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