Danyl Pérez Sánchez CIEMAT-SPAIN

Purpose of the Presentation A brief description of Model formulation and implementation process as part of safety assessment methodology In particular, it: Introduces the Model formulation and implementation process; and examines important consideration for conceptual model development and brief description of mathematical model development to implement in computer code and the parameter data collation

Safety Assessment Methodology The Safety Assessment methodology applied today in EU is based on the main guidelines currently developed inside international forums as those promoted by IAEA considering the ICRP Radiation Protection criteria. The common ISAM methodology is adapted to a process that identifies the following key components: Assessment context System description Development and Justification of Scenarios Scenario Evaluation Formulation and Implementation of Models Analysis of Result

Safety Assessment Process 1.Assessment context 2.Describe system 3.Develop and justify scenarios 4.Formulate and implement models 5.Run analysis 6.Interpret results 8. Adequate safety case 7.Compare against assessment criteria 9. Effective to modify assessment components 10.Review and modification Rejection Acceptance YES NO YES NO

In recent developments (ISAM, ASAM, PRISM, BIOMASS, EMRAS) it is recommended to establish first a methodology for the conceptual model development, specially in those cases when the assessment purpose is the estimations in future situations Safety Assessment Methodology

Safety Assessment Objectives The Safety Assessment methodology would be based on the main guidelines currently developed inside international forums as those promoted by IAEA The derivation of waste activity acceptance criteria for disposal Demonstration that an acceptable level of protection of human health and environment will be achieved both now and in the future

Model Formulation and Implementation

For Conceptual Model we Needs: Identification and characterization of the waste in terms of inventory Detailed information to adequate modelling of radionuclide's release Refine as more information on the waste and disposal system Characterization of disposal site Enhanced accumulation Information important to define pathways and receptors to develop a conceptual physical, chemical and biological model of the site Specification of facility design Specified design in term of the material used and the components of the system

INVENTORY ISOTOPIC CONTENT PHYSICO-CHEMICAL BEHAVIOUR RELEASE MECHANISMS WATE CHARACTERIZATION PHYSICO-CHEMICAL BEHAVIOUR UNDER DISPOSAL CONDITIONS WASTE PACKAGES DIMENSIONS BACKFILL MATERIALS STRUCTURAL MATERIALS ENGINEERED BARRIERS FACILITY System Description

SITE DESCRIPTION Socio-cultural and economical conditions Geology Hidrogeology Geochemistry Climatology Sismic and Techtonic conditions Biosphere system and ecosytems description Demography Population distribution Soil and water uses System Description

Scenario Development and Justification Hypothetical sequence of processes and events, devised for the purpose of illustrating the range of future behaviours of a repository system, for the purposes of evaluating a safety case Systematic approaches intended to provide a formal basis of the logic of the assumptions High level description of the expected temporal evolution of the disposal system (Reference Scenario) Initial screening and selection of FEPs High level description of alternative temporal evolutions of the disposal system

Conceptual Model Formulation Once that scenarios have been develop, the consequences in terms of the assessment context must be analysed A conceptual model can be defined as a set of qualitative assumptions used to describe a system or subsystem for a given purpose At a minimum, these assumptions concern the geometry and dimensionality of the system, initial and boundary conditions, time dependence, and the nature of the relevant physical and chemical processes The assumptions should be consistent with existing information within the context of the given purpose

Conceptual Model Formulation A conceptual model, which is a representation of the behaviour of the studied system that is fit for the particular purpose of the assessment as defined in the assessment context The conceptual model provides a description of the components of the system and the interactions between these components It also includes a set of assumptions concerning system geometry, the chemical, physical, hydro geological, biological, and mechanical behaviour of the system, consistent with the available information and knowledge

Goal of Conceptual Model Provide a framework that permit judgements to be made about behaviour of the total The model detail that mathematical models can developed to describe behaviour of the system and its components Estimate of the performance of the system over time. The model should be simple possible but include detail to represent system behaviour adequately to ensuring compliance with safety requirements. Specification of facility design Specified design in term of the material used and the components of the system

Conceptual Model Formulation A conceptual model should comprise a description of: the models Features, Events and Processes (FEPs); the relationships between these FEPs; and the models scope of application in spatial and temporal terms The model is a simplified representation of the natural processes and, so, the results from them are approximations Modellers frequently have developed the mathematical models and codes without any previous consideration of the conceptual models

