United States Nuclear Regulatory Commission - Protecting People and the Environment

Individual Plant Examination of External Events for Severe Accident Vulnerabilities (Generic Letter No. 88-20, Supplement No. 5)

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555-0001

September 8, 1995


NRC GENERIC LETTER 88-20, SUPPLEMENT 5:  INDIVIDUAL PLANT EXAMINATION OF
                                         EXTERNAL EVENTS FOR SEVERE ACCIDENT
                                         VULNERABILITIES


Addressees

All holders of operating licenses (except those licenses that have been
amended to possession-only status) or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to
(1) notify addressees of modifications in the recommended scope of seismic
reviews that are performed as part of individual plant examinations of
external events (IPEEEs) for the focused-scope and full-scope plants and (2)
provide guidance to licensees who wish to voluntarily modify their previously
committed seismic IPEEE programs.

Background

On June 28, 1991, NRC issued Generic Letter 88-20, Supplement 4, "Individual
Plant Examination of External Events (IPEEE) for Severe Accident
Vulnerabilities, 10 CFR 50.54(f)," (Reference 1), and NUREG-1407, "Procedural
and Submittal Guidance for the Individual Plant Examination of External Events
(IPEEE) for Severe Accident Vulnerabilities:  Final Report," (Reference 2). 
The generic letter requested all licensees to perform an IPEEE to find plant-
specific vulnerabilities to severe accidents caused by external events and
report the results to NRC.  Section 4.1 of Reference 1 and Chapter 3 of
Reference 2 address the seismic portion of the IPEEE.  The lists of review
level earthquakes (RLEs) and the review scope defined by the staff for all
U.S. sites are presented in Appendix 3 of Reference 1.  Plants in the central
and eastern U.S. have been assigned to appropriate review categories (plant
bins) primarily according to a comparison of available seismic hazard results.

The hazard results used in the binning process included those published in
1989 by Lawrence Livermore National Laboratory (LLNL) (Reference 3) and the
Electric Power Research Institute (EPRI) (Reference 4).  NRC established the
bins because of the large inherent uncertainties in the probabilistic
estimation of seismic hazard (Appendix A to Reference 2).  Using this
approach, the staff compared the relative seismic hazard of the 69 central and
eastern U.S. plant sites, and assigned each plant to one of four bins for the
seismic margins method (Reduced-Scope, 0.3g Focused-Scope, 0.3g Full-Scope,
and 0.5g bin).  Two plants in the 0.5g bin have committed to perform a seismic
probabilistic risk assessment (PRA) and have performed the assessment.

9508310033.                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 2 of 6


Description of Circumstances

In 1994, based on a re-elicitation of LLNL ground-motion and seismicity
experts, the staff published revised seismic hazard results in NUREG-1488
(Reference 5).  The new LLNL mean hazard estimates are lower than the 1989
LLNL results but higher than the EPRI estimates.  The Nuclear Energy Institute
(NEI), based on these revised hazard estimates, advocated that most focused-
scope plants should instead perform reduced-scope studies as part of the
seismic IPEEE (Reference 6).  NEI also stated that each licensee is
responsible for proposing the most cost-effective program to satisfy the
seismic IPEEE request consistent with the level of seismic hazard at the
specific site.  Seven licensees have informed NRC of their intent to revise
their IPEEE commitments.

These developments prompted NRC to revisit systematically the seismic IPEEE
program rather than to deal with each licensee individually. The staff stated
its intent to review LLNL's revised seismic hazard estimates and to determine
if it is appropriate to revise the seismic IPEEE scope in Information Notice
94-32, "Revised Seismic Hazard Estimates," (Reference 7).  The staff also
stated in Reference 7 that licensees who have not completed the seismic
portion of the IPEEE may continue with their programs and submit their
completed IPEEE based on References 1 and 2.

