This publication is part of the outcome of the IAEA's BIOMASS research coordination programme and it describes the work carried out during a four year period from 1996 to 2000 by the BIOMASS Dose Reconstruction Working Group. The objective of this work was to improve models for radioiodine environmental transport through model testing and intercomparison. The Hanford scenario modelling exercise described in this report provided an opportunity for model testing using measurement data from an acute release of I-131 to the environment from the Hanford plant in September 1963. This dose reconstruction exercise allowed model testing of atmospheric dispersion, environmental transport models and dose calculations. Six participants took part in this exercise and their experience, approaches and models are described. The information and the scenario itself are a valuable resource for other modellers to develop their methodologies further. This publication is primarily intended for use by experts in environmental modelling worldwide, operators of nuclear fuel cycle facilities, and regulatory bodies of IAEA Member States.