l-13 an approach to safety assessment of near surface ... of a near surface disposal facility...
TRANSCRIPT
R. R. Rakesh and P. K. Narayan
BARC- IAEA Regional Training Course
on
Development of a Near Surface Disposal Facility
Mumbai, India
February 15-19, 2010
An Approach to Safety Assessment of Near Surface Disposal Facility
L-13
Safety Assessment Concepts
Safety Assessment is an iterative procedure for evaluating the performance of a disposal system and its potential impact on human health and environment.
•What is safety assessment?
•Why and when is it undertaken?
•What impacts can it be considered to assess?
•What are the timescales over which these
impacts are assessed?
A. Performance analysis Quantitative analysis of at least some subset of processes relevant to the behavior of the disposal system and calculation of (at least) intermediate parameters of interest, e.g. thermal evolution, container life time, contaminant release from some subpart of the disposal system.
B. Safety analysis Quantitative analysis of a set of processes that have been identified as most relevant to the overall performance of the disposal system and calculation of a measure of overall performance relevant within the given national regulatory regime, e.g. individual dose to members of critical group, integrated total release of contaminants.
C. Performance assessment Includes A. In addition, comparison of intermediate parameters to appropriate criteria set by regulation or design targets, e.g. maximum allowable temperatures, minimum groundwater travel time, contaminant release from a subsystem.
D. Safety assessment Includes B. In addition, testing of arguments that sufficient subsets of processes have been analysed, appropriate models and data used, plus comparison of calculated measures of overall performance to regulatory limits and targets.
E. Safety case Includes C and D. In addition, a full trace of arguments and evidence that a sufficient set of processes have been analysed and appropriate models and data used; relevant overall measures of performance and safety are within acceptable ranges allowing for uncertainties. More qualitative, parallel lines of evidence and reasoning may be also used to support results of the quantitative modeling and to indicate the overall safety of the system; e.g. that the disposal system does not rely overly on one component and the analysis does not overly rely on particular data or methods.
Safety Terms
Why and When ?
What Impacts and Timescales ?
Impacts
• radionuclide fluxes from the disposal facility
• radio toxicity of water entering the biosphere
• radionuclide concentration in environmental materials
• doses to non-human biota
• individual dose to a potential/hypothetical exposure group member
• individual risk.
Time scales
• No fixed time scale: (depends on nature of the waste disposal system, geology and hydrogeological conditions of the site, longevity of the radionuclides present in the waste, the food habits of the people residing nearby and so on)
• General guide lines: time period corresponding to maximum possible radiological dose for the people residing nearby, through all possible exposure pathways
Safety Assessment Methodology
Assessment Context • What is being assessed? • Why is it being assessed?
Purpose
• Regulatory bodies • Operator of the facility • Waste producers • Politicians • Public (media)
Audience
• regulations of national or international agencies (IAEA) (non country-specific)
• example: safety guide on safety assessment on Near Surface Radioactive Waste
Regulatory framework
• Individual effective dose, flux of radionuclides from the facility, concentration
• Need to correspond with the purpose and the regulatory framework, and take into account the timescales.
Assessment end-points
• No fixed time frame for assessment impacts (depends on the site characteristics, nature of waste etc)
• General guide line: time of arrival of peak concentration or dose of radionuclide of concern in biosphereActive and passive institutional control period
Assessment timeframes
System Description
near-field
• waste and waste forms
• engineered barriers
geosphere
• lies between near-field and biosphere
• components (soil, rock, groundwater and their properties)
biosphere • climate and atmosphere
• water bodies
• human activity
Features, Events, Processes and Scenario
Scenario: A scenario is a possible future behaviour of a disposal system that may
occur due to interactions of FEPs. It handles future uncertainty directly by describing
alternative outcomes.
Scenario
Features
Events
Processes
• a prominent or distinctive part or characteristic of the facility or its environment (near field features, far field features, biospheric features)
Features • a qualitative or quantitative change or complex of changes located in a restricted portion of time and space (near field events, far field events, biospheric events)
Events • a phenomena that results due to gradual changes that leads towards a particular result (near field events, far field events, biospheric events)
Processes
Features, Events and Processes
• Features: Inventory, Waste form, Disposal modules
• Events: Precipitation, Human intrusion
• Processes: Erosion of the disposal modules, Degradation of waste form and disposal modules, Near field flow and transport
• Features: Rock formation and fractures present in the formation
• Events: Flooding (probability very less), Earthquake (impact on transport process will be negligible)
• Processes: Flow through fractures (transport of radionuclides); sorption and desorption of radionuclides during transport; Discharge into RPS lake: Dispersion and Dilution
• Features: Human Inhabitants (critical group), Atmosphere, Milk and Meat producing animals, Crops (Agriculture)
• Events: Watering of crops from down stream water of the dam, Bathing and swimming in the down stream river of the dam, Intrusion at the site , Use of RPS lake water for daily purpose
• Processes: Construction work at the site results into Internal Exposure (Ingestion, Inhalation), External Exposure
Scenario Generation
The approach to be used is based on the one proposed by the Scenarios Working Group of ISAM Programme:
Scenario development
Expert judgment
Influence diagram
Interaction matrix
Selection of scenarios provides appropriately comprehensive picture of the system, its possible evolutionary pathways, critical events for the purposes of the assessment. A limited number of representative scenarios.
