Abstract
A simplified model was developed to represent
radionuclide migration from a deep geological nuclear
waste repository system to the biosphere. The modelled
repository system is based on the concept of multiple
nested transport barriers. The model can be used to
assess migration and migration properties of single
nuclides (no decay chains) through the repository system.
Radionuclide transport processes included to the model
are diffusion and sorption in the repository nearfield
and advection, matrix diffusion and sorption in the
geosphere. A simplified approach to handle solubility
limited release of the nuclide from the waste canister is
included into the model.
The model treats transport barriers as wellmixed
volumes. It is also assumed that radionuclide outflow
from a barrier can be calculated by negleting
radionuclide concentration in the target barrier.
Radionuclide transport through the simplified system can
be calculated by applying formal analogy of the model to
the mathematical model of the radioactive decay chain.
Simplifying the barriers as wellmixed volumes suggests
that they can be characterised by simple performance
measures. Radionuclide outflow from the barrier can be
represented by an equivalent flow rate, which is an
apparent volumetric flow rate that combined with the
radionuclide concentration in the barrier gives the
outflow rate of the nuclide. Temporal behaviour of the
release rate can be described by two time constants: i)
compartment halflife of the nuclide concentration
calculated by dividing capacity of the barrier (the total
pore volume multiplied by the retardation factor) with
the equivalent flow rate and ii) delay time for start of
the outflow from barrier after beginning of the inflow to
barrier.
Performance of the simplified approach to produce actual
release rates for different nuclides was tested by
modelling C14, I129 and Pu239 using data from the
RNT2008 radionuclide migration analysis. Accuracy of the
simplified approach is challenged if the nuclide's
halflife is not long compared to the time required for
the development of perfectly mixed solute concentration
field in the barrier. The nuclide and barrier
combinations that are prone to this behaviour can be
identified by comparing the estimated compartment delay
time with the nuclide's radioactive halflife. The
simplified model performed well for the C14 and I129,
as expected based on the measures above. Early transients
of the concentration field in the buffer and in the
geosphere are important for the transport of Pu239 in
the calculated case. The simplified model gave results
for Pu239 that were roughly of the same order of
magnitude than the corresponding numerical results.
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Supervisors/Advisors 

Award date  29 Nov 2013 
Place of Publication  Espoo 
Publisher  
Print ISBNs  9789513880972 
Electronic ISBNs  9789513880989 
Publication status  Published  2013 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 nuclear waste
 repository system
 migration
 modelling
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Poteri, A. (2013). Simplifying solute transport modelling of the geological multibarrier disposal system: Dissertation. VTT Technical Research Centre of Finland. http://www.vtt.fi/inf/pdf/science/2013/S42.pdf