NRC contracted with Energy Research, Inc. (ERI) to do a seismic revisit study
to determine whether consideration of the new LLNL seismic hazard estimates
(1) would significantly change the original binning results and (2) would
warrant adjusting the seismic scope and guidelines of the seismic IPEEE
review.  The latter effort would also require the determination of how the
scope should be modified and the justification of such modifications.  ERI
completed the study and submitted two reports in September 1994 (References 8
and 9).  The staff held a public workshop on October 21, 1994, to discuss
these reports, present comments from a peer review group, determine issues to
be addressed, and solicit public input for developing the staff position on
the seismic scope modification.  The transcript of the workshop is available
in Reference 10.

NRC issued a draft of this supplement for public comment in January 1995 and
received written comments from seven organizations.  This supplement includes
changes resulting from the resolution of these public comments.  Comments on
draft Supplement 5 and the staff's resolution of the comments will be made
available in the Public Document Room. 

Discussion

The staff evaluated the ERI reassessment reports, the peer review group's
comments, the NEI white paper (Reference 6), and comments received at and
after the workshop.  The staff concludes that (1) licensees may use the
revised LLNL seismic hazard estimates instead of the 1989 LLNL seismic hazard
estimates in the seismic PRA and (2) the scope of the seismic IPEEE may be
modified for all focused-scope and full-scope plants by eliminating the need .                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 3 of 6


to calculate the capacity of certain generally rugged components or certain
site effects that would not be significant sources of contributors to seismic
severe-accident risk or would not result in cost-beneficial improvements.  The
justification for this reduction in the seismic review scope is that the
perceived seismic hazard estimates and associated risks have decreased. 
However, the examination process for the modified seismic IPEEE remains the
same process described in Supplement 4 to Generic Letter 88-20 and NUREG-1407.

The most significant comments and concerns with respect to reducing the scope
of the IPEEE seismic review which were raised at and after the workshop and
the associated resolutions are summarized in Attachment 2.  

The opinions expressed by certain utilities represented at the public
workshop, showed that the guidance provided in GL 88-20, Supplement 4, and in
NUREG-1407 is being interpreted in an unintended manner.  For instance,
certain utilities interpreted NUREG-1407 as needing a minimum number of
seismic margin capacity calculations (i.e., high confidence of low probability
of failure).  The NRC staff wants to reemphasize that the guidance in the
generic letter and NUREG-1407 does not preclude the use of well-based expert
judgment and efficient approaches that minimize the effort of conducting an
IPEEE.  In GL 88-20, Supplement 4, the staff stated:

      "The application of the above approaches involves considerable      
      judgement with regards to the requested scope and depth of the      
      study, level of analytical sophistication, and level of effort to      
      be expended."

The detailed guidelines presented in NUREG-1407 do not preclude use of this
type of judgment.  The use of judgment is further recognized in NUREG-1407 in
connection with the importance of the peer review.  Discussions at the
workshop indicated that some utilities did use such judgment, within the
framework of the current guidance as discussed, to reduce the cost of IPEEEs.

Modified Scope of Seismic Examination

The methods originally described and guidelines described in NUREG-1407
fulfill Supplement 4 to GL 88-20.  However, the results of the revised LLNL
seismic estimates indicate that the perceived seismic hazard has been reduced
for most plant sites in the central and eastern U.S.  Accordingly, NRC
proposes reducing the scope of the seismic IPEEE programs for licensees as
follows:

      (1) Licensees Performing a Seismic PRA

          The licensee can use the higher of the mean (arithmetic) seismic
          hazard estimates from the revised LLNL (Reference 5) or EPRI
          studies.
.                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 4 of 6


      (2) Focused-Scope Plants

          The seismic capacities for reactor internals and soil-related
          failures need not be evaluated for the seismic IPEEE.  Modifying the
          scope of the seismic IPEEE for focused-scope plants in this manner
          will make these evaluations equivalent to those for the
          reduced-scope plants, with additional evaluations of a few known
          weaker, but critical, components or items.  The rationale for
          retaining the evaluations of these critical components is provided
          in Attachment 1.

