NL-08-0031, Request to Implement an Alternative Source Term Response for Additional Information Regarding Atmospheric Dispersion Factors

Request to Implement an Alternative Source Term Response for Additional Information Regarding Atmospheric Dispersion Factors

ML080250254
Person / Time
Site:	Hatch
Issue date:	01/24/2008
From:	Stinson L Southern Nuclear Operating Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NL-08-0031
Download: ML080250254 (5)

Text

L M. Stinson (Mike)

Southern Nuclear Vice President Operating Company, Inc.

Fleet Operations Support 40 Inverness Center Parkway Post Office Box 1295 Birmingham. Alabama 35201 Tel 205.992.5181 Fax 205.992.0341 SOUTHERN A COMPAMY January 24, 2008 Energy to Serve }'ourWorld""

Docket Nos.: 50-321 NL-08-0031 50-366 U. S. Nuclear Regulatory Commission ATIN: Document Control Desk Washington, D. C. 20555-0001 Edwin I. Hatch Nuclear Plant Request to Implement an Alternative Source Term Response to Request for Additional Information Regarding Atmospheric Dispersion Factors Ladies and Gentlemen:

On August 29, 2006. Southern Nuclear Operating Company (SNC) submitted a request to revise the Edwin I. Hatch Nuclear Plant (HNP) licensing/design basis with a full scope implementation of an alternative source term (AST). By letters dated November 6,2006 and August 13,2007, SNC has submitted further information regarding atmospheric dispersion factors.

By letter dated January 9,2008, the NRC requested additional information concernil1g atmospheric dispersion factors. Enclosure 1 provides the SNC response to this request for additional information (RAI).

(Affirmation and signature are provided on the following page.)

U. S. Nuclear Regulatory Commission NL-08-0031 Page 2 Mr. L. M. Stinson states he is a Vice President of Southern Nuclear Operating Company, is authorized to execute this oath on behalf of Southern Nuclear Operating Company and to the best of his knowledge and belief, the facts set forth in this letter are true.

This letter contains no NRC commitments. If you have any questions, please advise.

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Enclosure:

1. Response to NRC RAI Regarding Atmospheric Dispersion Factors cc:

Southern Nuclear Operating Company Mr. J. T. Gasser, Executive Vice President Mr. D. R. Madison, Vice President - Hatch Mr. D. H. Jones, Vice President - Engineering RType: CHA02.004 U. S. Nuclear Regulatorv Commission Mr. V. M. McCree, Acting Regional Administrator Mr. R. E. Martin, NRR Project Manager - Hatch Mr. J. A. Hickey, Senior Resident Inspector - Hatch State of Georgia Mr. N. Holcomb, Commissioner - Department of Natural Resources

Edwin I Hatch Nuclear Plant Request to Implement an Alternative Source Term Response to NRC RAI Regarding Atmospheric Dispersion Factors Response to NRC RAI Regarding Atmospheric Dispersion Factors NRC Question 1 The Southern Nuclear Operating Company, Inc. (SNC) used a single set of atmospheric dispersion factors (X/Q values) as input in the control room dose assessment for all releases assumed to be ground level. SNC's letter dated November 6,2007, provided tables of values to show that the reactor building vent X/Q values were limiting. However, when the Nuclear Regulatory Commission (NRC) staff did comparison calculations using the inputs shown in the ARCON96 input files provided as Enclosure 2, the staff found that reactor building release X/Q values appeared to be more limiting than the reactor vent X/Q values. Therefore, please provide further quantitative information sufficient to enable the NRC staff to perform confirmatory calculations, if appropriate, to verify SNC's conclusion that the reactor building vent X/Q values are limiting.

SNC Response to NRC Question 1 The November 6, 2006 letter referenced in this question was submitted in response to questions that were transmitted to SNC on September 29,2006 via facsimile. NRC Question 2 in that facsimile transmittal stated "provide inputs to the ARCON96 computer runs or other methodologies (e.g., if a different methodology was used to calculate the elevated release X/Q values), along with a description of the assumptions used to make calculations for all the X/Q values." of the November 6, 2006 response provided the requested inputs to the ARCON96 computer runs that had been performed. The response to NRC Question 2 was provided on pages 2 and 3 of Enclosure 1. The response described the additional considerations relevant to the computer runs that were ultimately used to calculate the reactor building contribution to control room and Technical Support Center (TSC) dose.

