POSIVA Report 2016-1



Simplified Transport Modelling of a Disposal System and Doses Using Probabilistic Methods


Kupiainen, P., Nummi, O.



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ISBN 978-951-652-246-6 / ISSN 2343-4740


The disposal concept for radioactive waste is based on a nested barrier system where barriers confine radionuclides and limit their possible releases into the biosphere. Posiva's disposal facility includes repositories for spent nuclear fuel (SNF) and for low and intermediate level waste (LILW) generated during the encapsulation process. The radionuclide release and transport from the repositories is depicted by using a simplified transport model that also covers radionuclide migration in biosphere. The release rates from the two repositories and radionuclide migration in biosphere are calculated analytically with fractional mass transfer rates and delay times that are assigned for the release barriers and biosphere compartments. The doses to the most exposed individual are based on the resulting radionuclide concentrations in the biosphere compartments. The effect of variability and uncertainty in the model parameters on the release and dose rates are quantified with an uncertainty analysis. Moreover, the parameters causing the most uncertainty to the release and dose rates are identified by a sensitivity analysis.

The results of the simplified model compare well to the release and dose rates reported in TURVA-2012 safety case, despite of the biosphere model being rather stylized. The distributions for the maxima of the release rates to biosphere have about one order of magnitude difference between the 50% and 95% confidence levels for all nuclides. Similarly, same confidence levels of the total dose rate differ by slightly over one order of magnitude within the dose assessment period of the first 10 000 years. In most of the cases, the distributions of the release and dose rates are roughly log-normal. The parameters causing the most of the variability and uncertainty in the release rates are those related to the transport release barriers with the smallest fractional mass transfer rates (i.e. those with the highest retention capacity). For the SNF repository these are the parameters related to canister performance and for the LILW repository, the groundwater flow through the LILW hall. In the biosphere, the lake area causes most uncertainty to the total dose rate due to C-14 dose calculated with a specific activity model.


Nuclear waste, disposal system, radionuclide transport, biosphere, dose assessment, probabilistic simulation, sensitivity analysis.


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