      (3) Full-Scope Plants

          The seismic IPEEE need not include an evaluation of seismic
          capacities for reactor internals.  Soil-related failures should
          still be evaluated, but only for safety-related supporting systems
          and equipment that are founded on soil whose function might be
          affected by liquefaction or general instability of the soil.  The
          licensee may also need to evaluate the potential for such postulated
          soil failures or their consequences.  Reference 11 contains guidance
          for such evaluations; a review of appropriate design and
          construction records is adequate.

The staff is aware of recent observations of cracks associated with reactor
internals at some plants.  The NRC issued GL 94-03 (ref. 12) which requested
BWR licensees to inspect their core shrouds by the next outage and to justify
continued safe operation until inspections could be completed.  The staff has
concluded in all cases that licensees have provided sufficient evidence to
support continued operation of their BWR units to the refueling outages in
which shroud inspections or repairs have been scheduled.  ASME Code structural
margins, as required by 10CFR50.55a, have been maintained in all core shrouds
examined to date.  In addition, the industry BWR Vessel and Internals Project
has proactively been addressing internal cracking and developing evaluation
criteria, inspection methods, as well as repair and mitigation methods for all
BWR reactor internals.  The NRC proposes to conduct research regarding the
synergistic effects of multiple cracking in one or more internals components
(ref. 13).  The research will consider seismic loading as a part of the
program.  Therefore, eliminating this item will not detract from the IPEEE.
The remaining scope is the same as that outlined in Supplement 4 to GL 88-20
and NUREG-1407.  The staff reviewed discussions at the workshop, public
comments on the draft Supplement 5, and other information and has taken the
position that using appropriate judgment as allowed in the generic letter and
NUREG-1407 and eliminating detailed evaluations for soil-related failures and
reactor internals that may not lead to cost-beneficial improvements will
maintain the integrity of the IPEEE process while reducing cost.  However,
regardless of the category assigned to the plant, a careful and thorough
seismic walkdown remains the key element in examining seismic vulnerability.
.                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 5 of 6


Requested Information

Licensees of focused-scope and full-scope plants who voluntarily choose to do
seismic IPEEEs using the modified procedures described above must inform NRC
in writing of their intent to do so.  If the revised submittal schedule
differs from schedules previously committed to, the new proposed schedule must
be included in the response.  NRC will schedule meetings with the licensee, if
requested, to discuss subjects raised by licensees and to give necessary
clarifications.  

Licensees who do not modify their seismic IPEEEs are not expected to submit
any response to this generic letter supplement.  Licensees who previously
submitted their requests to modify their seismic IPEEEs may choose not to
submit any response to this generic letter supplement; should that be the
case, NRC will respond separately to their previous requests.

Requested Response

Within 60 days of the date of this generic letter, all addressees who
voluntarily choose to perform seismic IPEEEs using the modified procedures
described above are required to submit a response to the information requested
above.

Address the required written reports, if applicable, to the U.S. Nuclear
Regulatory Commission, ATTN:  Document Control Desk, Washington, DC 20555,
under oath or affirmation under the provisions of Section 182a, Atomic Energy
Act of 1954, as amended, and Section 50.54(f) of Title 10 of the Code of
Federal Regulations (10 CFR 50.54(f)).

Backfit Discussion

The evaluation required by 10 CFR 50.54(f) to justify the IPEEE information
request was included in Supplement 4 to Generic Letter 88-20.  This generic
letter supplement only provides information that addressees may use
voluntarily to reduce the scope of seismic IPEEEs using the modified
procedures described above.  Therefore, the generic letter does not involve
backfitting concerns and no backfit analysis was prepared by the staff.

Paperwork Reduction Act Statement

The information requested herein for voluntary submittal is covered by the
Office of Management and Budget Clearance No. 3150-0011, which expires July
31, 1997.  The public reporting burden for this voluntary collection of
information is estimated to average 20 hours for each 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 voluntary submission 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 .                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 6 of 6


the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202,
(3150-0011), Office of Management and Budget, Washington, DC 20503.

Compliance with the following request for information is purely voluntary. 
The following information would assist NRC in evaluating the costs and savings
of responding to this generic letter supplement:

(1)   the licensee staff time and costs to prepare the requested reports and
      documentation, and 

(2)   an estimate of the long-term costs or savings accruing from the response
      to this generic letter supplement.

