NRC basis for Rule Making (Linear Relationship)

[sandy_perle@email.fpl.com]

     We have had a lot of discussion regarding BRC, ALARA, regulatory 
     control and public influence. Below is the section from the 10CFR20 
     Final Rule preamble which addresses the scientific basis for the rule 
     and the Commission's interpretation of the scientific evidence. The 
     points made are interesting and detail why the limits are as they are 
     and why the linear relationship, without a threshold value was 
     adopted.
     
     In this same document (not provided in this post) the NRC also 
     addressed the 0.001 rem proposed BRC. They stated that the nuclear 
     power industry was in favor of a value, not the 0.001 value, but a 
     value to at least open the door to a concept being accepted, with a 
     reasonable limit. Many who responded during the public comment period 
     stated that they would find 0.010 rem acceptable. 
     
     Sandy Perle
     Supervisor Health Physics
     Florida Power and Light Company
     Nuclear Division
     
     (407) 694-4219 Office
     (407) 694-3706 Fax
     
     sandy_perle@email.fpl.com
     
     ----------------------------------------------------------------------
     
     B. Fundamental Radiation Protection Principles
     
        The radiation protection standards in this final rule are based 
     upon the assumptions that--
     
        (1) Within the range of exposure conditions usually encountered in 
     radiation work, there is a linear relationship, without threshold, 
     between dose and probability of stochastic health effects (such as 
     latent cancer and genetic effects) occurring;
     
        (2) The severity of each type of stochastic health effect is 
     independent of dose; and
     
        (3) Nonstochastic (nonrandom) radiation-induced health effects can 
     be prevented by limiting exposures so that doses are below the 
     thresholds for their induction.
     
        The first assumption, the linear nonthreshold dose-effect 
     relationship, implies that the potential health risk is proportional 
     to the dose received and that there is an incremental health risk 
     associated with even very small doses, even radiation doses much 
     smaller than doses received from naturally occurring radiation 
     sources.  These health risks, such as cancer, are termed stochastic 
     because they are statistical in nature; i.e., for a given level of 
     dose, not every person exposed would exhibit the effect.  The second 
     assumption means that when a stochastic effect is induced, the 
     severity of the effect is not related to the radiation dose received.  
     The third assumption implies that there are effects, termed 
     nonstochastic effects, for which there is an apparent threshold; i.e., 
     a dose level below which the effect is unlikely to occur.  An example 
     of a nonstochastic effect is the formation of radiation-induced 
     cataracts of the eyes.
     
        The above assumptions are necessary because it is generally 
     impossible to determine whether or not there are any increases in the 
     incidence of disease at very low doses and low dose rates, 
     particularly in the range of doses to members of the general public 
     resulting from NRC-licensed activities.  It is firmly established, 
     both from animal studies and human epidemiological studies (such as 
     those of the radium dial painters, radiologists, and the atomic bomb 
     survivors) that there is an increased incidence of certain cancers 
     associated with radiation exposure at high doses and high dose rates.  
     However, whether these effects occur at very low doses and, if they 
     occur, whether their occurrence is linearly proportional to dose are 
     not firmly established.  This creates considerable uncertainty in the 
     magnitude of the risk at low doses and low dose rates.  There is no 
     clear human evidence of radiation-induced genetic damage to the 
     children of irradiated parents.  Such effects are inferred from 
     studies of mice and nonmammalian species (e.g., fruit flies).
     
        In the absence of convincing evidence that there is a dose 
     threshold or that low levels of radiation are beneficial, the 
     Commission believes that the assumptions regarding a linear 
     nonthreshold dose-effect model for cancers and genetic effects and the 
     existence of thresholds only for certain nonstochastic effects remain 
     appropriate for formulating radiation protection standards and 
     planning radiation protection programs.