ML20059J589
| ML20059J589 | |
| Person / Time | |
|---|---|
| Site: | Shoreham File:Long Island Lighting Company icon.png |
| Issue date: | 03/30/1993 |
| From: | Landis M AFFILIATION NOT ASSIGNED |
| To: | Fauver D NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| NUDOCS 9311120276 | |
| Download: ML20059J589 (2) | |
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i4 March 30,1993 Dr. David Fauver Division of Imw-Level Waste Management and Decommissioning Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Rockville, MD 20852
SUBJECT:
ESSAP AND SHOREHAM CALIBRATION PROCEDURES
Dear Dr. Fauver:
A review and cross-comparison of direct measurement methodology was to be included as part of the confirmatory activities, conducted during the recent visit to the Shoreham facility. Initially, a comparison of pancake GM probe equipment and calibration was conducted. During this study, several significant differences in equipment and calibration methodology were identified; as a result of these differences and other possible factors, there was a wide discrepancy in the efficiency factors, determined for the ESSAP and licensee GM instruments.
ESSAP calibrates pancake GM detectors at contact using Tc-99 (for Shoreham). The detectors hase a fine mesh screen covering the face of the probe to reduce the chance of puncture. The efficiencies typically range for 0.20 to 0.25 cpm /dpm.
Based on our understanding, Shoreham calibrates pancake GM detectors at I cm from the source, using Co-60. The detectors have a coarse wire screen covering the face of the probe to reduce the chance of puncture. The average efficiency is 0.11 cpm /dpm.
In attempting to resolve the difference in efficiency factors, the following parameters which may effect the efficiencies have been evaluated in detail: (a) the technical specifications for the GM tubes; (b) electronic calibration of the scaler; (c) the effect of the fine mesh screens (ESSAP's) versus the coarse wire screens (Shoreham's); (d) the effect of calibration geometry-at contact and at I cm. Results are:
(a)
The pancake GM tubes are essentially identical in design. Both have window density 2
thickness of 1.4 to 2.0 mg/cm.
(b)
Electronic pulse generator tests of scalers have confirmed that they are operating properly.
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Dr. Fauver March 30,1993 (c). The efficiencies obtained using the fine mesh screens (ESSAP) were higher (by about 0.02 cpm /dpm) than those obtained using the coarse wire screens (Shoreham).
(d)
Efficiencies at contact, with Tc-99, were higher by 0.04 to 0.05 cpm /dpm, than those obtained at I cm.
4 These latter two items significantly effect the efficiencies.
In addition, during the process of conducting this study, we discovered that some of our older Tc-99 calibration sources have a 30%
error in the activity values provo.1 by the manufacturer. (These sources have been removed from service.) Using new ESSAP calibration sources and adjusting the efficiencies for the differences due to the protective screens and source-to-detector distance, ESSAP concurs with the average efficiency
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of 0.11 reported by Shoreham.
Because the ultimate objective of the confirmatory process is to compare survey data using statistical methodology, it appears that either the same instrumentation and procedures should be used by both organizations or that scaling factors should be applied to the final values to correct for differences in survey methodology. Both options have advantages and disadvantages. The former would be the simplest but would probably require that ESSAP duplicate Shoreham's instrumentation and procedures.
The latter would require a thorough evaluation of all instrumentation and procedures used by both organizations to develop the approoriate factors. Since the scaling factors would vary with the instrumentation used, the final survey data would have to clearly identify the instrumentation used for tne measurements.
Due to the difficulties encountered in the comparison of pancake GM detectors, the allocated time for on-site activities did not allow ESSAP to pursue comparisons of other instrument types. In addition to completing the comparison of all detector responses, we should also resolve issues related to survey 2
methodology, using probes larger than 100 cm, to assure that this will not adversely effect the ability to measure surface activity, relative to all applicable guideline values. It is our understanding that Shoreham is currently evaluating this particular matter. We suggest that these matters be resolved at the meeting to be scheduled in April.
Please call me at (615) 576-2908 if you have any questions regarding this information.
Sincerely, 4
Michele R. Landis i
Environmental Survey and Site Assessment Program MRL:kew cc:
T. Mo, NRC/NMSS, 6H3 D. Tiktinsky, NRC/NMSS, 6E6 S. Swift /F. Brown, NRC, 6H3 R. Nimitz, NRC/ Region I J. Berger, ESSAP T. Vitkus, ESSAP PMDA, 6E6 File /202