ML103570025

Additional Information Supporting License Amendment Request Regarding the Use of Neutron Absorbing Inserts in Unit 2 Spent Fuel Pool Storage Racks

ML103570025
Person / Time
Site:	LaSalle
Issue date:	12/22/2010
From:	Gullott D Exelon Nuclear, Exelon Generation Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
RS-10-216
Download: ML103570025 (5)

Text

www.exeloncorp.cotmi Cxelor Generation 43O+0Wirf eld Road Wamenville, ft 605jc Exekn.Nuclear RS-1 0-216 December 22, 2010 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 LaSalle County Station, Units 1 and 2 Facility Operating License Nos. NPF-11 and NPF-18 NRC Docket Nos. 50-373 and 50-374

Subject:

Additional Information Supporting License Amendment Request Regarding the Use of Neutron Absorbing Inserts in Unit 2 Spent Fuel Pool Storage Racks

References:

1.Letter from P. R. Simpson (Exelon Generation Company, LLC) to U.S. NRC,"License Amendment Regarding the Use of Neutron Absorbing Inserts in Unit 2 Spent Fuel Pool Storage Racks," dated October 5, 2009 2.Letter from J. L. Hansen (Exelon Generation Company, LLC) to U.S. NRC,"Additional Information Supporting License Amendment Request Regarding the Use of Neutron Absorbing Inserts in Unit 2 Spent Fuel Pool Storage Racks," dated December 14, 2010 In Reference 1, Exelon Generation Company, LLC (EGC) requested an amendment to Facility Operating License Nos. NPF-11 and NPF-18 for LaSalle County Station (LSCS), Units 1 and 2, respectively. The proposed change revises Technical Specifications (TS) Section 4.3.1,"Criticality," to address a non-conservative TS. Specifically, the proposed change addresses the BORAFLEXTM degradation issue in the Unit 2 spent fuel storage racks by revising TS Section 4.3.1 to allow the use of NETCO-SNAP-IN rack inserts in Unit 2 spent fuel storage rack cells as a replacement for the neutron absorbing properties of the existing BORAFLEXT"" panels.EGC provided additional information to support the NRC's review of the proposed license amendment in Reference 2. On December 21, 2010, the NRC provided verbal feedback regarding the NRC's review of Reference 2, and stated that clarification was needed. In response to this feedback, EGC is providing the attached information, which completely supersedes the Attachment to Reference 2. Revision bars are included in the Attachment to this letter to identify the portions of the responses that are being changed.

EGC has reviewed the information supporting a finding of no significant hazards consideration, and the environmental consideration, that were previously provided to the NRC in Attachment 1 December 22, 2010 U.S. Nuclear Regulatory Commission Page 2 of Reference 1. The additional information provided in this submittal does not affect the bases for concluding that the proposed license amendment does not involve a significant hazards consideration. In addition, the additional information provided in this submittal does not affectthe bases for concluding that neither an environmental impact statement nor an environmental assessment needs to be prepared in connection with the proposed amendment.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this letter, please contact Mr. Kenneth M. Nicely at (630) 657-2803.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 22nd day of December 2010.

Respectfully, David Gullott Manager - Licensing

Attachment:

Revised Response to Request for Additional Information cc:NRC Regional Administrator, Region III NRC Senior Resident Inspector - LaSalle County Station Illinois Emergency Management Agency - Division of Nuclear Safety ATTACHMENT Revised Response to Request for Additional Information Page 1 NRC Request 1 On page 5 of Attachment 1 to the supplement dated November 23, 2010 (Supplement), for the response to 3.a, confirm that "degradation" is thinning degradation. In addition, indicate the amount of areal density degradation that corresponds with each thinning degradation percentage (e.g., 57.5 percent, 79.8 percent, 68.1 percent, 65.3 percent, and 80 percent).

Response BORAFLEX TM loss manifests itself in three ways: (1) gap formation, (2) edge dissolution, and (3) panel thinning.

