BARC- IAEA Regional Training Course
on
Development of Near Surface Disposal Facility
Mumbai, India
February 15-19, 2010
Post-Closure Safety Assessment of a
NSDF: A case study
PK Narayan, RR Rakesh, RK Bajpai & RN Nair
Post-Closure Safety Assessment of a
NSDF: A case study
Operating NSDF In India 1. Seven NSDF operational
2. Built in remarkably different geological, hydrogeological and
climatologically varying domains
Coastal Facilities
Three NSDF located in weathered basalts, high rainfall, low recharge, gentle
slopes, highly sorptive soil
One in lateritic soil profile developed over crystalline rock, coastal humid
climate,high rainfall, poor sorption
One more located along the east coast in weathered profile over charnockite
rocks ,moderate rain fall, moderate sorption
Inland Facilities One located in flat bedded sandstones, thin soil cover,semi arid climate
One located in Alluviums of Northern Plains, high groundwater potential
Post-Closure Safety Assessment of a
NSDF: A case study
Safety Assessment Case
Study of a coastal site
Site specific charecterization &
Safety Assessment
Post-Closure Safety Assessment of a
NSDF: A case study
About the Facility
Location In Basaltic Terrain along the West Coast
Disposal Modules : Stone Lined Trench, RCT and Tile Holes
Waste Source : Operation of research reactors,
Various radiological laboratories and the reprocessing
plant .
Industrial and medical application of radiation sources
Waste volume disposed ( Till Dec. 1997) Approximately 20,500 m3
13925 m3 in Earth Trenches (ET)/Stone Lined
Trenches (SLT), 6525 m3 in RC Trenches (RCT)
and remaining 50 m3 in Tile Holes (TH)
Post-Closure Safety Assessment of a
NSDF: A case study
Site characterization
Regional Geological framework
Topography
Dominantly flat topography with a few high relief (110m elevation) hills
with steep step like slopes, occupied by layers of basaltic flows. The
succession consists of almost 40 flows with a total thickness of ~2km.
Regional Tectonics
Forms a part of Western Ghat Escarpment running along the West
Coast of India, Located to the west of a group of N-S trending stable
faults,Mild density of lineaments, high heat flow,
Seismicity
Lies in seismic zone II with max horizontal acceleration of 0.10g (IS
Code) , Maximum magnitude earthquake < 4
Post-Closure Safety Assessment of a
NSDF: A case study
Tsunamis
No reported Tsunamis so far, except mild inland sea water intrusions during 1615
Absence of high potential collision tectonics in Arabian sea
Presence of protective Western Ghat high relief geomorphic feature along the west coast
Geology
Mostly dominated by alternate amygdaloidal and basaltic flow ranging in thickness from a
few meter to tens of meter and separated by sedimentary interbeds. Top soil cover is about 2-
10m thick. Flows are undeformed and homogeneous.
Hydrogeology
Generally two aquifers represented by amagdaloidal basalt and vesicular basalt separated
by sedimentary interbeds.
Transmissivity of weathered basalts, vesicular basalts and fractured basalts of the Deccan
trap area range from 90 to 200 m2/day, 50 to 100 m2/day and 20 to 40 m2/day respectively.
Post-Closure Safety Assessment of a
NSDF: A case study
A typical cross section of the site showing subsurface geology and dip of the flows