Difference in Formulation for Waste Disposal Normal or Accidental Release Estimate Predictive Releases Disposal Systems Source TermReal and knownEstimate quantify (ft) Release PointReal and known Dependent on the media, transport agent and the disposal design System DescriptionArea to the release point Disposal system, current situation, possible future state of the system Models Calibrated with Measures Based experience and experiments (assumptions maximize) Results Compared against measures Verification with intercomparison of models or partial experiments (considerations of uncertainties)

Consideration for Model Development Models are developed at a level of detail that is fit-for-purpose, given the status of the waste management program, the assessment context and existing knowledge of the disposal system; The conceptual model provides a reasonable representation of the disposal system, and that the mathematical model adequately represents the conceptual model; Any alternative conceptual and mathematical models that have been considered or evaluated are documented in order to provide supporting arguments as to the adequacy of the selected models; Appropriate model verification and evaluation exercises are conducted and documented to build confidence in the fitness of the model for its intended purpose.

Conceptual Model Approaches Approaches have been used to facilitate the development of conceptual models in a traceable manner The SACO Approach The Interaction Matrix The influence Diagram

Conceptual Model Development Contaminant Release Mechanisms, transport media and Mechanisms, and Human Exposure Mechanism for Leaching Scenario

FEPs for Near Surface Repository Natural Processes Biological Intrusion Faulting Seismicity Meteorological processes and climate changes Fluid interactions Erosion Flooding Fluctuations in the water table Groundwater flow Seepage water Weathering Deterioration with time Freezing Wetting/drying

FEPs for Near Surface Repository Processes of the waste and the Repository System Obstruction of the drainage system Improper waste emplacement Failure of the top cover Presence or generation of chemical compounds that may disturb barrier performance Gas generation Waste and soil compaction Waste/soil interaction

FEPs for Near Surface Repository Human Activities Construction activities Farming Groundwater exploitation Habitation Reuse of disposed material Archaeology Other industrial activities

Conceptual Model by Interaction Matrix Identify detailed conditions, characteristics and processes for each Interaction Matrix element reviewed each Interaction Matrix element and identified key processes cross reference to where conditions, characteristics and data are defined (Design Scenario) summarise key processes, how they are to be modelled noting time and other dependencies

Interaction Matrix Development Representation of the Conceptual Model for a Leaching Scenario

Interaction Matrix Approach Interaction Matrix for Human Intrusion Scenario

Mathematical Model Mathematical models translate the assumptions of a conceptual model into the formalism of mathematics, usually sets of coupled algebraic, differential and/or integral equations with appropriate initial and boundary conditions in a specified domain These equations are solved to give the temporal and spatial dependence of the quantities of interest (such as radionuclide concentrations and doses to humans)

Data and Parameters for Models Once the conceptual and mathematical models have been developed it is necessary to assign values to the different parameters, a process which is here called model parameterization During model parameterization it should be ensured that: Parameter values used as inputs to the models and codes used in assessment calculations are documented. The model parameterization process should be traceable to source data Records are kept of how site and system specific characterization data has been used to derive parameter values used in the assessment calculations. Where a probabilistic approach has been used in the assessments, a justification of the selected probability distributions is provided

Summary in Model Formulation There is a need to make the process of formulating and developing models formal, defensible, and transparent to independent review Generation of conceptual models of the disposal systems using information from the assessment context, system description and scenario generation steps of the safety assessment procedure. A conceptual model can be defined as “a set of hypotheses or assumptions describing the physical and chemical processes that affect the development of the repository system and the surrounding environment, together with geometry, structure, properties, initial and boundary conditions of the system”

Summary in Model Formulation Implemented of these mathematical models in computer tools that are used then used to solve the mathematical models. Throughout this process, data are used to help develop the conceptual and mathematical models and provide input into the computer tools. The level of detail to which the models are developed will be a function not only of the assessment context but also the stage of iteration of the assessment process It must also be remembered that uncertainties are associated with all stages of model formulation and implementation. These uncertainties need to be identified, reduced and, as far as possible, quantified as part of the safety assessment

Key in Safety Assessment Process Procedures and tools such as the assessment context, ISAM FEP list, interaction matrix, etc. have proved very helpful It is useful to develop a summary flow diagram for each stage of the process and revise it as you work through it It is important to document all assumptions and decisions so that the safety assessment is logical and transparent and amenable to audit Many assumptions and decisions have been justified on the basis of the limited time, resources and information available

Key References