l09.- elements of the safety assessment (iii) · l09.- elements of the safety assessment (iii)...
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International Atomic Energy Agency
L09.- Elements of the safety assessment (III)
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OBJECTIVE
To identify the key elements in the development of the
safety assessment:
• Identification of Models and Data Needs
• Performing Dose Calculations
• Evaluation of results. Analysis of uncertainties
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Introduction
In the previous lecture (elements of the safety assessment)
you learned about:
• Why to do a safety assessment. Overall approach. Safetyassessment objectives.
• Identification of some key elements in the development of
the safety assessment:
✓ Assessment context. Safety criteria and end points.
✓ Description of the facility or activity
✓ Development and justification of scenarios
Now we will continue developing this issues
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Safety Assessment ProcessAssessment Context
Description of Facility or Activity
and Sources
Development and Justification of Scenarios
Identification of Hazards
Selection and Hazard Screening
Identification of Scenarios
Evaluation of results
Identification of Models and Data Needs
Performing Dose/Risk Calculations
Analysis of Safety Measures
Compare against Assessment Criteria
Analysis of Engineering
Adequate? Adequate?Review and
Modify
Independent
verificationAccept
Purpose, Regulations, Standards
Engineering Design, Safety Measures,
Other relevant information: Normal operational
procedures, Management systems operational
experiences, site data
Events, Initiating events,
Expert judgments
Models and Computer codes
Generic, Facility/Activity specific data
Yes Yes No
No
Safety
Assessment
Process
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Formulation and implementation of models
• Once the scenarios are defined (and not being screened
out), the corresponding assessments shall be carried out;
This is commonly done using assessment models.
• Assessment model includes:
Conceptual model
Provides description of the components of the system and the
interactions between them; Includes assumptions concerning the
system geometry, the chemical, physical, biological and mechanical
behavior.
Mathematical model
Is representation of the conceptual model using mathematical
equations.
Computer code
Is software implementation of mathematical model and facilitates
performing of calculations.
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Support for Models
Need for Clarity and Transparency
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Need for clarity and transparency
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Model Formulation and Implementation
Model
Data
Numerical models
Assessment
ContextScenarios
Conceptual
Models
Mathematical
Models
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Model Formulation and Implementation
Model
Data
Numerical models
Assessment
ContextScenarios
Conceptual
Models
Mathematical
Models
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Conceptual Models
✓ Can be defined as a set of qualitative assumptions used todescribe a system or subsystem for a given purpose.
✓ At a minimum, these assumptions concern the geometry anddimensionality of the system, initial and boundary conditions,time dependence, and the nature of the relevant physical andchemical processes.
✓ In safety assessment analysis, the conceptual model consists of:
✓ The model’s features, events and processes (PIEs);
✓ The relationships between these PIEs;
✓ The model’s scope of application in spatial and temporal
terms.
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Model Formulation and Implementation
Model
Data
Numerical models
Assessment
ContextScenarios
Conceptual
Models
Mathematical
Models
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Mathematical Models
Once the models have been developed it is necessary to
assign values to the different parameters.
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.
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Mathematical Models
– The mathematical model adequately represents the
conceptual model;
– For mathematical models and computer codes, verification
and validation exercises are conducted and documented
to build confidence in the fitness of the model for its
intended purpose.
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1.00E-12
1.00E-11
1.00E-10
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08
Time after closure (Years)
Ann
al In
divi
dual
Effe
ctiv
e D
ose
(Sv
y-1)
Normal - Liquid
Normal - Gas
Normal - Solid
Variant A - Intruder
Variant A - Site Dweller
Variant B - Bathtubbing
Variant C - Rapid Transport
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Factors affecting the complexity of the models
MODEL
Nature of the problem
Purpose of
the
Assessment
Data availabilityUncertainties
Regulatory Framework
(end points)
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Model Formulation and Implementation
Model
Data
Numerical models
Assessment
ContextScenarios
Conceptual
Models
Mathematical
Models
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Numerical Models / Computer Codes
Types of codes
✓Proprietary codes
Advantage that the codes have been previously developed and
checked, and have a history of application to a range of cases
✓Modified codes and codes specifically developed for
implementation of the chosen mathematical models
Need to be developed and checked, advantage of being tailored to the
needs of the specific problem to be addressed
Increase confidence in the codes through a process of verification,
calibration and validation
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One or more codes can be used, depending
of the complexity of the conceptual and
mathematical models
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Model Formulation and Implementation
Model
Data
Numerical models
Assessment
ContextScenarios
Conceptual
Models
Mathematical
Models
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• Data are important at all stages of model development.
• Consideration should be given to the treatment ofuncertainties associated with the parameter values.
• If the computer codes are to be used for probabilisticcalculations rather than deterministic calculations, thenparameter distributions need to be specified.
• Data required and their meaning within the context of themodel, should be documented to provide a basis forestablishing a model input parameter databases.
Data Requirements
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Data Sources
• Source characterisation programme
• Design data
• Literature searches
• R&D
• Separate calculations
• Expert elicitation
Targeted
further work
Safety
Assessment
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Models & Data - Summary
The process consists of three stages.
✓Conceptual models;
✓Mathematical models;
✓Numerical models/Computer codes;
Level of detail determined by the problem and theavailable data.
Identify and quantify uncertainties.
