reports to the U.S. Nuclear Regulatory
Commission, ATTN: Document Control Desk, Washington, D.C. 20555, under oath
or affirmation under the provisions of Section 182a, Atomic Energy Act of
1954, as amended, and 10 CFR Part 50.54(f). In addition, submit a copy to the
appropriate regional administrator.
Backfit Discussion
This generic letter only requires information from the addressees under the
provisions of Section 182a of the Atomic Energy Act of 1954, as amended, and
10 CFR Part 50.54(f). Therefore, the staff has not performed a backfit
analysis. The information requested will enable the NRC staff to determine
whether licensees are complying with the current licensing basis for the
facility with respect to GDC 62 for the prevention of criticality in fuel
storage and handling and 5-percent subcriticality margins either contained in
the technical specifications, or committed to in the updated FSARs, of plants
containing Boraflex in the spent fuel storage racks. The staff is not
establishing a new position for such compliance in this generic letter.
Therefore, this generic letter does not constitute a backfit and no documented
evaluation or backfit analysis need be prepared.
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June 26, 1996
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Federal Register Notification
A notice of opportunity for public comment was published in the Federal
Register (60 FR 56359) on November 8, 1995. Comments were received from
15 licensees and 1 industry organization. Copies of the staff evaluation of
these comments have been made available in the public document room.
Paperwork Reduction Act Statement
This generic letter contains information collections that are subject to the
Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.). These information
collections were approved by the Office of Management and Budget, approval
number 3150-0011, which expires July 31, 1997.
The public reporting burden for this collection of information is estimated to
average 150 hours per response, including the time for reviewing instructions,
searching existing data sources, gathering and maintaining the data needed,
and completing and reviewing the collection of information. The U.S. Nuclear
Regulatory Commission is seeking public comment on the potential impact of the
collection of information contained in the generic letter and on the following
issues:
1. Is the proposed collection of information necessary for the proper
performance of the functions of the NRC, including whether the
information will have practical utility?
2. Is the estimate of burden accurate?
3. Is there a way to enhance the quality, utility, and clarity of the
information to be collected?
4. How can the burden of the collection of information be minimized,
including the use of automated collection techniques?
Send comments on any aspect of this collection of information, including
suggestions for reducing this burden, to the Information and Records
Management Branch, T-6 F33, U.S. Nuclear Regulatory Commission, Washington, DC
20555-0001, and to the Desk Officer, Office of Information and Regulatory
Affairs, NEOB-10202 (3150-0011), Office of Management and Budget,
Washington, DC 20503.
The NRC may not conduct or sponsor, and a person is not required to respond
to, a collection of information unless it displays a currently valid OMB
control number.
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June 26, 1996
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If you have any questions about this matter, please contact one of the
technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
ORIGINAL SIGNED BY
Brian K. Grimes, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Larry Kopp, NRR
(301) 415-2879
Internet:lik@nrc.gov
Krzysztof Parczewski, NRR
(301) 415-2705
Internet:kip@nrc.gov
Lead Project Manager: Steven Bloom, NRR
(301) 415-1313
Internet:sdb1@nrc.gov
