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Home       > Electronic Reading       Room       > Document       Collections       > Generic       Communications >       Generic       Letters >       1996       > Gl96004 
				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9606240132 .                                                            GL 96-04                                                            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 .                                                            GL 96-04                                                            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 .                                                            GL 96-04                                                            June 26, 1996                                                            Page 5 of 7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..                                                            GL 96-04                                                            June 26, 1996                                                            Page 6 of 7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..                                                            GL 96-04                                                            June 26, 1996                                                            Page 7 of 7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