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Press Release
Table of Contents
Acknowledgements
Summary and Recommendations
1. Introduction
2. The concept of the critical group and the maximally exposed individual
3. Description of the subsistence farmer scenario
4. International use of the subsistence farmer approach
5. Reasonableness of the subsistence farmer scenario on occupational grounds
6. Relation of the subsistence farmer scenario to Radionuclide Soil Action Levels (RSALs) at Rocky Flats
7. Erosion of the subsistence farmer scenario
8. The Radioactive Wildlife Refuge
9. Enforcement for the eons
10. Conclusions and Recommendations
11. References
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2. The concept of the critical group and the maximally exposed individual
The concepts of the critical group and maximally exposed individual originated from discussions regarding the disposal of high-level radioactive waste. According to T.H. Pigford, who has long been involved in discussions involving radioactive waste, projects for long-term disposal of high-level radioactive waste have been planned with the following ethical goals in mind:
- Future people, of distant times, should be given the same health protection afforded to people living near nuclear facilities today.
- Present generations should be responsible for safely disposing of the radioactive waste that we have created.
- Future generations should not have to take conscious action to protect themselves from the radioactivity that we have created.
- Disposal systems should provide long-term security against weapons proliferation.6
The principal basis for radiation protection until recently has been to set limits on the maximum allowable exposures to individuals from man-made sources. For example, the overall individual dose limit for the general population from all man-made sources of radiation (other than medical) is 100 millirem per year. The limit for exposure due to emissions from specific facilities is generally in the range of 5 to 25 millirem per year.
Both the individual and population dose concepts are incorporated into current standards codified in federal regulations 40 CFR 191, which apply to all high-level waste repositories except Yucca Mountain.
The "maximally exposed individual" is a hypothetical construct, corresponding to a set of "reasonable" assumptions about human needs and activities. People who may be unusually sensitive to radiation or who have unusual habits are not used for standard setting. For example, a British inquiry omitted people who subsisted mainly on clams from its definition of the affected population because this diet was considered unusual.7
For the purposes of calculating radiation dose, a small, homogeneous group of individuals is used to define a "critical group." The concept goes back to at least 1977.8 The International Commission on Radiological Protection (ICRP) defines the critical group in the following manner:
"When an actual group cannot be defined, a hypothetical group or representative individual should be considered who, due to location and time, would receive the greatest dose. The habits and characteristics of the group should be based upon present knowledge using cautious, but reasonable assumptions. For example, the critical group could be the group of people who might live in an area near a repository and whose water would be obtained from a nearby groundwater aquifer. Because the actual doses in the entire population will constitute a distribution for which the critical group represents the extreme, this procedure is intended to ensure that no individual doses are unacceptably high." 9 (emphasis added)
Since an actual group can never be defined far into the future, it is generally necessary to define such a critical group in order to consider issues related to protection of local populations who may live in the area at that time. Since the critical group must be both small and homogenous, the concept essentially extends the idea of a maximally exposed individual, that is used for current operations, to people far into the future.
A description of the critical group is included in ICRP 26. This provides an explicit link between the critical group and the maximally exposed individual:
"It is often possible to identify population groups with characteristics causing them to be exposed at a higher level than the rest of the exposed population from a given practice. The exposure of these groups, known as critical groups, can then be used as a measure of the upper limit of the individual doses resulting from the proposed practice. When several practices may contribute significantly to the exposure of the same exposed population, either simultaneously or successively, the definition of critical groups must take account of these separate contributions."10 (emphasis added)
ICRP also recommends that critical groups be small so that they are homogenous with the upper limit to size usually being "up to a few tens of persons." They could be as small as only one person.11
"In an extreme case it may be convenient to define the critical group in terms of a single hypothetical individual, for example when dealing with conditions well in the future which cannot be characterized in detail." (emphasis added)
In this specific instance, the congruence of the critical group with a hypothetical maximally exposed individual is complete.
