ML083450035

Humbolt Bay Power Plant, Unit 3 - Approval to Use Effective Dose Equivalent Weighting Factors for External Radiation Exposure

ML083450035
Person / Time
Site:	Diablo Canyon, Humboldt Bay
Issue date:	12/22/2008
From:	Wang A Plant Licensing Branch IV
To:	Conway J Pacific Gas & Electric Co
Wang, A B, NRR/DORL/LPLIV, 415-1445
References
TAC MD8914, TAC MD8915
Download: ML083450035 (6)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555*0001 December 22, 2008 Mr. John Conway Senior Vice President - Station Generation and Chief Nuclear Officer Pacific Gas and Electric Company Diablo Canyon Power Plant P.O. Box 770000 San Francisco, CA 94177-0001

SUBJECT:

DIABLO CANYON POWER PLANT, UNITS NOS. 1 AND 2, AND HUMBOLT BAY POWER PLANT, UNIT 3 - APPROVAL TO USE EFFECTIVE DOSE EQUIVALENT WEIGHTING FACTORS FOR EXTERNAL RADIATION EXPOSURE (TAC NOS. MD8914 AND MD8915)

Dear Mr. Conway:

By the letter dated June 9,2008, as superceded by letter dated November 18,2008, Pacific Gas and Electric Company (PG&E, the licensee) requested approval by the US Nuclear Regulatory Commission (NRC) for the use of the weighting factors listed in Table 1 in the American National Standard Institute (ANSI) HPS N13.41-1997, "Criteria for Performing Multiple Dosimetry," (the Standard) for external radiation exposures when demonstrating compliance with total effective dose equivalent (TEDE), based on requirements in Title 10 of the Code of Federal Regulations (10 CFR), Part 20, for the Diablo Canyon Power Plant, Unit Nos. 1 and 2, and Humboldt Bay Power Plant, Unit 3.

This approval allows the licensee the option to control effective dose equivalent (EDE) using the weighted external exposure measurements, in those cases, where it is a more accurate predictor of the risk from occupational radiation exposure.

The NRC staff completed a technical and regulatory review of the proposed request and supporting documentation. The NRC staff concludes that the licensee's commitments to dose measurement using the multiple dosimetry method to determine EDE from external exposures (EDE ex ) by applying the weighting factors listed in Table 1 of ANSI/HPS N13.41-1997 is consistent with the ANSI/HPS standard and with the requirements of 10 CFR, Part 20, and is therefore acceptable for the purposes of demonstrating compliance with the TEDE based requirements in 10 CFR, Part 20.

J. Conway -2 The basis for the NRC staff conclusion is contained in the attached safety evaluation. If you have any questions, please contact me at 301-415-1445 or bye-mail at Alan.Wang@nrc.gov.

Sincerely,

();m WOJ1 .

Alan Wang, Project Plant Licensing

~~

Bran~tJ~ger Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-275, 50-323 and 50-133

Enclosure:

Safety Evaluation cc w/encl: Distribution via ListServ

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO APPROVAL TO USE EFFECTIVE DOSE EQUIVALENT WEIGHTING FACTORS FOR EXTERNAL RADIATION EXPOSURE PACIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON POWER PLANT, UNITS 1 & 2, AND HUMBOLDT BAY POWER PLANT, UNIT 3 DOCKET NOS. 50-275, 50-323, AND 50-133

1.0 INTRODUCTION

By the letter dated June 9, 2008 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML081700090), as superseded by letter dated November 18, 2008 (ADAMS Accession No. ML083370178), Pacific Gas and Electric Company (the licensee) requested approval by the U.S. Nuclear Regulatory Commission (NRC) for the use of the weighting factors listed in Table 1 in the American National Standard Institute (ANSI) HPS N13.41-1997, "Criteria for Performing Multiple Dosimetry," (the Standard) for external radiation exposures when demonstrating compliance with total effective dose equivalent (TEDE), based on requirements in Title 10 of the Code of Federal Regulations (10 CFR) Part 20, for the Diablo Canyon, Units 1 and 2, and the Humboldt Bay Power Plant, Unit 3. The Standard was approved in December 1996 by the NRC.

The licensee stated that accurate assessment of occupational exposure of workers from external sources of radiation in highly non-uniform radiation fields requires a method for assessing the effective dose equivalent (EDE) and that the use of the proposed weighting factors will improve the accuracy of the licensee's assessment of occupational exposure.

Therefore, the effect of granting this request would be to allow the licensee the option to control EDE using the weighted external exposure measurements in those cases where it is a more accurate predictor of the risk from occupational radiation exposure.

2.0 REGULATORY EVALUATION

The requirements for demonstrating compliance with TEDE dose limits are provided in 10 CFR 20.1201 (c). The regulations define Effective Dose Equivalent (EDE or HE) as Ii the sum of the products of the dose equivalent to the organ or tissue (HT) and the weighting factors (cuT)

=

applicable to each of the body organs or tissues that are irradiated (HE ~cuTHT)." Each tissue weighting factor is the proportion of the risk of stochastic effects resulting from the dose to that organ or tissue to the total risk of stochastic effects when the whole body is irradiated uniformly.

The weighting factors are applicable to the organs and tissues whether the dose results from radiation sources internal or external to the body.