BORAFLEX TM shrinks in a high radiation environment causing panel cracking and creating gaps. It has been conservatively assumed in the spent fuel pool criticality analyses that a 1.0 inch coplanar gap exists throughout the spent fuel rack BORAFLEX TM panels. Additionally, the spent fuel pool criticality analyses conservatively assume a 10 percent reduction in the width

of every BORAFLEX TM panel throughout the spent fuel rack due to edge dissolution. The resulting keff increases due to these assumptions have been included in the calculated in-rack k eff for each fuel assembly type, and provide additional margin to the criticality criterion (i.e., k eff < 0.95) as discussed below. In addition to the gap formation and edge dissolution effects on k eff for each fuel assembly type, BORAFLEX TM loss in the form of panel thinning will occur. BADGER test results and RACKLIFE calculations determine "equivalent panel loss," taking into account the panel thinning and edge dissolution effects of BORAFLEX TM degradation. For the purpose of determining an individual assembly's ability to be stored in a given rack location, a comparison is made between BADGER and/or RACKLIFE results, and the allowable degradation limit. By comparing BADGER and/or RACKLIFE equivalent panel loss results to the degradation (i.e.,

panel thinning) limits, as determined from the calculated in-rack keff results, additional margin to the criticality criterion (i.e., k eff < 0.95) is realized. Response 3.a in Attachment 1 of Reference 1 equates degradation to panel thinning only. Since the approach to determining the limiting degradation is based upon panel thinning only, the amount of areal density degradation is equal to the amount of panel thinning for each percent thinning indicated in Reference 1. The panel percent degradations indicated in Reference 1 are referenced to the initial, nominal panel areal density of 0.0238 grams 10 B/cm 2. NRC Request 2 In Attachment 3 to the supplement, indicate the amount of areal density degradation that corresponds with each thinning degradation percentage (e.g., 52.27 percent, 29.97 percent).

Response As noted in EGC's response to NRC Request 1, the approach to determining the limiting degradation is based upon panel thinning only. Therefore, the amount of areal density degradation is equal to the amount of panel thinning for each percent thinning indicated in ATTACHMENT Revised Response to Request for Additional Information Page 2 Reference 1. The panel percent degradations indicated in Reference 1 are referenced to the initial, nominal panel areal density of 0.0238 grams 10 B/cm 2. NRC Request 3 On page 1 of Attachment 3 to the Supplement, it states:

-Unit 2 spent fuel storage rack cells are declared UNUSABLE when the RACKLIFE projected degradation for the most limiting panel in a spent fuel pool cell exceeds 52.27 percent. It appears that a BADGER result would also be a necessary criterion to declare the cell UNUSABLE. Clarify if a BADGER measurement was to exceed the allowable degradation thresholds and the RACKLIFE prediction did not, or whether that cell would be declared RESTRICTED or UNUSABLE.

Response The highest degradation level observed either through BADGER measurement or by RACKLIFE prediction is used to characterize cell designation as UNUSABLE, RESTRICTED, or UNRESTRICTED. NRC Request 4 On page 3 of Attachment 3 of the supplement, from the table entitled "Summary of BADGER Test Results," clarify the meaning of "loss." Address whether this is thinning degradation or areal density degradation. Discuss whether the "loss" is from nominal thickness or nominal areal density or a different reference value.

Response BADGER measured loss is reported as percent areal density loss, which includes panel thinning and edge dissolution effects. This is effectively an equivalent uniform areal density loss averaged over the entire panel. The loss is reported relative to the nominal panel areal density. Thus, the current criticality analysis for the most reactive fuel type in the Unit 2 spent fuel pool (i.e., ATRIUM-10), the BADGER test results, and the RACKLIFE model results, are all referenced to the initial, nominal panel areal density of 0.0238 grams 10 B/cm 2. NRC Request 5 As discussed on page 3 of Attachment 3 of the supplement, explain how panels G74 North and G74 South are "reference panels" since they are being irradiated and exhibit "loss." Also, describe the use of these "reference panels."

ATTACHMENT Revised Response to Request for Additional Information Page 3 Response Panels G74 North and G74 South were selected based on their RACKLIFE calculated absorbed dose and boron carbide loss. They serve to provide a baseline areal density measurement by which to compare other panel measurements.

Reference panels may have received small amounts of radiation. However, doses received by panels G74 North and G74 South are below 1x10 9 rads, and dissolution of BORAFLEX TM is minor at these levels. Scatter in measured values between these panels is the result of BADGER measurement uncertainty and/or variations in as-built BORAFLEX TM areal density.

Reference 1. Letter from P. R. Simpson (Exelon Generation Company, LLC) to U.S. NRC, "Additional Information Supporting License Amendment Request Regarding the Use of Neutron Absorbing Inserts in Unit 2 Spent Fuel Pool Storage Racks," dated November 23, 2010