If you have any questions about this matter, please contact the technical
contact listed below or the appropriate Office of Nuclear Reactor Regulation
(NRR) project manager.

                              /s/'d by DMCrutchfield


                              Dennis M. Crutchfield, Director
                              Division of Reactor Program Management
                              Office of Nuclear Reactor Regulation





Technical contact:  John Chen, RES
                    (301) 415-6549

Attachments:
1.  Components Needing Capacity Evaluation and Basis
2.  Public Workshop Comments and Resolutions 
3.  References.                                                            Attachment 1
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 1 of 2


               COMPONENTS NEEDING CAPACITY EVALUATION AND BASIS

The components and issues identified as needing evaluation and the bases for
the retention are briefly described below: 

(1)   Relay Chatter 

While preparing the original guidance in NUREG-1407, the NRC staff developed
its position on the relay chatter issue after thoroughly discussing the issue
with industry and evaluating the results of previous studies.  The staff
drastically reduced the scope of relay chatter evaluation, retaining only the
identification of bad actor relays.  Since these relays are of low capacity,
their identification is considered to be minimum scope for the IPEEE review. 
The guidance does not preclude any efficient and expeditious means of
identifying these relays.

(2)   Masonry and Block Walls

Probabilistic risk assessments and margin studies have demonstrated that
failure of masonry or block walls may be a significant safety concern in
existing nuclear power plants.  The earthquake experience database and
analytical evaluations of seismic fragility demonstrate that masonry and block
walls without proper reinforcements are vulnerable to earthquake motion.  In
evaluating these walls, more lenient criteria were used; thus, the available
margins beyond the safe-shutdown earthquake may not be comparable to those of
other components of the plant.  Therefore, in doing the seismic IPEEE review,
the licensee should identify and evaluate masonry and block walls whose
failure may affect safety components required for safe plant operation.  The
licensee should correct, if warranted, any situation that may present a
significant threat to plant safety.

(3)   Flat-Bottom Tanks

Earthquake experience data and analytical fragility evaluations have
demonstrated that flat-bottom tanks with poor anchorage are vulnerable to
earthquake ground motion.  The typical failure mode of concern is buckling at
the base of the tank, causing the liquid contents to escape or the tank to
collapse.  If a flat-bottom tank fails, it could also flood surrounding areas
in the plant.  Past seismic studies of nuclear power plants have designated
flat-bottom tanks as low-capacity components.  Such components include the
refueling water storage tank and the condensate storage tank, whose failures
would often significantly affect plant safety.

The identification and evaluation of flat-bottom tanks should, therefore, be
included as a fundamental element of the seismic IPEEE review to correct any
situation that may threaten plant safety, if warranted..                                                            Attachment 1
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 2 of 2


(4)   Other Items

The licensee should also consider several other items that pertain to
inadequate anchorage and bracing, adverse physical interactions, building
impact, or pounding.  These items include the weaker components of the diesel
generators or pumps.  However, the licensee's seismic review team should
determine whether seismic capacities of those components need to be evaluated
in the seismic review.
.                                                            Attachment 2
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 1 of 3


                    PUBLIC WORKSHOP COMMENTS AND RESOLUTION

All significant comments and concerns raised at and after the workshop,
together with staff's response, are summarized below.

(1)   Candidate plant sites for seismic scope reduction:  The industry      
      suggested that candidate sites should not be limited to focused-scope
      plants.  

      Response:  In addition to modifying the scope for focused-scope plants,
      the staff reduced the scope of review for full-scope plants by
      eliminating the evaluation of reactor internals.

(2)   Use of absolute hazard or risk criteria for rebinning or sub-binning
      candidate sites:  The comments indicated that the absolute risk
      criterion should play a significant role in the seismic rebinning.  