-2 For the purposes of implementing workplace controls, and due to the difference in dosimetry methods, Part 20 breaks EDE into two components: dose resulting from radioactive sources internal to the body, and dose resulting from sources external to the body. The doses from external and internal exposures are then summed to obtain the TEDE. Several dose limits (such as those in 10 CFR 20.1201 (a)(1 )(i) and 20.1301 (a)) and other requirements in Part 20 are based on TEDE.

As of February 15, 2008, 10 CFR 20.1003 defines TEDE as the "sum of the effective dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures)." The committed effective dose equivalent is the sum of the products of the dose equivalents to each tissue, from radioactive material taken into the body, integrated over 50 years, and the weighting factor applicable to that tissue.

In most relatively uniform exposure situations, a single dosimeter, calibrated to measure Deep Dose Equivalent (DDE), worn on the whole body, provides a reasonably accurate estimate of the EDE from external exposures (EDE ex ). If the body is not irradiated uniformly, a single dose measurement cannot determine the dose to the various organs and tissues for an accurate determination of the EDE ex . To insure a conservative TEDE determination, 10 CFR 20.1201 (c) requires that when external exposure is determined by measurement with an external monitoring device, the DDE, measured for the part of the body receiving the highest exposure, must be used in place of the EDE ex , unless the EDE ex is determined by a dosimetry method approved by the NRC.

Using DDE in place of EDE ex can be overly conservative in extremely non-uniform irradiations (e.g., when only a small portion of the whole body is irradiated). As discussed in NRC Regulatory Issue Summaries 2002-06, 2003-04 and 2004-01, the NRC has approved several methods for determining EDE ex , and has encouraged the use of EDE ex in place of DDE for demonstrating compliance with the TEDE requirements in Part 20.

3.0 TECHNICAL EVALUATION

The NRC staff reviewed the technical approach for estimating EDE ex provided in ANSI/HPS N13.41-1997. This multiple dosimetry method divides the whole body into seven separate compartments. Each compartment, or combined compartment (since the ANSI IHPS N13.41 allows combining adjacent compartments), is monitored separately. The results of the dose measurement for each compartment are weighted with an associated "compartment factor."

The resulting weighted doses are then summed to determine the EDE ex for the whole body. As noted earlier, the Standard was approved by the NRC in December 1996.

The compartment factors are listed in Table 1 of the ANSIIHPS N13.41. The factor for each compartment was developed by summing the stochastic weighting factors given in Part 20 for all the organs located within that compartment. For each tissue that resides in more than one compartment (e.g., red bone marrow), the weighting factor was apportioned between the compartments based on the fraction of the total mass of the tissue residing in each, using the information in International Commission on Radiation Protection Publication 23.

Fundamental to the ANSI/HPS N13.41 multiple dosimeter method of determining EDE ex , are the assumptions that 1) the average dose to the tissues in each compartment can be reasonably measured (with one or more dosimeters), and 2) that the dose distribution across the

-3 compartment is sufficiently constant so that this average dose can be applied to each tissue in the compartment. The compartments defined in ANSI/HPS N13.41 are small enough so that under most exposure situations these assumptions are met and a single determination of DOE in each compartment is sufficient. However, this may not be the case in those unusual situations where a significant dose gradient exists across the compartment (particularly the thorax and abdomen compartments). In these cases, dosimeter placement, in each compartment becomes critical to ensuring that the EDEex is not underestimated.

The licensee stated in its application that in uniform radiation fields, the dosimeter used to measure whole body dose is normally worn on the chest. To ensure that the estimates of EDEex are conservative, the licensee stated in its application that the dose to each compartment (and/or combined compartment) would be measured by locating the dosimeter, calibrated to DOE, at the highest exposed portion of that compartment, consistent with the dosimeter placement criteria in NRC Inspection Procedure 71121.01.

4.0 CONCLUSION

S The NRC staff concludes that the licensee's commitment to dose measurement using the multiple dosimetry method to determine EDE ex by applying the weighting factors listed in Table 1 of ANSI/HPS N13.41-1997, as discussed above in Section 3.0 of this safety evaluation, is consistent with the ANSI/HPS standard and with the requirements of 10 CFR Part 20, and is therefore acceptable for the purposes of demonstrating compliance with the TEDE based requirements in 10 CFR Part 20.

Principal Contributor: R. Pedersen, NRR Date: December 22, 2008

necember 22, 2008 The basis for the NRC staff conclusion is contained in the attached safety evaluation. If you have any questions, please contact me at 301-415-1445 or bye-mail at Alan.Wang@nrc.gov.

Sincerely, IRAI Alan Wang, Project Manager Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-275, 50-323 and 50-133

Enclosure:

Safety Evaluation cc w/encl: Distribution via ListServ DISTRIBUTION:

PUBLIC RidsNrrDorlLpl4 RPedersen, NRR LPLIV Reading RidsN rrLAG Lappert RidsAcrsAcnwMailCenter RidsN rrPM DiabloCanyon RidsNrrDirsltsb RidsOgcRp RidsNrrDorlDpr RidsRgn4MailCenter ADAMS Accession Nos.: ML083450035 *Concurrence via email OFFICE NRR/LPL4/PM NRR/LPL4/LA NAME GLappert*

DATE 12/10108