[Federal Register Volume 60, Number 216 (Wednesday, November 8, 1995)]
[Notices]
[Pages 56359-56361]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-27624]



-----------------------------------------------------------------------


NUCLEAR REGULATORY COMMISSION

Proposed Generic Communication; Boraflex Degradation in Spent 
Fuel Pool Storage Racks (M91447)

AGENCY: Nuclear Regulatory Commission.

ACTION: Notice of opportunity for public comment.

-----------------------------------------------------------------------

SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing to issue 
a generic letter concerning Boraflex degradation in spent fuel pool 
storage racks. The purpose of the proposed generic letter is to request 
that licensees who use Boraflex as a neutron absorber in their spent 
fuel storage racks (1) assess the capability of the boraflex to 
maintain a 5 percent subcriticality margin and (2) submit a plan of 
action if this subcriticality margin cannot be maintained by the 
Boraflex material because of current or projected degradation. The NRC 
is seeking comment from interested parties regarding both the technical 
and regulatory aspects of the proposed generic letter presented under 
the Supplementary Information heading.
    The proposed generic letter was endorsed by the Committee to Review 
Generic Requirements (CRGR) on September 26, 1995. The relevant 
information that was sent to the CRGR will be placed in the NRC Public 
Document Room. The NRC will consider comments received from interested 
parties in the final evaluation of the proposed generic letter. The 
NRC's final evaluation will include a review of the technical position 
and, as appropriate, an analysis of the value/impact on licensees. 
Should this generic letter be issued by the NRC, it will become 
available for public inspection in the NRC Public Document Room.

DATES: Comment period expires December 8, 1995. Comments submitted 
after this date will be considered if it is practical to do so, but 
assurance of consideration cannot be given except for comments received 
on or before this date.

ADDRESSEES: Submit written comments to Chief, Rules Review and 
Directives Branch, U.S. Nuclear Regulatory Commission, Mail Stop T-6D-
69, Washington, DC 20555-0001. Written comments may also be delivered 
to 11545 Rockville Pike, Rockville, Maryland, from 7:30 am to 4:15 pm, 
Federal workdays. Copies of written comments received may be examined 
at the NRC Public Document Room, 2120 L Street, N.W. (Lower Level), 
Washington, D.C.

FOR FURTHER INFORMATION CONTACT: Laurence I. Kopp (301) 415-2879.

SUPPLEMENTARY INFORMATION:

NRC Generic Letter 95-XX: Boraflex Degradation in Spent Fuel Pool 
Storage Racks (M91447)

Addressees

    All holders of operating licenses for nuclear power reactors.

Purpose

    The U.S. Nuclear Regulatory Commission (NRC) is issuing this 
generic letter to request that each addressee that uses Boraflex as a 
neutron absorber in its spent fuel storage racks (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.'' The Electric Power Research Institute (EPRI) has 
been studying the phenomenon of Boraflex degradation for several years 
and recently issued EPRI TR-103300, ``Guidelines for Boraflex Use in 
Spent-Fuel Storage Racks,'' December 1993, identifying 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 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 

[[Page 56360]]
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. 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. No other storage cells exhibited as significant 
a 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 2 x 10\10\ cGy (1 to 2 x 10\10\ rad). The application of realistic 
assumptions based on these tests has demonstrated that the reactivity 
effects of Boraflex shrinkage and gaps are very small and can generally 
be accommodated within the existing design basis of most storage racks 
(EPRI TR-101986, ``Boraflex Test Results and Evaluation,'' February 
1993).
    Data from laboratory tests and spent fuel pool silica measurements 
have identified a second factor that could affect storage rack service 
life: 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's 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 depletion of boron carbide 
from Boraflex. The loss of boron carbide from Boraflex is characterized 
by slow dissolution of the silicon polymer 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 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 TR-103300 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. Once silica 
reaches an equilibrium value, the rate of dissolution essentially 
stops. However, when water purification systems are used to remove 
silica from the pool water, the solubility equilibrium becomes 
unbalanced and panel dissolution resumes.
    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 requires 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 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.

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. 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 
5 percent subcriticality margin assumed in the analysis, the generally 
lower reactivity of stored fuel than that assumed in the safety 
analysis, and, in the case of PWRs, the presence of borated water in 
the pool. 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 spent fuel pool storage racks 
containing the neutron absorber Boraflex are requested to provide a 
description of the physical condition of the Boraflex, including any 
deterioration, on the basis of current as well as future projected 
accumulated gamma exposure and possible water ingress to the Boraflex 
and state whether a subcritical margin of 5 percent can be maintained 
for the life of the racks in unborated water. 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 will be taken in the event it cannot be maintained. 
Licensees should describe the results from any previous blackness tests 
and state whether blackness testing will be periodically performed. Any 
abnormal pool silica levels should also be described. All licensees are 
requested to submit the information to the NRC to 

[[Page 56361]]
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 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 50.54(f). In addition, submit a 
copy to the appropriate regional administrator.

Backfit Discussion

    This generic letter only requires information from the addresses 
under the provisions of Section 182a of the Atomic Energy Act of 1954, 
as amended, and 10 CFR 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.

Federal Register Notification

    (To be completed after the public comment period.)

Paperwork Reduction Act Statement

    The information collections contained in this request are covered 
by the Office of Management and Budget clearance 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. Send comments 
regarding this burden estimate or any other aspect of this collection 
of information, including suggestions for reducing this burden, to the 
Information and Records Management Branch, (T-6F33), 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.

    Dated at Rockville, Maryland, this 2nd day of November, 1995.

    For the Nuclear Regulatory Commission.
Dennis M. Crutchfield,
Director, Division of Reactor Program Management, Office of Nuclear 
Reactor Regulation.
[FR Doc. 95-27624 Filed 11-7-95; 8:45 am]
BILLING CODE 7590-01-P