45 and USEV models at P1, which are more similar. In the case of P2, results from these three models are all similar. The NTUA model results do not seem consistent, since the model calculates very low concentrations in the bottom water and, simultaneously, concentrations in sediment are much higher than those of the other models (out of the scale range). In the case of point P3, the new values defined for the di?usion coe?cients lead to a very weak signal. 137Cs does not seem to reach either the bottom water or the sediment. Again, the NTUA model predicts an instantaneous arrival of the radionuclide signal and very high concentrations in sediments at the initial time, which even reduce in time. These results again seem to be inconsistent. 3.4.4. 137Cs dispersion and constant and uniform SPM distribution in the water column As mentioned previously, this experiment was designed with a constant and uniform suspended matter concentration in the water column equal to 5 mg/l. Results for the three points are presented in Figures 30–32. Essentially, the results are the same as in the previous exercise without SPM. This is not surprising given the relatively low a?nity of 137Cs to be fixed to solid particles. This a?nity is quantified by the partition coe?cient, which is defined as [65, 66]: ?? = ? ? ? ?? ? ??? (7) where SPM is the suspended matter concentration and kd the corresponding distribution coe?cient of the radionuclide. This coe?cient gives the fraction of radionuclides remaining dissolved, under equilibrium conditions, for a given kd value and suspended matter concentration. For SPM = 5 mg/L and kd = 2 × 103 L/kg, which are the values fixed for the exercise, a result of PC = 0.99 is obtained, indicating that most of 137Cs remains in solution (99%), and is not significantly adsorbed on suspended particles. Hence, the contamination of bed sediments caused by deposition processes is also negligible. Contamination of bed sediments is mainly produced by direct adsorption of dissolved radionuclides present in the bottom water. The water–sediment interface may be considered as a high suspended matter environment [67]. Thus, the corresponding PC value would be significantly lower here and a significant fraction of 137Cs would be adsorbed on bed sediments. Maps showing the computed distribution of 137Cs in surface water and sediment are presented in Figures 33 and 34, respectively, for the JAEA and KAERI models as examples. At a general level, it can be observed that the behaviour of the radionuclide patch is very similar, even in the case of sediment. For surface water, there is a remarkable agreement between both models. Essentially the same radionuclide patches are produced for surface water by these models.