Danyl Pérez Sánchez
CIEMAT-SPAIN
[email protected]
http://www.ciemat.es
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
10. Review and
modification
5. Run analysis
7. Compare against
assessment criteria
9.
YES
Effective to
modify
assessment
components
6. Interpret results
Acceptance
YES
8.
Adequate
safety
case
NO
NO
Rejection
Safety Assessment Methodology
 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 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
System Description
WATE
CHARACTERIZATION
WASTE PACKAGES
FACILITY
• INVENTORY
• ISOTOPIC CONTENT
• PHYSICO-CHEMICAL BEHAVIOUR
• RELEASE MECHANISMS
• PHYSICO-CHEMICAL BEHAVIOUR UNDER DISPOSAL
CONDITIONS
• DIMENSIONS
• BACKFILL MATERIALS
• STRUCTURAL MATERIALS
• ENGINEERED BARRIERS
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
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 Term
Real and known
Estimate quantify (ft)
Release Point
Real and known
Dependent on the media, transport
agent and the disposal design
System Description
Area 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