NUMERICAL MODELLING OF RADIONUCLIDE MIGRATION IN A NSDF USING PORFLOW
In any Near Surface Disposal Facility (NSDF) radioactive wastes are immobilized in matrices such as cement, glass matrices and are disposed in various engineering modules depending upon activity level of waste form. As long as these engineered barriers are intact, the release of the radioactive waste from the disposal facility does not occur. However, releases to environment due to degradation of barriers caused by certain low probability external / internal environmental factors are postulated by designers. It provides inputs for the radiological impact assessment of a NSDF for obtaining regulatory clearances. In the wake of the March 2011 Fukushima NPP accidents, such studies have now assumed greater importance for radiological impact assessment at a site. This paper presents one such study on the migration of radionuclides within a typical NSDF site due to external environmental factors.
A typical low and intermediate level radioactive waste disposal site is considered for investigation. The facility consists of top cover and vault system along with various soil and concrete layers. A 2-D model of this facility is developed using CFD software PORFLOW. The geometrical details of the facility, radionuclide inventory, properties of the soil layers etc. are given as input to the code. Appropriate boundary conditions are applied with the objective of obtaining the molar flux of radioisotopes through the various layers due to water ingress from top soil layer and infiltration into the concrete vault. Simulations are carried out for a period of 104 yrs. Most of the radionuclides such as 55Fe, 58Co, 60Co and 144Ce are found to decay within the vault itself. The influence of soil layers on water transport and radionuclide migration are brought out in this paper.