RECENT DEVELOPMENTS IN BIOKINETIC MODELS AND THE CALCULATION OF INTERNAL DOSE COEFFICIENTS

In most cases the measurement of radioactivity in an environmental or biological sample will be followed by some estimation of dose and poss ibly risk, either to a population or an individual. This will normally involve the use of a dose coefficient (dose per unit intake value) ta ken from a compendium. In recent years the calculation of dose coeffic ients has seen many developments in both biokinetic modelling iind com putational capabilities. ICRP has recommended new models for the respi ratory tract and for the systemic behavior of many of the more importa nt elements. As well as this, a general age-dependent calculation meth od has been developed which involves an effectively continuous variati on of both biokinetic and dosimetric parameters, facilitating more rea listic estimation of doses to young people. These new developments wer e used in work for recent ICRP, IAEA and CEC compendia of dose coeffic ients for both members of the public (including children) and workers. This paper presents a general overview of the method of calculation o f internal doses with particular reference to the actinides. Some of t he implications for dose coefficients of the new models an: discussed. For example it is shown that compared with data in ICRP Publications 30 and 54: the new respiratory tract model generally predicts lower de position in systemic tissues per unit intake; the new biokinetic model s for actinides allow for burial of material deposited on bone surface s: age-dependent models generally feature faster turnover of material in young people. All of these factors can lead to substantially differ ent estimates of dose and examples of the new dose coefficients are gi ven to illustrate these differences. During the development of the new models for actinides, human bioassay data were used to validate the m odel. Thus, one would expect the new models to give reasonable predict ions of bioassay quantities. Some examples of the bioassay application s, e.g., excretion data for the plutonium model, are discussed briefly .