1 SUMMARY This report describes the work undertaken by the Modelling of Marine Dispersion and Transfer of Radionuclides Accidentally Released from Land-based Facilities Working Group 10 (WG10) of the IAEA MODARIA Programme. Two marine scenarios have been studied: (i) the dispersion of Chornobyl accident fallout in the Baltic Sea [1]; and (ii) the dispersion of radionuclides released from the Fukushima Daiichi nuclear power plant (NPP) accident into the Pacific Ocean [2]. For the modelling of radionuclides within the Baltic Sea, four models were used to compare the distribution of 137Cs through time and space after five years of simulation. Significant conceptual, numerical and parameterization di?erences between models in addition to the complexity of the Baltic Sea hydrodynamics contribute to differences in model outputs. However, generally there is good agreement between the models and measured data. For the modelling of radionuclides within the Pacific Ocean, a sequential chain of dispersion exercises was carried out using selected models in which a progressive harmonization of models (understood as using the same forcing and parameterizations) was performed. This allowed the causes of variability between model results to be analysed. Model harmonization was required to achieve a relatively close agreement between model outputs for the Pacific Ocean radiological scenario. The coastal waters of Fukushima represent a more dynamic system, therefore, the output of models is found to be very dependent on the ocean model used. Some general conclusions close this report with a comparison of model performances when applied to the Baltic Sea and Pacific Ocean radiological scenarios. The di?culties of developing operative modelling systems for supporting decision making in cases of emergencies in highly dynamic environments is highlighted. It is concluded that a multi-model approach may be valuable when environmental processes are complex. Through this approach, the predictions that obtain the greatest degree of consensus among modellers are made evident and the aspects that are subject to disagreement, and which ought therefore to be handled carefully, also become clear. Thus, the report is divided into four parts, beginning with an introductory section. Section 21 describes the Baltic Sea radiological scenario. Section 3 summarizes the Pacific Ocean radiological scenario and Section 4 provides the general conclusions. Models which have been applied are summarized in the corresponding Annexes I–XI, where full details on each model may be consulted in the references given. 1 The core of the work presented in Sections 2 and 3 of this report is based on research papers published during the course of the project but has been expanded to include new material and more detailed information. Where applicable, the content from the research papers has been reproduced with permission from Elsevier Ltd. The two relevant papers are A comparison of marine radionuclide dispersion models for the Baltic Sea in the frame of IAEA MODARIA program, Journal of Environmental Radioactivity 139 (2015) 66–77 and A new comparison of marine dispersion model performances for Fukushima Daiichi releases in the frame of IAEA MODARIA program, Journal of Environmental Radioactivity 150 (2015) 247–269.