      Response:  The staff considered absolute seismic hazard and risk
      criteria when it reconsidered seismic rebinning.  However, the inherent
      uncertainty in the absolute number would affect decision making, because
      small variations in the core damage frequency (CDF) threshold or in the
      approximately calculated CDFs of candidate plants would significantly
      affect the binning for many plants.  No consensus was reached on the
      specific risk criterion that should be selected for the rebinning
      process.  Therefore, the staff did not recommend using an absolute risk
      criterion when determining whether to reduce the seismic scope. However,
      licensees may use numerical values in determining which plant-specific
      improvements should be implemented.

(3)   Overall reduction of seismic scope for all candidate sites:  The 
      suggested reduction as presented in the ERI report, with the exception
      of reactor internals, would not reduce the scope of seismic review.  

      Response:  Past experience has demonstrated that certain weaker
      components need to be retained in the IPEEE.  The rationale for
      retaining the evaluations of those critical components and issues is
      provided in Attachment 1.

(4)   Role of the licensee's seismic review team (SRT):  Certain utilities
      expressed concern that the role of the licensee's SRT in decision making
      is not clear.

      Response:  The staff wants to emphasize that the SRT has an important
      role in determining how to implement the IPEEE program, i.e., selecting
      the most cost-effective and expedient approach for the IPEEE program.
      The importance and flexibility of the SRT has been stated clearly in the
      IPEEE guidance, NUREG-1407, which allows for the use of judgment and
      latitude in implementing the IPEEE program.    
.                                                            Attachment 2
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 2 of 3


(5)   Evaluation of the effects of soil-related failures:  No simple or cost-  
      effective improvements may be available for plants.

      Response:  Although simple or cost-effective improvements may not be
      available for low seismic hazard sites to deal with the effects of
      soil-related failures, soil-related failures are still considered to be
      important for relatively high seismic hazard sites in the seismic IPEEE.
      
      Therefore, the staff concludes that the licensees of focused-scope
      plants may eliminate the evaluation of soil-related failures from their
      seismic IPEEE programs.  However, to gain insight, the full-scope plants
      should continue evaluating the effects of soil-related failure.  The
      evaluation effort should focus on safety-related supporting systems and
      equipment that are founded on soil and whose function might be affected
      by soil-related failures. 

(6)   Cost savings:  The potential cost savings associated with eliminating
      certain evaluations described in the in the NEI white paper (Reference
      6) are high.

      Response:  The experience gained at certain plants indicates that the
      potential cost savings may be substantially lower than those presented
      in the NEI paper.  Some of the savings cited by the utility personnel
      can be achieved without changing scope, since NUREG-1407 allows
      flexibility such as in eliminating detailed evaluation of reactor
      internals and using an alternate approach as to bad actor relay
      assessment.

(7)   Seismic capacity evaluation of reactor internals:  Should the evaluation
      of reactor internals be eliminated? 

      Response:  The results of a few seismic PRAs have indicated that
      uncracked reactor internals are inherently rugged (having seismic
      capacities well beyond the requested earthquake review level of 0.3g)
      and do not contribute significantly to the core damage frequency. 
      However, a significant effort is involved in calculating the fragility
      or capacity of the reactor internal components.  On the basis of earlier
      study results (assuming uncracked reactor internals) and the perceived
      reduction of seismic hazard estimates and associated seismic risk, the
      staff concluded that the cost of the evaluation outweighs the risk of
      the failure of reactor internal components and proposes to eliminate
      them from the examination.  However, the staff is aware of recent
      observations of cracks associated with reactor internals at some plants.
      
      The NRC issued GL 94-03 (ref. 12) which requested BWR licensees to
      inspect their core shrouds by the next outage and to justify continued
      safe operation until inspections could be completed.  The staff has
      concluded in all cases that licensees have provided sufficient evidence
      to support continued operation of their BWR units to the refueling .                                                            Attachment 2
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 3 of 3


      outages in which shroud inspections or repairs have been scheduled. 
      ASME Code structural margins, as required by 10CFR50.55a, have been
      maintained in all core shrouds examined to date.  In addition, the
      industry BWR Vessel and Internals Project has proactively been
      addressing internal cracking and developing evaluation criteria,
      inspection methods, as well as repair and mitigation methods for all BWR
      reactor internals.  The NRC proposes to conduct research regarding the
      synergistic effects of multiple cracking in one or more internals
      components (ref. 13).  The research will consider seismic loading as a
      part of the program.  Therefore, eliminating this item will not detract
      from the IPEEE. 

