ML20116M077

Provides Suppl Response to Re Insp Rept 50-267/96-01 in Jan 1996 Re NRC Concerns About Fsv Final Survey Program.Specifically,Bias in Instrumentation Response Overestimating Amount of Contamination Present

ML20116M077
Person / Time
Site:	Fort Saint Vrain
Issue date:	08/14/1996
From:	Borst F PUBLIC SERVICE CO. OF COLORADO
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
P-96067, NUDOCS 9608190295
Download: ML20116M077 (2)

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S eubiic 16805 WCR service-191/2; Platteville, Colorado 80651

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August 14, 1996 Fort St. Vrain P-96067 U.S. Nuclear Regulatory Commission A'ITN: Document Control Desk Washington, D.C. 20555 Docket No. 50-267

SUBJECT:

Supplemental Response to Inspection Report Concerns (NRC Inspection Report No. 50-267/96-01)

REFERENCES:

1. NRC Letter, Scarano to Crawford, dated March 5,1996 (G-96019)

2. PSCo Letter, Borst to Document Control Desk, dated April 8,1996 (P-96025)

Gentlemen:

This letter provides supplemental information regarding Public Service Company of Colorado's (PSCo) response to NRC concerns regarding the Fort St. Vrain final survey program. The NRC's concerns were identified during a January 1996 inspection (Reference 1), and PSCo's response was provided in Reference 2. During subsequent discussions with NRC representatives, additional clarification was requested regarding i PSCo's response to the following concern:  ;

NRC Concern 1:

Determine whether there is a bias in instrumentation response which overestimates the amount of contamination present.

As indicated in Reference 2, the basis for this concern is that during side-by-side measurements performed by PSCo's decommissioning contractor (the Westinghouse Team / Scientific Ecology Group - WT/SEG), and by the NRC's contractor (the Oak ,

Ridge Institute for Science and Education - ORISE), ORISE's measurements were about 18 percent lower than WT/SEG's. I O In Reference 2, PSCo and WT/SEG indicated that much of this difference could be O!

attributed to a difference in the method of determining detection efficiency. Based on 1f '

a review of over 1800 source test results performed during the Fort St. Vrain final i

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• l P-9dO67 August 14,1996 l Page 2 survey and during additional testing conducted by WT/SEG, it is concluded that our method of determining detection efficiency will result in a slight overestimate of the true

! activity for ideal field measurement conditions. A brief description of the differences between ORISE's and WT/SEG's methods is provided below.

! WT/SEG positions a single source at multiple positions under the detector (Front, center, and rear) at a distance of approximately 1/8-inch from the face plate of the detector and the detector efficiency is determined using the mean of the source counts from the multiple positions. The outer edges (front and rear) of the detector are less sensitive than the center which is the most sensitive detector position. It is our understanding that

! ORISE places a single source at a single center position, on contact with the protective l screen and the detector efficiency is determined using the single position.

Based on testing conducted by WT/SEG, placing the source on contact with the protective screen at the center of the detector can increase the value for detection efficiency by as much as 10% of the efficiency value obtained with a centered source,

. 1/8-inch away from the faceplate. In addition, accounting for the outer edges of the detector (front and rear) can increase the value for detection efficiency by an additional 3% of the efficiency value. This accounts for a totalincrease of approximately 13% of the efficiency value obtained using the WT/SEG method (e.g., from 20% to 22.6%).

Other differences cannot be completely explained and may be attributed to measurement technique which includes the standard error associated with counting equipment and the use of radioactive source material, and inherent differences between SEG's and ORISE's detectors and electronic settings.

PSCo and WT/SEG acknowledge that under ideal field measurement conditions (i.e.,

detector centered over localized activity on a smooth surface) our method of determining i detection efficiency will result in a slight overestimate of the true activity. However, considering the variety of surfaces in the Fort St. Vrain facility, we believe that the extent of overestimation will be reduced for actual field measurements and is not significant. We consider that our method of efficiency determination is prudent to avoid underestimating activity for field measurements, and is appropriate for the Fort St. Vrain project.

If you have any questions regarding this information, please contact Mr. M. H. Holmes at (303) 620-1701.

Sincerely, hhs 0@~

Frederic J. 3orst Decommt oning Program Director

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P-96067 August 14, 1996 Page 3 l

l l FJB/SWC l .

cc: Regional Administrator, Region IV Mr. Robert M. Quillin, Director l Radiation Contml Division l Colorado Department of Public Heal h and Environment

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