Modelling the radionuclide transport behaviour in groundwater at a near-surface disposal facility using SPRING

Simone Tillmann1, Torben Weyand2, Guido Bracke2, Barbara Reichert1
1 Institut für Geowissenschaften, Abt. Geologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Nussallee 8, 53115 Bonn
2 Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Schwertnergasse 1, 50667 Köln

14.10 in Young Hydrogeologists Forum

25.03.2020, 17:30-17:45, Telemann-Saal

The strategy for management of radioactive waste (RW) in Ukraine includes its disposal in near-surface facilities. The long-term safety of such facilities must be demonstrated by the licensee in the safety case. Modelling methods of radionuclide transport and analysis of potential radiation exposure in the long-term period are used to assess the long-term safety.

A typical scenario for long-term safety analysis is the potential exposure of people by consumption of contaminated groundwater from wells located at various distances. Specific data of the engineered near-surface RW disposal facility (ENSDF) at Vector site in the Chernobyl exclusion zone is taken to simulate radionuclide transport with a numerical model.

This study modelled the transport of the radionuclides Am-241 and Np-237 in groundwater released from ENSDF to hypothetical wells in order to analyse the potential risk of human exposure by consumption of water. At the Chernobyl zone, Am-241 and Np-237 are some of the highest contributors to potential exposure of humans. The numerical model considered the hydrogeological characteristics of the area around the ENSDF by using the code “Simulation of Processes in Groundwater” (SPRING by delta h).

The modelled scenario includes leaching of radionuclides by infiltrated precipitation and migration from the facility through the unsaturated zone to the aquifer. The transport to the well is modelled as advective-dispersive flow through the Quaternary shallow aquifer. Sorption and decay are included in all simulation calculations. As a result, the radionuclide activity concentrations in the different wells are computed up to 300,000 years after closure of the facility. Using these results, the potential radiation exposure of members of the public can be calculated and assessed applying Ukrainian and international regulations. With a sensitivity analysis, parameters and processes having the most significant impact on the transport results were determined.

The results indicate a higher retardation of Np-237 compared to Am-241. Smaller peak concentrations of Np-237 near to the disposal facility are affiliated to the origin as daughter nuclide, whereas decay of accumulated Am-241 causes comparatively higher Np-237 concentrations with increased observation distance.  Comparing the calculations with variating parameter values, the model is found to be especially sensitive to changes in Kd-value as well as dispersivity. Recommendations therefore include sampling campaigns and laboratory investigations to enlarge the site-specific data set. Further modelling should focus on the total anticipated activity of all radionuclides, also considering the other disposal facilities of the Vector site to obtain more detailed transport results for safety assessments.

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