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3.4. EXERCISE 3: HOMOGENIZATION OF WATER–SEDIMENT INTERACTIONS
3.4.1. Introduction
This exercise is designed as in the previous cases. A constant release of 1.0 × 106 Bq/s of a long
lived radionuclide (radioactive decay can be neglected) is used. The release starts on 26 March
2011 and the time frame of calculations extends until 30 May 2011. However, exactly the same
bathymetric file was used for all models. The computational grid is presented in Figure 25. In
addition, the same di?usion coe?cients have been used. Constant and uniform reasonable
values for diffusion coefficients have been defined. The purpose of using constant and uniform
values is to remove additional variability between models which would be introduced if, for
example, a Smagorinsky’s scheme were to be used to compute the horizontal di?usion
coe?cient and/or any turbulence model were to be applied to calculate the vertical di?usivity.
The fixed values for the horizontal and vertical di?usion coe?cients, Kh and Kv, are:
?? = 10 ?? ? ? (5)
?? = 1.0 × 10???? ?? (6)
Exercise 3 consists of three parts, which are briefly summarized below and are described in
more detail in the following sections.
(1) Tracer: This considers the performance of models using a perfectly conservative
radionuclide, as was done for Exercises 1 and 2.
(2) 137Cs and no SPM in the water column: It is equivalent to the 137Cs case of Exercise 2. As
previously, in order to simplify the problem, it was considered that bed sediments are
uniform over the whole model domain and that they are composed entirely of fine
material (clays) with mean size 10 µm. A uniform porosity of 0.6 was assumed and,
finally, the thickness of the sediment which interacts with water was set to 10 cm.
In addition, the same parameters were used to simulate uptake/release interactions with
bed sediments. Models may use a distribution coe?cient or, alternatively, kinetic transfer
coe?cients. Values for the two options have been defined and the equilibrium distribution
coe?cient is 2.0 m3/kg. This is a mean value taken from Ref. [64] for open ocean water
and in agreement with measurements taken o? Fukushima [52]. The rate describing
release from bed sediments is k2 = 1.16 × 10-5 s-1. The kinetic rate describing uptake (k1)
is derived from k2 and the distribution coe?cient [65].
(3) 137Cs with constant and uniform SPM in the water column: SPM concentration has been
set as 5 mg/l and resuspension of particles from bed sediments back to the water column
is not included in the calculations. Other parameters are required if SPM is included in
the calculations. They are as follows:
? Particle diameter: 1 µm;
? Particle density: 2600 kg/m3;
? Settling velocity: derived from Stoke’s law;
? Critical deposition stress: 0.5 N/m2.
End points of calculations are the same as defined previously, i.e. time series of radionuclide
concentrations at points P1, P2 and P3. Radionuclide concentration for surface water is