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Safety Assessment Process
Assessment Context
Description of Facility or Activity
and Sources
Development and Justification of Scenarios
Identification of Hazards
Selection and Hazard Screening
Identification of Scenarios
Evaluation of results
Identification of Models and Data Needs
Performing Dose/Risk Calculations
Analysis of Safety Measures
Compare against Assessment Criteria
Analysis of Engineering
Adequate? Adequate?Review and
Modify
Independent
verificationAccept
Purpose, Regulations, Standards
Engineering Design, Safety Measures,
Other relevant information: Normal operational
procedures, Management systems operational
experiences, site data
Events, Initiating events,
Expert judgments
Models and Computer codes
Generic, Facility/Activity specific data
Yes Yes No
No
Safety
Assessment
Process
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Interpretation
• Once the calculations have been undertaken, the
results need to be collated, interpreted and compared
against the relevant assessment criteria.
• Ensure that an understanding of the results from each
component of the assessed system is developed.
• Intermediate calculation outputs should be analysed,
as well as those relating to endpoints such as dose
and/or risk.
Evaluation of results
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Performing calculations and analyzing their results
Management of uncertainties• Sources of uncertainty – model, data, parameters,
human errors, etc.
• Uncertainty and sensitivity analyses.
• Treatment of uncertainties.
Analysis of assessment results▪ Comparison against assessment criteria.
▪ Identification of safety measures (passive, active).
▪ Engineering analysis.
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Independent verification of safety assessment results
Review and modification of the assessment models
Evaluation of results
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Comparisons:
• Release pathway;
• Concentration vs. background;
• Time of peak dose;
• Against previous assessments.
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Evaluation of results
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When presenting the output from safety assessment
calculations, sufficient results should be provided;
▪ e.g. both needed for comparison with the ultimate assessment
endpoints and with any alternative or sub-system performance
criteria.
Guidance on the use of the safety assessment results should
be provided;
▪ For example, it should be explained whether the safety
assessment results (endpoints) will be compared directly with
regulatory criteria (e.g. safety targets) or whether these will be
used for illustrative or other purposes.
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Evaluation of results
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Safety Assessment Process
Assessment Context
Description of Facility or Activity
and Sources
Development and Justification of Scenarios
Identification of Hazards
Selection and Hazard Screening
Identification of Scenarios
Evaluation of results
Identification of Models and Data Needs
Performing Dose/Risk Calculations
Analysis of Safety Measures
Compare against Assessment Criteria
Analysis of Engineering
Adequate? Adequate?Review and
Modify
Independent
verificationAccept
Purpose, Regulations, Standards
Engineering Design, Safety Measures,
Other relevant information: Normal operational
procedures, Management systems operational
experiences, site data
Events, Initiating events,
Expert judgments
Models and Computer codes
Generic, Facility/Activity specific data
Yes Yes No
No
Safety
Assessment
Process
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Evaluation of results: Uncertainties
• In view of the complexity of facilities or activities, efforts
should be undertaken during the assessment to understand
the significance of the uncertainties and to reduce or bound
uncertainties;
• The analysis of uncertainties should be an integral part of
the dose or risk calculation process;
• Whenever possible, reported results should include ranges
of possible values (indicating what each range represents)
rather than single point values;
The analysis of uncertainty should be adequate for the
purpose of the assessment.
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Evaluation of results: Uncertainties
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Future
UncertaintyModel
Uncertainty
Data/parameter
Uncertainty
Subjective
Uncertainty
When defining a strategy for the treatment of uncertainties, it
is convenient to differentiate between:
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Evaluation of results: Uncertainties
Model Uncertainty
✓uncertainty in the conceptual model
✓mathematical model uncertainty
✓numerical model uncertainty
• Model uncertainty could arise from imperfect knowledge of the
processes, which leads to imperfect conceptual models;
• The mathematical representation of the conceptual models
may be approximate or over-simplified, also contributing to
model uncertainty;
• Imprecision in the numerical solution of mathematical models is
another source of uncertainty in this category.
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Evaluation of results: Uncertainties
Data/parameter Uncertainty
• Refers to the uncertainty in the used parameters values.
• This category often includes uncertainty in the:
– Source characteristics;
– Process characteristics;
– Measurement procedures – clearance / discharge
measurement procedures;
– Receptor characteristics – exposure time.
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Evaluation of results: Uncertainties
Future Uncertainty
• Future uncertainty is associated with the evolution of the
facility or activity over the timescales of interest.
• Examples include stochastic events such as earthquakes,
climate change and human intrusion.
• These uncertainties are usually considered through the use
of scenarios
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Evaluation of results: Uncertainties
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• For each specific scenario it is
necessary to deal with uncertainties in
the models and parameter values
used;
• A commonly used approach to
address model uncertainties is to
perform inter-comparisons between
alternative models, and in some cases
also between model predictions and
empirical observations.
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Evaluation of results: Uncertainties
• Sensitivity and/or uncertainty analyses may demonstrate
that a given uncertainty is not significant to the safety.
– Sensitivity study may show that the models are not sensitive
to some parameters, even when varying these over the whole
range of possible values;
– Uncertainty analysis may show that some parameters, may
have a small contribution to the overall uncertainty of the
model predictions.
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Evaluation of results: Uncertainties
The graded approach to safety assessment
applies equally to the treatment of uncertainty.
For example, a commonly used approach to
treat uncertainty is to:
✓ use conservative (cautious) assumptions
when simplifying the models;
✓ assign conservative values to model
parameters.
This approach has several advantages, in
particular for the demonstration of compliance
with regulatory criteria.
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Evaluation of results: Uncertainties
However, it should be taken into account that:
✓ In some cases the conservative assumptions may lead to
assessments representing situations that are extremely
unrealistic or impossible and, therefore, difficult to interpret
and communicate;
✓When conservative values are assigned to several
parameters, the results of the calculations might be over
conservative due to magnification of errors, and would
provide a poor basis for decision making;
✓ An assumption that is conservative in one scenario, or for one
radionuclide, might not be so for another.
The conservatism of the assumptions should be justified in
relation to their impact on the assessment endpoints.
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Thank you!