O.P.SINGH and V.PADMARAJ

Transoft International (P) Ltd.,

Analysis of transport of radioactive contaminates in a layered aquifer system still poses significant challenges, although considerable progress has been made in the literature towards the understanding of this problem., Fluidyn - POLLUSOL is a quasi three dimensional solute transport computer model for predicting the fate of conservative as well as non conservative contaminants in subsurface.

In this paper Tritium (HTO) is selected to test the validity of the model, for Tritium has negligible sorption, making our simulation simpler and more efficient. When it contaminates aquatic ecosystem, although discharged within tolerable limits, it has a complex pattern of movement and distribution and long residence time. Its capability of getting incorporated into biological compounds and transferred along food chain make the study important.

The model results show that non-conservative contaminant like Tritium can be efficiently dealt (with the help of model - Fluidyn - POLLUSOL) in complex natural subsurface hydro - geographical conditions.

Nuclear fuel cycle includes processes like mining,milling,fuel fabrication, fuel fabrication, fuel consumption in reactors, fuel reprocessing, waste solidification, and burrring the solidified waste or unprocessed spent fuel in deep geological repositories. Each of these steps contributes directly or indirectly to increase level of radioactive contamination of the earths environment. In developed countries, four main hydrogeologic alternatives are being investigated for suitability for repository development.

1. Deep salt beds

2. Thick unsaturated zone in arid regions

3. Deep crystalline igneous rock

4. Deep shale formations

But these options are still to be evaluated for their capability to isolate the radioactive waste from the biosphere for an acceptable period of time. The objectives of terminal storage of these wastes can be achieved by dumping the waste in Antarctic ocean floor at location where natural burial beneath migrating continental plates will occur or rocket transport beyond the earth's gravitational field. But these methods are often very expensive and impractical. The standard practice of managing reactor waste is to put them to shallow burial sites, these mounting wastes can be seen as a potential source of contamination of ground water.

Groundwater contamination is difficult to deal with because of heterogeneity of subsurface system, zones of degradation are difficult to detect, once subsurface contamination has been detected ,it requires long periods of ground water flow to be flushed and sometimes contamination of aquifer often results in aquifer or part of aquifer being damaged beyond repair.

FLUIDYN - POLLUSOL is a software which predict the fate of the contamination in the subsurface both unsaturated and saturated zones. This user friendly CFD package is capable of handling irregular domain with different combination in the subsurface flow and conservative as well as in conservative pollutants.

In this paper only few hypothetical simple test cases are dealt. Tritium is selected ti illustrate model capabilities.

FLUIDYN - POLLUSOIL is designed to simulate quasi 3-D flow and contaminated transport in anisotropic and heterogeneous soils. For unsaturated zone Richard's equation is solved. Galerkin finite element method is applied to numerically solved by finite difference method. Modified Strongly Implicit Procedure (MSIP) is used to solve these system of equations. MSIP requires less computer time and has fewer numerical difficulties than other numerical methods, especially when Solving large complex problem (Trescott et.al 1977). The solute transport equation is solved using M o d i f i e d M e t h o d of characteristics (Fried ,1975). Neuman boundary conditions are imposed by suitably selecting the system's parameters at that specified spatial locations. The effect of c o n s e r v a t i v e and non-conservative pollutant is duly taken care of by different in-built options designed in the software. A data base for various species is provided.

Consider a hypothetical case of groundwater flows. The flow field is stabilised between two constant head boundaries. It is assumed that on a single plane, there exist two types of zones - one having sandy clay with very low value of hydraulic conductivity and other is sand mixed with broken rocks with a high value of hydraulic conductivity. Tritium with a half life period of 12.3 years is dumped. Depending upon the location of the dump the effects are studied.

Fig.1 to Fig.4 represent the temporal and local distribution of the contamination when dump site is in clay zone, while Fig.5 to Fig.8 show the same representation when dump site is the sand zone.

If a cursory glance is cast on Fig.1 to Fig.4 it clearly indicates that there is not much variation in the concentration distribution. The convective terms are very low, hence transport of the contamination is minimized. On the other hand Fig.5 to Fig.8 give and an indication that convective terms are more effective in sand zone and thus there is appreciable amount of migration of contamination.

Fig.1 when compared to Fig.5 illustrates an important point that clay having better absorption capacity than sand, the maximum concentration at the end of first year is 167 mg/l compared to 180 mg/l in sand - zone.

However with passage of time the distribution in the sand zone is more spatially spread, thus the maximum concentration decreases. One important point that draws our attention is that due to heterogeneous effects in the sand zone, the concentration plume may get separated into pockets after sufficient time(see fig.8).

FLUIDYN - POLLUSOL is quite precise and logical in predicting the movement of radioactive pollutant in groundwater. Simulation results can suggest some of good remedial measures to handle the problem of radioactive wastes. Because of economical prediction and versatile capabilities.

FLUIDYN - POLLUSOL is a helping hand for most of industries, research organizations and consultants, who are directly or indirectly effected by problems of subsurface pollution in complex heterogeneous actual field conditions.