ML20072V494

Forwards Summary of Existing & Planned Work for Activation Verification,910415, Describing Analysis of Wire & Charpy Specimens from Prestressed Concrete Reactor Vessel Top Head Near Liner

ML20072V494
Person / Time
Site:	Fort Saint Vrain
Issue date:	04/15/1991
From:	Crawford A PUBLIC SERVICE CO. OF COLORADO
To:	Weiss S NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation
Shared Package
ML20072V499	List: ML20072V494 ML20072V503
References
P-91137, NUDOCS 9104220231
Download: ML20072V494 (3)

Text

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P.O. Box 840 April 15, 1991 conver co 80201 0840 Fort St. Vrain Un:t No. 1 A, Clegg Crawford P-91137 Q'j'fgnj,,

U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D. C. 20555 ATTN: Mr. Seymour H. Weiss, Director Non-Power Reactor, Decommissioning and Environmental Project Directorate Docket No. 50-267

SUBJECT:

PCRV ACTIVATION ANALYSIS VERIFICATION

REFERENCE:

1) NRC Memorandum, Erickson to Weiss, dated February 15, 1991 (G-91064)

Dear Mr. Weiss:

This letter provides an interim report of Public Service Company of Colorado's (PSC's) efforts to verify the Fort St. Vrain (FSV)

Prestressed Concrete Reactor Vessel (PCRV) Activation Analysis, in addition to the activities already in progress, PSC undertook added efforts as a result of a meeting with the NRC on February 11, 1991 (Reference 1). The attached report, " Summary of Existing and Planned

- Work For Activation Verification", describes the current status of these efforts and provides an overall assessment, based on results obtained to date, of the PCRV concrete removal plans and associated cost estimate information previously provided to the NRC.

As discussed during the February 11, 1991, meeting, FSV decommissioning cost estimates for concrete removal and disposal during the dismantlement of the PCRV have been based on the activation analysis. The disposal volumes used in PSC's cost estimates are based on conservative assumptions of the neutron flux levels and the concrete and rebar composition.

As described in the attached report, PSC's neutron flux verification efforts have included analysis of wire and Charpy specimens from the PCRV top head near the liner, and tendon wire samples from a vertical tendon located 32" from the PCRV liner. Based on a preliminary analysis, the measured results compare quite well with predicted activities near the liner. Also, the activity levels ir. the tendon wires are reasonable, considering the difficulty in predicting neutron flux levels at that distance from the core.

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P-91137

. .Page 2 April 15, 1991 During. the current planning phase for FSV decommissioning, PSC's efforts to determine concrete and rebar composition are limited to  !

materials which can be removed < rom the PCRV without affecting-its integrity. In the absence of concrete core samples, PSC- is evaluating the effects of worst case concentrations of_ trace element impurities, sM as cobalt, europium, silver, and niobium, on PCRV activity levels and on the amount of concrete that would have to be removed. This effort includes taking PCRV concrete surfue samples  ;

at various locations- to confirm the conservatism of the detivation analysis assumptions. After the decommissioning implementation phase is entered, several planned activities-involve core drilling into the PCRV. Core samples from-these activities will be analyzed to further confirm the activ'ation analysis and support disposal efforts.

As: described in the attached report, PSC has determined that the amount of-concrete that must be removed to achieve an unrestricted use dose 1 rate of 5 microR/hr is calculated to be 21", taking into account that (1) there will be no dose contribution from the top head or core support floor, and (2) the concrete need not be considered a

, homogeneous concrete /rebar mixture. PSC considers that the activation analysis is conservative; however, in a worst case scenario, the_ actual thermal flux _and the actual trace element concentrations could be greater than PSC's assumed _ values. These

, worst case -actual conditions could require additional concrete L removal .of 4" and 2", respectively, for a total required concrete i

removal depth-of 27".

l During actual PCRV dismantlement, PSC plans to remove concrete-from L the PCRV sidewalls by using a diamond wire cutting device, threaded through. every third tube.of-the= inner row of vertical tendon tubes.

This results in a concrete removal depth that varies from 27" to 32".

l This will ensure adequate concrete removal, even in the worst case scenario described above.

PSC's on-going- efforts to verify the activation analysis--include-obtaining independent - confirmatory laboratory analysis- of the-L activity levels -in the material samples, and analyzing concrete impurity samples from various PCRV surface locations. These efforts are -expected to be completed within four months and PSC will1 provide theLresults to the NRC by August 15, 1991.

As discussed in Appendix A-- of the attached report and during the-February 11, 1991, meeting. . tritium in concrete is best treated by ,

pathways analysis and application of the 10 mrem / year _ criterton_. PSC does not-anticipate-tritium to be a dominant dose contributor or affect concrete -removal, _and has therefore not included tritium in the planned verification activities'.

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P-91137 Page 3 April 15, 1991 The information provided in the attached summary report c:,toblishes conservative bounding conditions for the activation analysis assumptions, and provides reasonable verification of the activation analysis results. All of the concrete removal activities discussed herein are within the scope of the concrete removal activities described in the Proposed Decommissioning Plan, and can be accomplished within the total decomr.issioning costs previously identified. Furthermore, the removal and disposal of concrete volumes required in the worst case scenarios is bounded by the cost estimate.

If you have any questions regarding the information provided in this submittal, please contact Mr. M. H. Holmes at (303) 480-6960.

Very truly yours, dY- $ 1 A. Clegg Crawford Vice President Nuclear Operations ACC/SWC/Imb cc: Regional Administrator, Region IV Mr. J. B. Baird Senior Resident Inspector

. Fort St. Vrain Mr. Robert M. Quillin, Director Radiological Control Division Colorado Department of Health

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