Screening of Scenario
Scenario
Natural evolution scenario Altered evolution scenario Stochastic
events scenario
(Probability of occurrence (Probability of occurrence low) (Probability of occurrence
reasonable) very low)
(Groundwater scenario, (Human intrusion scenario, (Meteorite impact,
Marine water scenario) Flooding) earthquake)
Conceptual Models
Model Formulation and Implementation
A conceptual model is the set of qualitative assumptions used to describe a system or subsystem for a given purpose. Important to document the identification of the various processes affecting the release, migration and fate of radionuclides and decide which processes are more relevant. A conceptual model should comprise a description of: •the model’s basic FEPs; •the relationships between the FEPs; and •the model’s scope of application in spatial and temporal terms (i.e. its domain).
Rainwater infiltration
Unconsolidated waste
Washout release
Unsaturated soil
Saturated porous media
Migration to
Biosphere
Plant uptake
Resuspension
Runoff, erosion
Soils/
Groundwater
Surface soils
Soils/
Groundwater
Air
Surface soils
Air
Surface water
Advection, diffusion, dissolution
Advection, diffusion
Advection, diffusion
Diffusion
Burrowing animals
Liquid
Gaseous
Solid
State of release Release Mechanism Primary receiving media
Release Mechanism
Mathematical Models
Mathematical models
Source Term
models
Diffusion release model
Dissolution release model
Rinse release model
Wash out model
Transport models
Transport through
groundwater
Transport through unsaturated porous
media
Transport through saturated porous
media
Transport through fractured rock media
Transport through
Surface water
Biospheric models
Radilogical dose through groundwater
pathway
Radilogical dose through
human intrusion pathway
Radilogical dose through
agriculture route pathway
Mathematical Models
•Source term modelling; •Transport modelling; and •Biospheric modelling. Transport model
where, C = concentration of radionuclide in groundwater Dx = longitudinal dispersion coefficient Dy , Dz = lateral dispersion coefficients u = groundwater velocity in x – direction v,z = groundwater velocity in y and z directions respectively = radioactive decay constant Rd = retardation factor b = bulk density of soil Kd = distribution coefficient = porosity of the migrating media.
CR
wC
zR
vC
yR
uC
xz
C
R
D
zy
C
R
D
yx
C
R
D
xt
C
dddd
z
d
y
d
x
Mathematical Models
• Radiological dose calculation:
• Where, D = radiological dose (Sv/y)
Dext= radiological dose due to external exposure (Sv/y)
Dinh= radiological dose due to inhalation exposure (Sv/y)
Ding= radiological dose due to ingestion exposure (Sv/y)
General formula for radiological dose from any radionuclide is
Where, A = any transfer coefficient, connected with food chain, shielding factor, breathing rates, transfer soil/plant factor, transfer aquatic animal/plant factor and so on. (Kg./year)
C = activity concentration in water and in soil (Bq/Kg)
F = dose conversion factor (dose coefficient) (Sv/Bq)
inginhext DDDD
Ground water Crop Animal Man Irrigation Food Food
Drinking
FCAD inginhext ,,
Data for Model Development Data type Parameter description
Geology - Local geology; - Thickness and extent of aquifers
Disposal facility dimensions - Length; - Width; - Depth;
- distance between different disposal modules; Inventory & Waste form - Total activity and radionuclides present;
- Waste forms and its dimensions - Density and porosity of waste form
Geotechnical Properties of the soil/rock
- Bulk density; - Porosity; - Void ratio; - Grain size analysis; - Plasticity index; - Mineralogical composition;
Hydro-geological data - Groundwater velocity (range & direction) - Seasonal water table variation - Recharge and discharge rates - Types of aquifer, Aquifer thickness - Aquifer characteristics (coefficient of permeability, porosity)
Geochemical data - Cation Exchange Coefficient (CEC) - Distribution coefficient (Kd) for different radionuclides with the site
specific soil Features available at that site
Water bodies existence if any with direction and distance w.r.t NSDF location.
