Global environmental transport models for tritium

In this paper we discuss some of the obstacles to the construction of credible models of tritium transport for use in dose assessments. We illustrate these difficulties by comparing model predictions of environmental tritium levels with measurements. Environmental monitoring of tritium has shown that specific activities in precipitation over land are typically higher by a factor of three to four than those in precipitation over the oceans. Experience with modeling CO2 turnover in the oceans has led to the conclusion that two-box reservoir models of the ocean often give unsatisfactory representations of transient solutions. Failure to consider these factors in global models can lead to distorted estimates of collective dose and create difficulties in validation of the model against real data. We illustrate these problems with a seven-box model recommended by the National Council on Radiation Protection and Measurements (NCRP), in which we forced the atmospheric compartment to reproduce an exogenous function based on historic observations of HTO in precipitation at 500N