52 3.5. EXERCISE 4: COMPARISONS WITH FIELD DATA 3.5.1. Source term In order to compare model predictions with radionuclide concentration measurements in the marine environment o? Fukushima, a realistic 137Cs source term needs to be used. During the Fukushima Daiichi NPP accident in 2011, radionuclides entered into the Pacific Ocean both from deposition on the sea surface of radionuclides previously released to the atmosphere and because of direct release of contaminated water into the sea. The reconstruction of these two source terms are described in the following two sub-sections. 3.5.1.1. Atmospheric deposition Two atmospheric dispersion models were applied to simulate the dispersion of radionuclides released to the atmosphere and to evaluate the subsequent deposition to the sea surface. These models were developed by KAERI and JAEA. The output from both models were compared and the average from both taken as the best estimate of deposition. Atmospheric dispersion models The main characteristics of the applied atmospheric dispersion models are presented in Table 15 and both models are brie?y described below: ? KAERI model, LADAS (Long-range Accident Dose Assessment System): After the accident, significant amounts of radioactivity were released to the air, which was transported inland and to the near shore of the Fukushima NPP between 12 March and 30 April 2011. During the early phase of the accident, i.e. 15–31 March 2011, radionuclides were deposited on the sea surface due to aeolian fallout, mainly in the northeast direction from Fukushima. Therefore, atmospheric deposition on the sea surface could a?ect dispersion patterns of radionuclides in water and sediment. The 137Cs atmospheric fallout rate, as a function of time and space, was calculated from the long range atmospheric transport model LADAS developed by KEARI [68, 69]. The particle tracking method was used in LADAS for estimating the concentration distribution of radioactive material released into the atmosphere. The model was designed to estimate air concentrations and dry deposition, as well as wet deposition at distances of up to several thousands of kilometers from the release point in a horizontal direction. The turbulent motion of the particle is considered separately within and above the atmospheric mixing layer and particles are released in order to evaluate the transport and di?usion process of a pollutant in the atmosphere. The concentration is calculated by tracking the trajectory of each particle. Lagrangian type models can treat a rapid concentration gradient near a source point easily and do not introduce numerical dispersion. A particle is advected by the averaged wind components and dispersed by a turbulent motion in a 3-D space. The movement of the particle is represented by the sum of the movements due to the advection and the turbulence. This model was connected with 3-D meteorological forecasts from the KMA (Korea Meteorological Administration) in order to obtain the required wind fields [68, 69]. ? JAEA model, WSPEEDI-II (Worldwide Version of System for Prediction of Environmental Emergency Dose Information): To simulate atmospheric dispersion of radionuclides released from the Fukushima NPP over eastern Japan, the WSPEEDI-II was used [70]. The simulation system WSPEEDI-II calculates air concentration and