Local Geology
Located on the southern slope of
a EW trending hill with max
elevation of 110m ,
Rocks are dominantly shallow
dipping basalt flows overlain by
thick soil cover, Three dominant
fracture trends N-S. NE-SW and
E-W
N
Post-Closure Safety Assessment of a
NSDF: A case study
Local Hydrogeology
1. Weathered zone (top 10m) constitutes the most productive water
bearing horizon Fractured basalts at deeper level are water
bearing ,Contacts of flows are also water conducting pathways
2. Absence of sea water intrusion in coastal aquifer as indicated by
ground water chemistry
3. Transmissivity of basalts 0.1-500m2/day
4. Aquifer thickness 2-8m
5. Water table fluctuates from 0.5 to 10 m bgl.
6. Water flows towards south with a velocity of 0.1 to 0.5m/day
7. Infiltration rates are of the order of 10cm/year
8. The annual average precipitation from 1959 to 2001 is 2376 mm
Post-Closure Safety Assessment of a
NSDF: A case study
Host rock/soil characteristics
Porosity: Weathered basalt ~ 34%
Vesicular basalt ~ 50%
Fractured Basalt ~ 15%
Massive basalt <2%
Hydraulic conductivity: 0.01-1 m/d
Mineralogy: Important mineral constituents are as under
Basalt: Plagioclase, biotite, augite, magnetite, sphene
with secondary zeolite, palagonite, aragonite
Weathered basalt Sericite, smectite, chlorite, leucoxene,
Soil Clays, organic matter,
Distribution coefficient for key radionuclides (ml/g)
90Sr 140-300
137Cs 600-1000
Post-Closure Safety Assessment of a
NSDF: A case study
Important techniques and tools used
1.Geological mapping on hill slope, pits, trenches and outcrops
2.Ground Penetrating Radar Surveys
3.Borehole and auger hole drilling
4.Pump and Slug test for aquifer parameter determination
5.Field Tracer Test for RN transport and infiltration studies
6.Laboratory based testing of rock and soil samples
Post-Closure Safety Assessment of a
NSDF: A case study
Details of emplaced waste ET/SLT : mainly metallic scrap, PVC, rubber, processed cake, HEPA
filters, glass wares, concrete chippings, soil, and other assorted
materials.
RC Trench mainly of solidified mucks/flocs sludges.
Tile holes depleted and defective sources, high radiation solid wastes
(Beta-Gamma) and suitable for disposal in NSDF within
permissible limits.
Estimated inventory for 35 years
Post-Closure Safety Assessment of a
NSDF: A case study
Sl.No. Properties Cemented waste form
1. Waste loading (%) 66.66
2. Compatibility With alkaline waste
3. Specific activity(Bq/g) 1.48-2.22x104
4. Density(g/cc) 1.8-2.0
5. Compressive strength(MPa) 10-15
6. Leach Rate(g/cm2/day) 10-4-10-5
7. Radiation stability No damage up to 108 Rads
8. Homogeneity Very good
9. Thermal stability Very good upto 100○C
Characteristics of the cement waste forms
The radioactive sludge cakes are immobilized in cement matrix.
Waste pack with cement matrix has a diameter of 600 mm and height of 900
mm. The spent resin is stored in 200 litre steel drum and is not immobilised
Post-Closure Safety Assessment of a
NSDF: A case study
Safety Assessment
an exercise undertaken to evaluate the performance of a disposal system
under a given set of conditions to make an assessment of its impact on human
health and environment. In the following discussions various components of a
typical safety assessment case as applied to the above site discussed.
Assessment Context
The purpose :
understand the behaviour of site with respect to groundwater flow and
migration of radionuclides.
suggest further data acquisition programme in order to improve the overall
safety wherever required.
Post-Closure Safety Assessment of a
NSDF: A case study
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
FEP’S
OF
COASTAL
SITE
Post-Closure Safety Assessment of a
NSDF: A case study
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)
Contd...
Post-Closure Safety Assessment of a
NSDF: A case study
Contd...
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)
Post-Closure Safety Assessment of a
NSDF: A case study
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
3.3 Radiological 3.3.1 Material property change
Contd....
Post-Closure Safety Assessment of a
NSDF: A case study
System Description Near-field
waste
the disposal area,
the engineered barriers (waste packages, disposal modules, disposal facility
cover, etc.)
the disturbed zone of the natural barriers that surround the disposal facility
Geosphere
soil,
unsaturated zone and the saturated zone
hydrogeology, geochemistry
tectonic and seismic conditions
Biosphere
climate and atmosphere,
Human population residing nearby and its activity,
sea
Post-Closure Safety Assessment of a
NSDF: A case study
Near Field FEPs Features Waste Inventory, engineering Barriers (Waste forms, Disposal Modules such as Earth Trench
(ET)/Stone Line Trench (SLT), Reinforced Concrete Trench (RCT) and Tile Holes (TH)).
Events •Precipitation;
•Human intrusion
oExcavation-inhalation
oDwelling-inhalation
Processes
Erosion of disposal modules
Degradation of engineered barriers
Near Field Flow and Transport (infiltration of groundwater, diffusion, dissolution and
transport in unsaturated zone):
Post-Closure Safety Assessment of a
NSDF: A case study
Features aquifer, sea, dug well
Events flooding,
earthquake,
landslide,
tsunami.
All the above are having a very small probability of occurrence and their impact on
transport process will be negligible for this site.
Processes: Flow and radionuclide transport through aquifer
Sorption and desorption during transport through aquifer
Discharge into sea through geologic media:
Dilution and dispersion of the radionuclides in the sea
Far Field FEPs
Post-Closure Safety Assessment of a
NSDF: A case study
Features Human inhabitants (critical group)
Atmosphere (air, groundwater) and
Marine foods (fish, invertebrate, salt etc.)