Institutions in countries other than the United States have also adopted the ICRP recommendations on the critical group concept. The United Kingdom's National Radiological Protection Board (NRPB) says:
"...it is appropriate to use hypothetical critical groups. For the purposes of solid waste disposal assessments, these are assumed to exist, at any given time in the future, at the place where the relevant environmental concentrations are highest, and to have habits such that their exposure is representative of the highest exposures which might reasonably be expected."12
The device of a small critical group is used to represent the maximally exposed individual for regulatory purposes. In practice, the maximally exposed individual should be in the critical group. Once the exposure scenario for the maximally exposed individual is selected, then it is possible to derive secondary standards for limiting concentrations of radionuclides in air, water, and soil. These secondary standards, if adhered to, would result in compliance with the primary dose standard.
The concept of the maximally exposed individual has existed for quite some time, although over time the terminology has changed. The roots of this concept can be traced back in part to the 1958 version of the Atomic Energy Commission's AEC Manual chapter 0524, where it was expressed in very rudimentary form, without the use of that expression.13 It was in this document that the AEC discussed the idea that limiting doses near sites from its operations would be expected to produce lower average individual doses in the general population. This document was updated and renamed in 1963.14 These documents first established radiation protection standards for populations located in uncontrolled areas outside of and around nuclear sites. To limit offsite doses, the maximum allowable concentrations of radionuclides were specified at the site boundary. This concept was also implicit in other regulations that were put into effect in the late 1960s and 1970s. Regulatory Guides 1.3 and 1.4 do not use the term maximally exposed individual, but their assumptions for calculating potential doses after a loss-of-coolant accident are designed to assess the maximum theoretical dose an individual could receive.15,16 Regulatory Guide (revision 1) 1.109 of 1977 explicitly uses the term "maximum exposed individual." In this document, dose estimates are given to assess the dose to the hypothetical "maximum exposed individual" in the absence of hard data.
Regulatory Guide 1.109 reads:
"...the NRC staff has made use of the maximum exposed individual approach."
"Maximum [exposed] individuals are characterized as 'maximum' with regard to food consumption, occupancy, and other usage of the region in the vicinity of the plant site and as such represent individuals with habits representing reasonable deviations from the average for the population in general."17
It is inherent in these definitions that these individuals' doses would be higher, possibly far higher, than those of the general population. The basic concept of this hypothetical construct clearly pre-dated these documents. For example, the Hanford environmental and evaluation staff would sum exposures from various sources "in a manner which tends to maximize the total dose."18 This is essentially calculating the exposure that the maximally exposed individual would receive. One can use documents such as this and the ones mentioned above to create a rough lineage of the model in regulatory literature.
The concept of the maximally exposed individual, which is at the heart of current radiation protection regulations for present populations, goes back to about the early 1960s and has come into general use. For example, it is used in the implementation of the Clean Air Act. A hypothetical person living at the site boundary for 24 hours a day and 365 days a year, without any building shielding factor is specified as the basis for compliance with the maximum allowable dose of 10 millirem per year. The reasoning is that if the hypothetical individual at the site boundary gets less than the maximum allowable dose, then every other person in the population would get less than that and therefore have a risk of cancer lower than that implicit in the standard.
But even a situation that seems straightforward - that of protecting offsite populations from radiation emitted by current operations - the actual problem is often more difficult than this scenario would make it appear. Implicit in such a scenario is the assumption that the location of the maximally exposed individual does not change during the year. Yet, changes of operations, accidents, sudden releases during cleanup operations, etc. could result in higher doses at other locations. There are examples when someone walking by a facility that has low normal emissions but is having an accident or an abnormal operation might receive a greater dose than a hypothetical maximally exposed individual whose location might be elsewhere based on routine operations. Hence, in order to determine who is really at risk requires a detailed knowledge not only of routine operations but also of extraordinary occurrences, possible accidents, and unanticipated events.19
If protecting people to pre-specified levels is difficult for the present generation, matters are far more complex and uncertain for future ones. The concept of the critical group, which is an extension of the concept of the maximally exposed individual, was created as a minimal, essential tool to assist in what might otherwise become an arbitrary exercise in wishful thinking.
Next: 3. Description of the subsistence farmer scenario 
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Institute for Energy and Environmental Research