Bio-spherical data Location of nearby population with respect to NSDF location, Crop production and food habits
Typical Inventory Estimation for 2X235 Mwe
PHWR Reactor
Radioisotopes Half life
(y)
Inventory of RAPS 3&4
(from reactor operation) at
closure of disposal modules
(MBq)
Inventory from Coolant
channel replacement of
RAPS 3&4 at closure of
disposal modules (MBq)
Total inventory at
the closure of dispoal
(MBq)
137Cs 30.2 4.33E+07 4.33E+07
134Cs 2.3 1.46E+06 1.46E+06
60Co 5.2 2.02E+06 1.26E+08 1.28E+08
90Sr 28.8 6.08E+05 6.08E+05
3H 12.3 2.00E+05 2.00E+05
65Zn 0.67 1.97E+04 1.97E+04
59Fe 0.12 1.28E+04 1.28E+04
54Mn 0.82 2.23E+04 1.82E+02 2.25E+04
144Ce 0.79 2.18E+04 2.18E+04
125Sb 2.4 5.07E+04 6.03E+06 6.09E+06
95Nb 0.96 1.27E+04 2.98E-40 1.27E+04
103Ru 4.16E-3 1.27E+04 1.27E+04
95Zr 0.17 2.22E-19 2.22E-19
113Sn 0.31 2.74E-08 2.74E-08
51Cr 0.08 4.69E-55 4.69E-55
Total
inventory 4.77E+07 1.32E+08 1.80E+08
Implementation on the Code and Run Analysis
1 + 1 = 3 ?
Once the scenarios and associated conceptual model and mathematical models have been developed, they need to be implemented in software tools (mathematical models) and associated data to perform the calculations.
Analysis of Results
Don’t accept the result 1 + 1 = 3 without analysis
The results need to be collated, analysed and presented.
While interpretation check should be done for
All the process simulation
input of initial and boundary conditions
Simulation of all the processes
Input of actual field parameter values with units and
parametric variations with time and space
Comparison of results
Compare the results with regulatory limits, if required run the analysis again
by reviewing the system description, scenarios formulation, formulation of
conceptual and mathematical models, run the analysis again, interpret the results
and then decide what to do………???
Uncertainty Analysis and Presentation of Results
• Sources of uncertainty
– Uncertainty in scenario selection
– Uncertainty in conceptual model
– Uncertainty in parameters
• Current practices in handling of parametric
uncertainty
– Conservative value
– Best estimate
– Sensitivity analysis
– Probabilistic analysis
Results should be discussed with operators and expert
groups before submitting to regulatory authority
Computer Tools
Computer codes
FEFLOW
MODFLOW
PORFLOW
AMBER
VS2DT
GOLDSIM
A number of commercial computer codes can be used to solve the mathematical models
Confidence Building
Following Steps must be followed for confidence building in the Safety Assessment •use of a systematic approach;
•peer review of the assessment (regulatory, academic and / or
independent);
•quality assurance at all stages of the assessment;
•verification, calibration and, if possible, validation of
models;
•consideration of relevant natural and man-made analogues to
disposal systems and / or its components;
•involvement of stakeholders;
•consideration of the various sources of uncertainties; and
•presentation of results.
Scenario Generation by Expert Group Phenomena FEP Number Features, Events, Processes
1. Natural Phenomena
1.1 Geological 1.1.1 Soil heterogeneity
1.2 Climatological 1.2.1 Precipitation, temperature and soil water balance
1.3 Geomorphological 1.3.1
1.3.2
Denudation, eolian fluvial
Chemical denudation
1.4 Hydrological 1.4.1 Recharge to ground water
1.4.2 Ground water discharge, exploitation GW well
1.4.3 Ground water condition
1.4.4 Saline or sea water intrusion
1.4.5 Effects at saline - fresh water interface
1.5 Transport and
Geochemical
1.5.1 Advection and dispersion
1.5.2 Diffusion
1.5.3 Matrix diffusion
1.5.4 Solubility limit
1.5.5 Sorption
1.5.6 Dissolution, precipitation and crystallization
1.5.7 Colloid formation, dissolution and transport
1.5.8 Complexing agents
1.5.9 Accumulation on soils and organic debris
1.5.10 Mass, isotopic and species dilution
1.5.11 Chemical gradient (electrochemical effects, osmosis)
1.6 Ecological 1.6.1 Plant uptake, (aquatic plant)
1.6.2 Animal uptake, (biota)
1.6.3 Uptake by rooting species, burrowing animal
1.6.4 Soil and sediment biturbation
1.6.5 Weathering, erosion and deposition
2. Human
Activities
2.1 Design and
construction
2.1.1 Common cause failure
2.1.2 Poor quality construction
2.1.3 Chemical effect (oxidation of soil)
2.2 Operations and
Closure
2.2.1 Heterogeneity of waste form (chemical, physical)
2.3 Post Closure 2.3.1 Ground water abstraction
2.4 Post Closure Surface
Activity
2.4.1 Altered soil or surface water chemistry
2.4.2 Land use change
2.4.3 Agriculture and fisheries practice changes
2.4.4 Demography change, urban development
3. Waste and
Repository
Effects
3.1 Chemical 3.1.1 Interaction of waste and repository materials with host material
3.1.2 Metallic corrosion
3.1.3 Interaction of host materials and groundwater with repository material,
3.1.4 Microbiological effects
3.2 Mechanical 3.2.1 Nil
3.3 Radiological 3.3.1 Material property change
Scenario Generation by Interaction Matrix
Component
A
1,1
Influence
of A on B
1,2
Influence
of B on A
2,1
Component
B
2,2 Inventory
1,1
Leaching
1,2
Groundwater
2,1
Geosphere
B
2,2