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Abstracts
SESSION H7: Performance Assessment Modeling and Parameters
1) 40305 Keynote – Development of a Realistic Repository Performance Assessment Method
Joonhong Ahn, UCB (USA)
Abstract Not Available
2) 40204 – Integrated model for the near field of a repository in granite host-rock. Probabilistic approach
Lara Duro, Alba Valls, Olga Riba, Jordi Bruno, Amphos XXI Consulting S.L.(Spain);
Aurora Martinez-Esparza, ENRESA (Spain)
The application of probabilistic approaches to the performance of underground repositories for long-lived
radioactive waste has received special attention in the last years. Numerous exercises have been developed in order to
elicit the Probability Distribution Functions (PDFs) of the several parameters needed for these developments. Several
integrated models allow the implementation of PDFs in the long-term simulations needed for Performance Assessment.
In this work we present how the deterministic compartmental model for a repository of high level nuclear waste (HLNW)
located in a crystalline host-rock has been modified to include PDFs for some of the parameters. The implementation of
probabilistic approaches gives also information on the most influencing parameter on the migration of the different
radionuclides from a deep repository concept.
3) 40017 – Spatial Variability and Parametric Uncertainty in Performance Assessment Models
Osvaldo Pensado, James Mancillas, Scott Painter, Southwest Research Institute (USA);
Yasuo Tomishima, AIST (Japan)
The problem of defining an appropriate treatment of distribution functions (which could represent spatial variability
or parametric uncertainty) is examined based on a generic performance assessment model for a high-level waste
repository. The generic model incorporated source term models available in GoldSim®, the TDRW code for contaminant
transport in sparse fracture networks with a complex fracture-matrix interaction process, and a biosphere dose model
known as BDOSE(TM). Using the GoldSim framework, several Monte Carlo sampling approaches and transport
conceptualizations were evaluated to explore the effect of various treatments of spatial variability and parametric
uncertainty on dose estimates. Results from a model employing a representative source and ensemble-averaged pathway
properties were compared to results from a model allowing for stochastic variation of transport properties along
streamline segments (i.e., explicit representation of spatial variability within a Monte Carlo realization). We concluded
that the sampling approach and the definition of an ensemble representative do influence consequence estimates. In the
examples analyzed in this paper, approaches considering limited variability of a transport resistance parameter along a
streamline increased the frequency of fast pathways resulting in relatively high dose estimates, while those allowing for
broad variability along streamlines increased the frequency of "bottlenecks" reducing dose estimates. On this basis,
simplified approaches with limited consideration of variability may suffice for intended uses of the performance
assessment model, such as evaluation of site safety.
4) 40203 – Development of a Radiolytic Model for the Alteration of Spent Nuclear Fuel. Incorporation of
non-oxidative matrix dissolution and hydrogen oxidation inhibition effect
Lara Duro, Alba Valls, Olga Riba, Jordi Bruno, Amphos XXI Consulting S.L.(Spain);
Aurora Martinez-Esparza, ENRESA (Spain)
In the last years, there have been numerous efforts from national waste management agencies to develop models
able to predict the dissolution behaviour of spent nuclear fuel under interim and/or long-term storage conditions. One of
the most evolved models is the so called Matrix Alteration Model (MAM), which is based on the radiolytic oxidative
dissolution of UO2 (Martínez-Esparza et al., 2004) and which has been applied to different experimental results with
certain level of agreement. The calibration of the MAM model in front of new experimental data has resulted in the
identification of some important drawbacks that may result in limited applicability of the model as a predictive tool. In
this work we present the modifications made to the MAM in order to improve it and expand its range of applicability: -
Incorporation of the non oxidative alteration of the matrix. Ignoring the incorporation of the non-oxidative alteration of
the matrix has proved to be non-conservative over long-term experiments, resulting in an underestimation of the actual
concentrations of uranium and radionuclides dissolving congruently with the matrix (Bruno et al., 2009). The
incorporation of this mechanism in the MAM has been done by considering the different rates of irradiated and
unirradiated UO2+x determined under reducing conditions and published in the open scientific literature. The modified
MAM is able to reproduce experimental data gathered under a diverse range of experimental conditions. Incorporation of
the catalytic effect of the surface on hydrogen activation. Hydrogen can be generated by radiolysis of water in the vicinity
of the spent nuclear fuel as well as by anaerobic corrosion of metallic components forming on the container of the fuel
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