Events
Human intrusion at the site(construction activities at the site)
Processes
Internal Exposure
Ingestion: Consumption of drinking water, marine food such as fish,
invertebrate, salt etc.
External Exposure: Swimming activity on beach
Biospheric FEPs
Post-Closure Safety Assessment of a
NSDF: A case study
Scenario Development
Near field scenario:
a) Failure of top cover of RC trench and direct inflow of rainwater
b) Failure of bottom cover of RC trench/tile holes
c) Failure of top and bottom cover
d) Failure of complete RC trench/tile holes
e) Degradation of waste form
Far field scenario: A human habitation and a well are postulated in south-east
direction at a distance of about 800 meters from the fence of the facility and outside
the boundary of the centre. The groundwater flows towards the bay in the southward
direction. The modification of the groundwater direction in future is a low probability
event. However, in the altered evolution scenario of groundwater flowing towards the
postulated habitation, the drinking water pathway has also been considered.
Post-Closure Safety Assessment of a
NSDF: A case study
Waste forms in Near Surface Disposal Facilities
Radioactivity release in local aquifer
Drinking underground
water from a well
Flux into the sea
Internal exposure External exposure
Swimming
Ingestion
Inhalation
Beach activity
Intrusion after institutional
control (300 years)
Dose to individual
Complete failure of disposal modules
Radioactivity release in near field by
diffusion and dissolution mechanism
Fish
Invertebrate
Salt
Excavation
Inhalation
Dwelling
Inhalation
Pathways and Scenarios
Post-Closure Safety Assessment of a
NSDF: A case study
Conceptual modeling of the site & the far
field scenario
Post-Closure Safety Assessment of a
NSDF: A case study
Mathematical model
Source Term Modeling: Leaching from cylindrical waste form has
been calculated by diffusion and dissolution mechanism. The
diffusion coefficient considered for this analysis is retarded diffusion
coefficient The release mechanism of radionuclides from spent resin
and spent source is governed by solubility-limited release.
Radionuclides Half life
(years)
Diffusion coefficients (cm2/sec.)
Cs137 30.2 5.60x10-06
Sr90 28.8 1.15x10-08
Co60 5.3
Post-Closure Safety Assessment of a
NSDF: A case study
Dissolution and Diffusion Phenomenon considered in
the model
Dissolution of
cement waste
from due to
interaction
with
groundwater Diffusion
of RN
Through
matrix
Groundwater
Radionuc
lides
Spent resin Spent source Sludge cakes 137Cs 90Sr 60Co 192Ir 137Cs 90Sr
Inventory
(Bq)
9.32×1
013
1.04×1
013
4.67×1
013
- 1.64×1
013
1.82×
1013
Total
inventory
1.04×1014 4.67×1013 1.82×1013
Inventory of radionulcides considered in the
safety assessment
For governing
equation of
these processes
pl refer to
training notes
Sl. No. Properties Cemented waste form
1. Waste loading (%) 66.66
2. Compatibility With alkaline waste
3. Specific activity(Bq/g) 1.48-2.22x104
4. Density(g/cc) 1.8-2.0
5. Compressive strength(MPa) 10-15
6. Leach Rate(g/cm2/day) 10-4-10-5
7. Radiation stability No damage up to 108 Rads
8. Homogeneity Very good
9. Thermal stability Very good up to 100○C
Post-Closure Safety Assessment of a
NSDF: A case study
Estimation of dose to critical group
through groundwater pathway
Code used : PLAFLX
Assumption : Isotropic K, Kd
Parameters Values
Groundwater Velocity (m/day) 0.5
Porosity (%) 30
Bulk density (g/ml) 1.7
Kd (137Cs, ml/g) 600
Kd (90Sr, ml/g) 140
Retardation factor for 137Cs 3400
Retardation factor for 90Sr 793
Aquifer thickness (m) 6.0
Length of the area source (m) 200.0
Width of the area source (m) 30.0
NW Parameters of the geo-
spherical transport model
Post-Closure Safety Assessment of a
NSDF: A case study
Radionuclide
concentration in
groundwater at
different
distances
Radionuclide
Concentration (Bq/m3)
200 m
distance 800 m distance
Maximum
permissible for
drinking
purpose
90Sr 7.36 x 10-1 8.04 x 10-21 3.7 x 103
137Cs negligible negligible 4. x 105
ciRii DCCD
Conversion of radionuclide concentration in
groundwater into radiological dose using
the equation Radionuclide Ingestion Inhalation
137Cs 1.3x10-8 3.9x10-8
90Sr 3.1x10-8 1.6x10-7
Dose conversion factors (Sv/Bq)
where,
Di = dose due to ith radionuclide (Sv/y)
Ci = concentration of ith radionuclide in groundwater (Bq/m3)
CR= annual consumption rate of groundwater for drinking purpose (m3/y)
Dci = dose conversion factors of ith radionuclide (Sv/Bq.)
