ML20116A451

Requests NRC Approval of Proposed Method to Fsv Final Survey Plan to Determine Exposure Rates in Prestressed Concrete Reactor Vessel

ML20116A451
Person / Time
Site:	Fort Saint Vrain
Issue date:	07/19/1996
From:	Borst F PUBLIC SERVICE CO. OF COLORADO
To:	Weber M NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
Shared Package
ML20116A455	List: ML20116A451 ML20116A466
References
P-96058, NUDOCS 9607260097
Download: ML20116A451 (3)

Text

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16805 WCR Service' 191/2; Platteville, Colorado 80651 July 19,1996 Fort St. Vrain l P-96058 U.S. Nuclear Regulatory Commission )

l A'lTN: Document Control Desk Washington, D.C. 20555 ATrN: Mr. Michael F. Weber, Chief  ;

Decommissioning and l Regulatory Issues Branch j Docket No. 50-267

REFERENCE:

PSCo Letter, Borst to Weber, dated May 17,1996 (P-96039)

SUBJECT:

ESV Final Survey Plan, Proposed Method to Determine Exposure Rates in the Prestressed Concrete Reactor Vessel l

Dear Mr. Weber:

This letter requests NRC approval of a proposed method for determining exposure rates in the Fort St. Vrain (FSV) Prestressed Concrete Reactor Vessel (PCRV). In the referenced letter, Public Service Company of Colorado (PSCo) proposed use of an in-situ gamma spectroscopic device, the Microspec-2, for determining exposure rate measurements at FSV. This method was developed to overcome the problem of highly variable exposure rate backgrounds at FSV. At that time, PSCo also indicated that the Microspec method would not be suitable for use in areas which are heavily influenced by activated concrete, such as the interior of the PCRV, and that we were reviewing alternate methods for this area.

l After further review, PSCo and our decommissioning contractor, the Westinghouse l Team / Scientific Ecology Group (WT/SEG), propose to determine exposure rates due to

! licensed material inside the PCRV as follows:

1. Use the Microspec-2 to determine gross exposure rates, and then I(

! 2. Subtract background exposure rates calculated from the activities of MJO representative concrete samples, using MicroShield and an appropriate model of the PCRV.

9607260097 960719 POR ADOCK 05000267 W PDR

P-96058 July 19,1996 Page 2 The two key features of this proposal are described and justified in the Attachment to this letter, PCRVFinalSurwy Exposure Rate Measurements, and are summarized as follows:

1. The Microspec-2 will be used for measuring exposure rates within the PCRV because it is available, it is used in other areas of the FSV Reactor Building, and measurement results do not require adjustment to account for the energy response characteristics of typical Nal dose rate instruments. The Microspec-2 was described in the referenced letter as a method for directly measuring exposure rates due to licensed materials. <

However, this method is not suitable for measurement locations that are heavily influenced by activated concrete, particularly when europium-152

is present. Concentrations of europium-152 interfere with the cobalt-60

( spectral region ofinterest (ROI) used by the Microspec-2. The proposed method uses the Microspec-2 to obtain the total spectrum exposure rate, instead of a partial spectrum or ROI exposure rate, as used in areas l outside the PCRV. As further discussed in the Attachment, the proposed l method is technically acceptable and is preferred for determining gross i exposure rates within the PCRV.

2. Background exposure rates are calculated because there is no unaffected structure on-site or off-site constructed similarly to the PCRV, using .

l concrete with the same aggregate source where background exposure rates l can be measured. WT/SEG obtained samples of concrete from various locations outside the PCRV and analytically determined the activity from naturally occurring radionuclides in these samples. These activities were then used in a MicroShield calculation, with a suitable analytical model ,

that accounts for the various PCRV geometries, to calculate background j exposure rates at all PCRV internal locations. These background values  :

are provided in the Attachment. Upon completion of PCRV l decontamination, WT/SEG recently obtained 15 samples from internal i PCRV surfaces, and confirmed that their activities compare very well with l those used in the background exposure rate calculations.

This method for determining background exposure rates has been satisfactorily demonstrated in an empty concrete room at Fort St. Vrain,  !

a diesel generator room which contains no licensed material. In this case, the background exposure rate was calculated from sample activity results, l using MicroShield, to be 14.2 prem/hr (conservatively not taking the cosmic contribution into account). This is within 5% of the measured l exposure rate of 14.9 rem /hr, determined with the Microspec-2.

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P-96058 July 19,1996 Page 3 It is noted that if there are any locations where activated concrete radionuclides such as europium-152 do not affect exposure rate measurements, the method described in the referenced letter would be suitable and we propose that either method would be acceptable in these locations.

• In conclusion, PSCo considers that the method described in this letter will provide an accurate determination of exposure rates within the PCRV. Use of sample analysis and l analytical modeling to determine background exposure rates will eliminate any costly developmental efforts associated with other survey techniques and allows continued use ,

of the Microspec-2 technology.

PSCo/WT are currently undertaking the use of this methodology for determining exposure rates within the PCRV, and would appreciate your approval as soon as practicable. If you have any questions regarding this information, please contact Mr. M.

H. Holmes at (303) 620-1701.

Sincerely,

&dvJd fgV Frederic J. Borst Decommi ioning Program Director FJB/SWC/ Attachment cc: Regional Administrator, Region IV l l

Mr. Robert M. Quillin, Director Radiation Control Division Colorado Department of Public Health and Environment

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