23 resulted from the Fukushima Daiichi NPP accident is rapidly diluted within the Kuroshio system over a timescale of a few months [49, 50]. Over the subsequent decades, radionuclides derived from the Fukushima Daiichi NPP accident will spread across the North Pacific basin. These model estimates have found that a component of 137Cs radionuclides will be injected into the interior ocean via subduction, before eventually returning to the surface by coastal upwelling along the west coast of North America. Other modelling results have found the residence time of 137Cs in the continental shelf to be 43 ± 16 days [51]. The relevance of atmospheric deposition was studied, finding that relatively high 137Cs concentrations detected in surface water north of 40°N and one month after the accident are due to atmospheric deposition [52]. More recently, a number of model sensitivity analyses have been made and have found that a tuning of the wind drag coe?cient was required for a better reproduction of 137Cs measurements [53]. However, for modelling purposes, a direct release source term of 27 PBq was used [54], which is considered a conservative high estimate [51]. In addition, some other modelling studies have had the objective of determining radionuclide releases from the Fukushima Daiichi NPP accident into the ocean, using inverse modelling [41, 45, 55]. All modelling studies mentioned previously, which is not a comprehensive list, present the common point that 137Cs is treated as a tracer and is a perfectly conservative radionuclide which does not interact with sediments. The first models including 137Cs contamination of bed sediments were described in Refs. [56, 57]. In the first case, a local study was carried out covering only the coastal region of Japan [56]. A larger domain was considered in the second study [57]. In both cases, calculated and measured 137Cs concentrations in bed sediments were compared. Furthermore, water–sediment interactions were described in a dynamic way in both studies. Adsorption by bottom sediments have also been studied [58]. All of these papers agree that a significant adsorption occurs in the first months after the accident, with most of the radionuclides staying on the seabed once they have been adsorbed. More recently, a box model (POSEIDON-R) has been used to perform a radiological assessment of the accident over the period of 2011–2040 [59]. This model includes not only adsorption to sediments, but also the transfer of radionuclides through the marine food web, using a dynamic food-chain model, and subsequent doses to humans. Some exercises comparing model performances when applied to simulate releases from the Fukushima Daiichi NPP accident have been carried out [60], with most of the discrepancies between the five participating models due to the di?erent calculated current fields in the coastal waters of Japan, o? Fukushima, which lead to di?erent radionuclide distributions. Di?erences in current fields are caused by the di?erent models and model settings used by the research groups. However, a systematic assessment aimed at investigating the reasons for di?erences was not carried out. The Science Council of Japan carried out a similar intercomparison study, with eleven models involved [61]. Significant di?erences between models were found and the models used were di?erent in concept (Eulerian versus Lagrangian), with di?erent settings and even di?erent source terms. Thus, it was concluded that a simple comparison of model results is not straightforward and that detailed systematic comparison studies, such as ones that use the same radionuclide forcing with di?erent models and/or the same model with di?erent forcing scenarios, are required. As described in the following sections, the objective of the work done in MODARIA consists of making such a systematic study [62].