In Enclosure 2 of the November 6, 2006 response, the ARCON96 input files included a case titled "CR X/Q Estimates for Reactor Building Release (Release Point - R1 )." Note, however, that the Question 2 response does not refer to this release point. As part of the engineering evaluation process used to develop the models for control room and TSC doses, an ARCON96 computer run was performed for a model of the reactor building as a point source directly across from the control room air intake on the side of the turbine building. This calculation provided comparative data. However, it was recognized that modeling of a reactor bUilding as a point source will result in an overly conservative value.

This conclusion is validated by the guidance provided in Section 3.2.4 of NRC Regulatory Guide (RG) 1.194 which indicates that a secondary containment structure (reactor bUilding) can be modeled as a diffuse area source. The 0 - 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> X/Q value for the reactor building vent release point is 1.26 E-03 as compared to a value of 5.44 E-04 when modeling the secondary containment as a diffuse area source using the guidance in Section 3.2.4 of RG 1.194. SNC chose to conservatively use the reactor building vent release point to model the dose contribution of the reactor building to control room and TSC dose.

NL-08-0031 E1 - 1 Response to NRC RAI Regarding Atmospheric Dispersion Factors NRC Question 2 NRC Regulatory Issue Summary (RIS) 2006-04, "Experience with Implementation of Alternative Source Terms," states that when running the control room atmospheric dispersion model ARCON96, two or more files of meteorological data, representative of each potential release height, should be used if X/a values are being calculated for both ground-level and elevated releases. SNC used the 60-meter wind and 60- and 1O-meter delta temperature measurements to generate control room X/a values for postulated releases from the 120-meter main stack because the wind data recovery rate at the 100-meter level during 1996 was below the 90-percent goal cited in Regulatory Guide 1.23, "Onsite Meteorological Programs." The licensee stated that use of the 60-meter data rather than the 1OO-meter data would result in conservative X/a values since the average wind speeds are lower at the 60-meter level.

The NRC staff agrees that average wind speeds would typically be lower at the 60-meter level, but notes that when using ARCON96 the highest X/a values are not usually associated with the lightest wind speeds, but are a function of the frequency of occurrence of moderate speed winds. Therefore, please provide further quantitative information sufficient to enable the NRC staff to perform confirmatory calculations to verify SNC's conclusion that use of the 60-meter data resulted in X/a values that are similar to or more limiting than those that would be generated using the 1OO-meter data.

SNC Response to NRC Question 2 SNC provided the 120-meter main stack release X/a values to NRC using the 60 meter level wind data instead of the 1OO-meter wind data because among the three years (1996-1998) of meteorological data used in the ARCON96 modeling runs, the 100-meter level 1996 data recovery rate was slightly lower than the 90%

goal cited in Regulatory Guide 1.23. The composite 3-year data recovery rate was about 93%.

Since the 3-year composite 1OO-meter level meteorological data recovery rate is higher than 90%, the 1OO-meter data set is considered representative of the site dispersion conditions for comparison purposes. To demonstrate that the maximum reported X/a value is still bounding, the 120-meter main stack release X/a values were re-generated using the 3-year composite 100-meter level wind data set. The ARCON96 modeling results show that for the main stack release, the 1OO-meter X/a values for all the averaging periods are lower than the reported 60-meter X/a values. Specifically, the 60-meter 0-2 hours main stack X/a value is 3.76E-06, reference table 4 of the August 29, 2006 AST submittal, while the corresponding 100-meter main stack X/a value is 1.09E

06. Therefore, the xla values produced by the 60-meter wind data are more limiting as stated in the response to NRC question 5 in the referenced August 13, 2007 letter.

NL-08-0031 E1 - 2