123 X-2. PACIFIC OCEAN APPLICATION Water circulation has been obtained from the JCOPE2 hydrodynamic model. Daily 3-D currents are imported to solve the advective transport of radionuclides. The model was also adapted to run with HYCOM water circulation. Nevertheless, it was concluded that, in general, model results were in better agreement with measured 137Cs concentrations in water and sediments with JCOPE2 circulation than if HYCOM is used [X-5]. Consequently, USEV model results with HYCOM circulation are not presented in this report. Concerning the dispersion of 137Cs released from the Fukushima Daiichi NPP accident, the two- step model reproduced measured 137Cs concentrations in bed sediments better than the one-step model [X-5]. Consequently, the two-step model has been used in all Fukushima exercises involving this radionuclide, except when common parameters for all models are defined for Exercise 3 and Exercise 4a as summarized in Table X-1. Some parameters are required in order to simulate 137Cs dispersion. Rates k2, k3 and k4 are taken from previous work dealing with dispersion of this radionuclide [X-6–7]. Although it is true that kinetic rates are site specific, there is no information available about them in Japanese Pacific Ocean coastal waters. Thus, representative values already used in the English Channel and Western Mediterranean Sea have been used as a first order approximation. As discussed previously [X-4; X-6–8], the exchange velocity ? can be deduced from the rates mentioned above and the radionuclide distribution coe?cient kd (see Eq. V-20). The mean value of the measured Cs distribution coe?cient is 2.1 × 103 [X-9], comparable to the IAEA’s recommended value [X-10]. Fixed kd = 2 × 103 was used to deduce ? following the procedure described in such references. The distribution of fine sediments in the seabed, described by parameter f, was reconstructed from information given in Ref. [X-11]. Fixed L = 0.05 m has been used. This parameter typically ranges from 0.035 m [X-6] to 0.10 m [X-7–8; X-12]. Also, ? = 0.1 was used for the sediment correction factor [X-7–8; X-12]. A representative value R = 10 µm was used for the mean particle size. TABLE X-1. KINETIC MODELS APPLIED BY USEV 3-D MODEL IN THE PACIFIC OCEAN RADIOLOGICAL SCENARIO Exercise Features Kinetic model 1 Own circulation and parameters none (only tracer) 2 Same circulation, own parameters 2-step 3 Same circulation and parameters 1-step 4a Same circulation and parameters 1-step 4b Own circulation and parameters 2-step