Home > Research Activities > Development of Models > Regional radionuclide transport model

 

  Many of the nuclear power plants and reprocessing plants around the world are placed in the coastal locations because they need large amounts of water. The authorized routine releases (e.g. past releases from Sellafield) and accidental releases (recent Fukushima-daichi accident) both have potential to contaminate marine environment. The dispersion of radionuclides in the sea is governed by complex interrelated processes: transport by currents, interaction with suspended and bottom sediments and uptake by biota. The tides stir radionuclide plume and resuspend contaminated sediments. To assess the pathways of radioactivity in the coastal seas the accurate description of abovementioned processes in linked chain of hydrodynamics, sediment transport and radioactivity transport models should be used. Key processes of radionuclide transport involved in THREETOX system of IMMSP, Ukraine are shown below.

 

  The modeling system under development is described along with short-term regional-scale simulation of the radionuclide dispersion in coastal seas. The system includes 3D hydrodynamic and sediment transport models coupled with Eulerian and Lagrangian radionuclide transport models.

 

  The model SELFE, originally proposed by Zhang and Baptista (2008) is adopted for the circulation in coastal sea regions of the Yellow Sea and in the vicinity of Fukushima Daiichi NPP. This 3D hydrodynamic model uses unstructured triangular grid in the horizontal and hybrid terrain-following coordinates S-Z coordinates in the vertical. An efficient semi-implicit time stepping without external-internal mode splitting is used. The model can deal with, if necessary, the wetting and drying in a natural way.

 

  Application to the Yellow Sea is briefly described. The Yellow Sea is a shallow shelf sea where seasonal circulation is driven by monsoon and affected by riverine inflow. The tidal currents dominate causing large variations of the sea level. The typhoons results in surges. These peculiarities of the sea regime demand a  fine resolution modeling of the sea hydrodynamics to assess routine and potential accident releases from existed and planned power plants and other nuclear installations. Here we apply modeling system to simulate dispersion of Cs 137 in the Yellow Sea where it was released from Younggwang NPP. The Yellow Sea was covered by triangular grids with resolution around 50 m in vicinity of NPP. The circulation was forced by wind in winter 2007 and tides. The open boundary conditions were provided by ROMS of SNU, Korea with resolution of 3km. Suspended sediment and radionuclide models based on Lagrangian technique were used in this application

 

  The Lagrangian sediment transport model simulates transport of non-cohesive, cohesive sediments and mixture of fractions of different size of cohesive/non-cohesive sediments in 2D and 3D modes along with deposition and resuspension.

 

  The model of radionuclide transport includes the advection-diffusion equations for dissolved radioactivity for adsorbed by suspended sediment in the water column and the equations for concentration of the dissolved and adsorbed radioactivity in the bottom deposits. The exchanges between different phases are described by diffusion, sorption and sedimentation-resuspension processes. Basically we adopted particle tracking approach by Perianez and Elliott (2002).

 

  The scenario of hypothetical release from outlet of Hanbit NPP 3-7 December of 1× 10 15 Bq of 137Cs was used.The results of simulation are presented on the right where positions of particles at two week after beginning of release are shown. The figure shows that wind and tidal driven residual current transports radioactivity to the south. The particles follow isobaths and plume of radionuclide in solute does not coincide with bottom area of contamination. The estimation of budget of radioactivity gives that at after two weeks 2%, 6% and 92% of activity is concentrated in the suspended sediments, bottom sediments and in water, respectively.

 

 

  Similar application was made to Sanmen NPP, China using an Eulerian model. Preliminary results of dissolved, particulated and bottom-deposited Cs137 distributions are shown.

 

 

  Trough the cooperation with researchers from IMMSP, Ukraine, the Lagrangian modelling approach was also used to simulate the transport of Cs 137 directly discharged into the coastal waters from the Fukushima Daiichi NPP.  The circulation was calculated using MOM3 operating in KIOST. The concentration of suspended sediment was assumed for simplicity.

 

Computed distribution of Cs 137 released from Fukushima Daiichi NPP.