BIOMOVS II: An international test of the performance of environmental transfer models

The Biospheric Model Validation Study-Phase II (BIOMOVS II) was an international cooperative program that tested the accuracy of predictions of environmental assessment models. Model evaluation was based on calculations made by individual participants for 10 test scenarios that addressed both short- and long-term releases of radioactivity from facilities such as power reactors, solid waste disposal repositories and uranium mill tailings. Model predictions were compared with each other and, where possible, with independent field observations, and reasons were sought for any differences that arose. Qualitative topics were also considered, including development of systematic methodologies for radiological assessments. This paper addresses conclusions arising from the study as a whole. Confidence intervals on predictions and differences between predictions and observations were often less than a factor of 10, although there was much variability among models and scenarios. Model performance depended critically, not only on the formulation and parameter values of the model itself, but also on the experience and assumptions made by the user. The study demonstrated the need to better explain and justify all aspects of model structure and application and to assess all sources of uncertainty. A key recommendation was that assessments should not be undertaken in isolation by one individual using one model.