## Pathway of Radionuclides and Transport Processes in the Atmosphere

Many radionuclides can be created from a nuclear weapons test, by the processes through nuclear fission, nuclear fusion, and neutron activation. The neutrons produced in fission and fusion can induce nuclear reactions that produce radioactive isotopes. One example of this neutron activation process is the reaction with atmospheric nitrogen producing 14C (carbon-14) with a radioactive halflife, t1/2 = 5730 years. The physical and chemical form of radionuclides may vary depending on the conditions of release and transport in addition to the element's properties. They can be gases, aerosols, and particulate material. The fate of radionuclides that are emitted into the atmosphere is determined by various physical processes of transport, removal via dry deposition or wet deposition, and turbulent mixing that govern the atmospheric flow and diffusion. Radionuclides that are deposited on the land surface can be resuspended into the atmosphere through the processes of resuspension that depend on the surface conditions and atmospheric wind over lands and on seaspray if the radionuclides are deposited in the sea.

Atmospheric transport and deposition models were generally used to study the transport processes that control the distribution of radionuclides in the atmosphere. These models were constructed to satisfy the condition of mass conservation and boundary and initial conditions and are governed by the following basic equation of atmospheric transport-diffusion of mean concentration [C = C(x, y, z, t)] for a specific radionuclide, i, in the vector form of three-dimension (x, y, z):

where V(x, y, z, t) is a vector of the mean wind velocity, K(Kx, K., Kz) is a diagonal matrix of the turbulent eddy diffusivity, \i is the decay coefficient of the ith radionuclide, D(x, y, z, t) is the deposition due to dry and wet removal, and S(x, y, z, t) is the source term including resuspension. Under the assumptions of homogeneous turbulence over a flat terrain in a large diffusion time, the above equation can be simplified to have a Gaussian solution, which serves as a basis for the Gaussian plume model. This Gaussian plume model requires only horizontal wind speed and direction at the release location along with the estimates of atmospheric stability and source term and can be quickly performed to have results of distributions of radioactivity in a gross view. Generally, having a more realistic view, the above equation is numerically solved by difference equation that can take into account the various effects of terrain and spatially varying turbulence and meteorology, including varied wind, temperature, and precipitation. The model obtained is then used for evaluating the consequences and predicting the distributions of an atmospheric release of radioactivity. But the model requires validation before it can be used. The common approach for model validation is to use the natural radionuclides, as described below.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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