ML083430517
| ML083430517 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon, Humboldt Bay |
| Issue date: | 12/05/2008 |
| From: | Pedersen R Division of Operating Reactor Licensing |
| To: | Wang A Plant Licensing Branch IV |
| grozan,T | |
| Shared Package | |
| ML083430470 | List: |
| References | |
| TAC MD8914, TAC MD8915 | |
| Download: ML083430517 (3) | |
Text
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, UNITS 1 & 2 HUMBOLDT BAY, UNIT 3 DOCKET NOS. 50-275, 50-323, & 50-133
1.0 INTRODUCTION
By the letter dated June 9, 2008, as superceded by letter dated October 20, 2008, Pacific Gas and Electric Company (the licensee) requested approval by the U.S. Nuclear Regulatory Commission (NRC) for the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 & 2, the Humboldt Bay Power Plant, Unit 3. The Standard was approved December 1996.
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
Part 20 defines Effective Dose Equivalent (EDE or E) as the sum of the products of the dose equivalent to each tissue (T) and the weighting factors (T) applicable to each of the body organs or tissues that are irradiated (E = TTT ). 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.
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 total effective dose equivalent TEDE. Several dose limits (such as those in 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 (EDEex). If the body is not irradiated uniformly, a single dose measurement can not determine the dose to the various organs and tissues for an accurate determination of the EDEex. 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 EDEex, unless the EDEex is determined by a dosimetry method approved by the NRC.
Using DDE in place of EDEex 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 EDEex, and has encouraged the use of EDEex in place of DDE for demonstrating compliance with the TEDE requirements in Part 20.
3.0 TECHNICAL EVALUATION
The NRC staff has reviewed the technical approach for estimating EDEex 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 /HPS 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 EDEex for the whole body.
The compartment factors are listed in Table 1 of the ANSI/HPS 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 reside in more than one compartment (i.e., 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 EDEex, 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 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 DDE
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.
To ensure that the estimates of EDEex are conservative, the licensee has committed to measuring the dose to each compartment (and/or combined compartment) by locating the dosimeter, calibrated to DDE, at the highest exposed portion of that compartment. The dosimeter location for each compartment will be subject to the dosimeter placement criteria in NRC inspection procedure IP 71121.01.
The licensee has stated that in uniform radiation fields, the dosimeter used to measure whole body dose is normally worn on the chest.
4.0 CONCLUSION
S The NRC staff concludes that the licensees commitments to dose measurement using the multiple dosimetry method to determine EDEex by applying the weighting factors listed in Table 1 of ANSI/HPS N13.41-1997, as discussed above in Section 3.0 of this report, 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 Contributors: R. Pedersen, NRR Date: December 5, 2008