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SECY-02-0214

December 4, 2002

• PURPOSE
• BACKGROUND
• DISCUSSION
• RECOMMENDATION

PURPOSE:

The Commission, in Staff Requirements Memorandum SRM-SECY-01-0127, directed the staff to "...develop guidance that specifies when it is appropriate to use effective dose equivalent rather than deep-dose equivalent for assessing the dose from external sources of radioactivity."

The purpose of this paper is to obtain the Commission's approval to issue a proposed Regulatory Issue Summary (RIS) on the appropriate use of effective dose equivalent for assessing dose from external sources of radiation.

BACKGROUND:

The use of Deep Dose Equivalent (DDE), as specified in Part 20, has provided a satisfactory method for licensees to show compliance with regulatory requirements in most ordinary exposure situations involving personnel monitoring. However, difficulties sometimes arise because the stochastic risk from radiation exposure is correlated with the effective dose equivalent, but not with the DDE. The DDE will provide a reasonable indication of risk only when its numerical value is close to that of the effective dose equivalent under the specified exposure conditions. This is the case when the whole body is exposed in a uniform or nearly uniform radiation field. It is not the case, however, when the radiation field is significantly non-uniform, or when parts of the body are shielded, or when the radiation is incident from an unusual angle, such as from underfoot or overhead. In such situations, the numerical value of the DDE is significantly different, and usually much higher, than that of the effective dose equivalent under the given exposure conditions.

An extreme example of the differences between the DDE and the effective dose equivalent is the case of localized contamination on the skin, such as that from a hot particle, or exposure to a narrow external radiation field. In such cases, the DDE at the exposure location may be very high, but the corresponding effective dose equivalent, and hence the stochastic risk, is low. Another, less extreme, example is the exposure of medical personnel wearing protective aprons during radiology procedures. Measuring DDE at the point of highest exposure may in this case overestimate the effective dose equivalent, and hence the radiation risk, by up to an order of magnitude or more outside the apron.

In an attempt to correct this situation, several methods have been developed that use the DDE as measured by one or more dosimeters, together with a suitable formula, to obtain a dose value that is numerically much closer to the effective dose equivalent. Underlying all of these methods is the idea that, regardless of the method used to assess and assign doses, the ultimate objective of any dose assessment situation is to obtain the most accurate estimate of the effective dose equivalent without underestimating that value. This is a necessary condition for any risk-based practice.

The DDE was defined by the International Commission on Radiation Units and Measurements (ICRU) to serve as an operational quantity and a surrogate to be used to provide a conservative estimate, based on measurements, of the effective dose equivalent, which cannot be measured directly. (Note: the ICRU calls the DDE the personal dose equivalent, but the definitions are identical). Therefore, in situations that do not involve personnel monitoring, as for example when the dose is calculated, the use of DDE to assess doses from external radiation fields is not justified under any circumstances. In such cases the effective dose equivalent is calculated directly.

DISCUSSION

The attached RIS is written for the audience of radiation safety professionals, and thus uses the terms and acronyms specific to complying with the requirements of 10 CFR Part 20. The guidance encourages licensees to use the effective dose equivalent in all situations that do not involve dose measurements using personnel dosimetry. Such situations include environmental assessments, dose calculations for effluents, dose calculations in connection with proposed licensing actions, calculations to show compliance with non-occupational dose limits and, in general, any dose assessments based on calculations. Such calculations of the effective dose equivalent are in fact now used in many regulated activities, both by the NRC and by licensees, because the dose conversion factor tables that are used in these calculations, or that are incorporated into the computer codes used to perform these calculations, are in fact effective dose equivalent factors, and not DDE factors.

The RIS also specifies that licensees must still use the DDE, measured at the location of highest exposure on the body, whenever the doses are being assessed on the basis of the readings of personnel dosimetry. Exceptions would be considered and approved on a case-by-case basis. These exceptions include any method that is capable of providing more accurate estimates of the effective dose equivalent. One such exception has already been approved for use by medical personnel wearing protective aprons during radiology procedures. Another method was recently approved for use at the Entergy reactor sites.

RECOMMENDATION

The staff recommends that the proposed Regulatory Issue Summary (RIS) be issued to all NRC licensees.

Attachment: Draft Regulatory Issues Summary

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