Section IV: What About the Subsistence Farmer?

Could the NAS committee's recommendation of limiting risk to individuals be compatible with allowing high doses of radiation to maximally exposed individuals, and in particular to subsistence farmers? And are the committee majority's recommendations in conformity with the recommendations of the ICRP? These questions are at the heart of the dispute between the committee majority and the lone dissenter, Professor Pigford. Appendix C of the report, where the majority specifies the method to be used for calculating exposures, also appears to make contradictory statements (see below).

Here is my understanding of the eight-step process of determining the exposure of the critical group as described in Appendix C by the NAS committee majority. My own comments are italicized in parentheses.

1. Identify the population which contains the people at risk of getting the highest doses. "For purposes of illustration, this example assumes a farming community in the Amargosa Valley." (The term "farming community" could include many occupations, not just subsistence farmers. It could be a large, inhomogeneous group, which would be incompatible with ICRP's recommendation for a critical group, or a small, homogenous group. For instance, it may consist of farmers, casino operators, and defense workers or it may have farmers only (see NAS report page 145. The farmers may or may not be subsistence farmers.)
2. Quantify the demographic and geographical characteristics of the population so as to determine what areas in the region "have the potential for farming and groundwater use." If possible, limit the area for exposure analysis by excluding some areas, such as those not likely to be farmed or where groundwater might be too deep. (On this basis, the area and groundwater in the immediate vicinity of the Yucca Mountain repository could be excluded from the calculations.)
3. Identify the intersections of those areas that might be farmed and those beneath which radioactively contaminated water would be present at some time.
4. Model the release of radionuclides from the repository and take into account that the plume of contamination passes through various areas at different times, limiting exposure in this way. Model various possible ways in which the contaminated plume of groundwater might travel (these are called "plume realizations"). People living in such areas before the plume is directly under them will be "at no risk" during these periods.
5. Calculate doses for a large variety of possible conditions and times, sampling from among the various plume realizations. (This step acknowledges, in contradiction to the one just above, that people "outside the area overlying the plume" could be exposed due to "local export of water or food.")
6. Calculate the times at which the groundwater under various exposed populations would be most contaminated.
7. Divide the results of each groundwater contamination ("plume realization") into geographical subareas in which doses are to be arithmetically averaged. The population of each subarea should be large enough "to allow computation of a meaningful average dose." Then define a "critical subgroup" consisting of all subareas with average risks within a factor of ten of the "maximum average" subarea risk. (The term "meaningful average" is not defined. This requirement could, in some cases, conflict with the ICRP recommendation that the critical group be small.)
8. Average the average doses for the critical subgroups in Step 7 for each plume realization. This final average of averages is defined by the committee majority to be the "technically appropriate representation for the critical-group risk."

"We recommend that the critical-group approach be used in the Yucca Mountain standards. (Text underlined in original.)

"The critical group has been defined by the International Commission on Radiological Protection (ICRP) as a relatively homogeneous group of people whose location and habits are such that they are representative of those individuals expected to receive the highest doses..."

"critical group for risk should be representative of those individuals in the population who, based on cautious, but reasonable, assumptions, have the highest risk resulting from repository releases. The group should be small enough to be relatively homogenous with respect to diet and other aspects of behavior that affect risks." (p. 53)

• Part V: Professor Pigford's Dissent
  
• Part VI: Not Mathematically Valid?
  
• Part VII: Implications Beyond High-Level Waste Disposal
  
• Return to beginning of article.
  

Institute for Energy and Environmental Research

Comments to Outreach Coordinator, Pat Ortmeyer: ieer@ieer.org
Takoma Park, Maryland, USA

Revised March 21, 1996