Monitoring methods and dose assessment for internal exposures involving mixed fission and activation products containing actinides.

Internal dose assessment for intakes of radionuclide mixtures is a difficult task. When the radionuclide mixture contains both the easy to detect gamma emitters, e.g., 60 Co and 95 Zr, and difficult to detect alpha emitters such as 239 Pu and 241 Am, a single monitoring method, such as in-vivo counting, is inadequate for detection and dose assessment. Recent experience with task related monitoring for such radionuclide mixtures at Ontario Power Generation CANDU nuclear power plants has offered an opportunity to review this topic and suggest a strategy for monitoring that involves a combination of in-vivo and in-vitro methods. Using the radionuclide composition data in a mixture from an actual case as an example, this paper describes a monitoring strategy for mixed fission and activation products, including the advantages and pitfalls of reliance on surrogate radionuclides for signaling the presence of actinides in the mixture. The described monitoring strategy is consistent with the recommendations of ICRP Publication 78, which advocates a combination of techniques so as to make the best possible evaluation of an unusual situation, for example, a programme of both body activity and excreta measurements. The use of experience and professional judgement for interpreting the combined in-vivo and in-vitro data for interim and ultimate intake and dose assessment is discussed and emphasized.