(8)   Generic seismic fragilities used in seismic rebinning:  Seismic
      rebinning on the basis of generic seismic fragilities, as was done in
      the ERI's study, would result in anomalous results.

      Response:  The staff concurs that seismic rebinning solely on the basis
      of generic seismic fragilities could result in anomalous results, since
      such items as the plant design basis and vintage of the plant may not be
      appropriately included.  For instance, plants located at the same site
      were put in different bins (Salem and Hope Creek), and the plants near
      the New Madrid area were placed in the modified-scope bin.  These
      observations contributed to the staff's decision to eliminate the use of
      an absolute risk criterion in the seismic scope modifications.

(9)   Information exchange through a workshop on lessons learned from IPEEE:
      Information exchange workshop on IPEEE lessons learned was suggested.

      Response:  An information exchange workshop on IPEEE lessons learned to
      discuss the experience gained about more practical or efficient ways of
      carrying out the seismic IPEEEs (e.g., with respect to the relay chatter
      issue) would benefit both industry and staff.  The staff will consider
      holding such a workshop in the future.
.                                                            Attachment 3
                                                            GL 88-20, Supp. 5
                                                            September 8, 1995
                                                            Page 1 of 1


                                  REFERENCES

[1]   U.S. Nuclear Regulatory Commission, Generic Letter 88-20, Supplement 
      No. 4, "Individual Plant Examination of External Events (IPEEE) for
      Severe Accident Vulnerabilities -- 10 CFR 50.54(f)," June 1991.

[2]   NRC, NUREG-1407, "Procedural and Submittal Guidance for the Individual
      Plant Examination of External Events (IPEEE) for Severe Accident
      Vulnerabilities," Final Report, June 1991.

[3]   NRC, NUREG/CR-5250, "Seismic Hazard Characterization of 69 Nuclear Power
      Plant Sites East of the Rocky Mountains," January 1989.

[4]   Electric Power Research Institute (EPRI), NP-6395-D, "Probabilistic
      Seismic Hazard Evaluation at Nuclear Plant Sites in the Central and
      Eastern United States: Resolution of the Charleston Issue," April 1989. 

[5]   NRC, NUREG-1488, "Revised Livermore Seismic Hazard Estimates for 
      69 Nuclear Power Plant Sites East of the Rocky Mountains," April 1994.

[6]   Letter from W. Rasin (NEI) to A. Thadani (NRC), "NEI White Paper,
      `Justification for Reduction in IPEEE Program Based on Revised LLNL
      Seismic Hazard Results,'" April 5, 1994.

[7]   NRC IN 94-32, "Revised Seismic Hazard Estimates," April 29, 1994.

[8]   Energy Research, Inc. (ERI) Report (ERI/NRC 94-502), "A Proposed
      Approach to Seismic Scope Re-assessment for Individual Plant Examination
      of External Events (IPEEE)," Final Draft, September 1994

[9]   ERI/NRC 94-504, "Approaches for Proposed Modifications of Seismic IPEEE
      Guidelines for Focused-Scope Plants," Final Draft, September 1994.

[10]  NRC Transcript, "Workshop in Seismic IPEEE Revisit," October 21, 1994.

[11]  EPRI NP-6041, "A Methodology for Assessment of Nuclear Power Plant
      Seismic Margin," October 1988.

[12]  NRC Generic Letter 94-03, "Intergranular stress Corrosion Cracking of
      Core Shrouds in BWR Reactors," July 25, 1994.

[13]  NRC memorandum from W. Russell to E. Beckjord, "NRR User Need Request
      for Support of Resolving Problem of Stress Corrosion of Reactor Vessel
      Internal Components," December 2, 1994.

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