MODEL FOR ESTIMATING Pii TRANSPORT AND DOSE 505 



10,000 



20,000 



DAYS 



Fig. 8 Predicted cumulative doses due to ^ Pu in different organs of Standard Man 



concentrations (MFC's) of radionuclides in air and water. The principal steps involved are 

 (1) identification of the critical exposure pathway, (2) identification of the critical organ 

 or organs, (3) selection of maximum-permissible-dose criteria, (4) calculation of the 

 corresponding MFC of plutonium in soil (MPC)s, and (5) comparison of the (MPC)s with 

 estimated inventories of plutonium in the surface soils of contaminated areas at NTS. 



Critical Pathway 



The estimated plutonium ingestion rate for a hypothetical Standard Man living in a 

 contaminated area at NTS is about 100 times the estimated inhalation rate, but, owing to 

 the very small fraction of plutonium transferred from the GIT to blood (3 x 10~^), the 

 GIT is the only organ that receives a significant dose from ingested plutonium. The 

 preferred dose-estimation model, based on ICRP recommendations (the Task Group 

 model. Fig. 4), shows that inhalation accounts for 100% of the plutonium deposited in 

 the lungs and thoracic lymph nodes, and, for an ingestion/inhalation ratio of 100, 

 inhalation accounts for about 95% of the plutonium in bone, liver, and kidney after 50 yr 

 of chronic exposure (Table 6). Clearly, inhalation is the critical pathway. 



Critical Organ 



According to th*^ Task Group model (Fig. 4), thoracic lymph nodes receive the highest 

 dose (Figs. 8 and 9), but the critical organs recognized by ICRP (International 