Post-Closure Safety Assessment of a
NSDF: A case study
where,
Q = maximum flux (Bq.m-2.y-1)
(40.269 for Sr90 at 200 m distance)
W = width of the facility (m) 600
H = thickness of the aquifer (m) 2
V = bay volume (m3) 4.5x107
r/V = fractional renewal rate in the sea (18.25 y-1)
= radioactive decay constant.
The radioactive flux entering into sea is
translated into radionuclide concentration
in the sea, CW (Bq/m3), using the equation
V
rV
HWQCW
.
Marine exposure pathway
Cw for Sr90 = 5.876 x 10-5 Bq/m3
Post-Closure Safety Assessment of a
NSDF: A case study
Dfish=Cw x Dci x Tf x Crf
Where,
Cw= Concentration of radionuclides in sea water=5.876x10-5Bq/m3
Dci=Dose conversion factor due to ingestion=3.1x10-8Sv/Bq for Sr90
Tf =Transfer factor for fish(ranges between 82x10-3to 14000x10-3 m3/kg)
For gold fish Tf = 9.33x10-3 m3/kg
Crf = consumption rate of fish=36.5 kg/y
Which gives
Dfish = 6.20x10-13Sv/y
A simple illustrative example of
dose due to the fish intake from
marine pathway
Estimation of dose to critical group through
Marine pathway
Post-Closure Safety Assessment of a
NSDF: A case study
Parameters used in the marine exposure pathway
Consumption rates
Fish (g/y) 3.65x104
Invertebrate (g/y) 1.83x104
Salt (g/y) 5.48x103
Inhalation (m3/y) 7.3 x103
Occupancy
Swimming (hr/y) 500
Beach activity (hr/y) 1750
Other parameters
Contaminated food fractions 0.5
Fraction of activity in air 0.05
Suspended sediment in air (g/m3) 1x10-4
Salt particles in air (g/m3) 1x10-5
Ingestion dose coefficient (Sv/Bq)
Cs 137 1.3x10-8
Sr 90 2.8x10-8
Inhalation dose coefficient (Sv/Bq)
Cs 137 3.9x10-8
Sr 90 1.6x10-7
Post-Closure Safety Assessment of a
NSDF: A case study
Where,
Sp. act = Radionuclide sp. Activity (Bq/g)
dustout door = Out-door dust concentration (g/m3)
Oout door = Out-door occupancy (fraction)
Aair = Activity in air (fraction)
Ir = Inhalation rate (m3/y)
Dc = Dose conversion factor (Sv/Bq)
Dustcon = Dust Concentration in air (g/m3)
Occupancy = Occupancy at the site (hr/y)
Radiological dose to Critical Group is calculated for human intrusion scenario after
300 years of closure of radioactive waste disposal facility. Dose is calculated for (a)
dwelling inhalation in both out-door and in-door conditions, and (b)
excavation/construction scenarios using following expression.
Human intrusion pathway
Post-Closure Safety Assessment of a
NSDF: A case study
Dwelling - Inhalation
Inhalation Rate (m3/hr) = 1.2
Indoor dust concentration (g/m3) = 50
Outdoor dust concentration (g/m3) = 100
Indoor occupancy fraction = 0.80
Outdoor occupancy fraction = 0.20
Fraction of activity in air = 0.05
Excavation - Inhalation
Occupancy (hr/y) = 100
Dust concentration in air (mg/m3) = 1.0
Fraction of activity in air = 0.33
Post-Closure Safety Assessment of a
NSDF: A case study
Radiological dose (Sv/y) to critical group by drinking
water pathway, marine exposure pathway and human
intrusion pathway
Radionuclides
Pathways
Groundwater
drinking
pathway
Marine exposure pathway Human intrusion pathway
Ingestion Ingestion Inhalation External Dwelling
inhalation
Excavation
inhalation
Cs137 --- 2.60 x 10-20 2.66 x 10-26 7.50 x 10-20 1.26 x 10-10 1.88 x 10-10
Sr90 1.82 x 10-28 2.55 x 10-7 4.17 x 10-12 8.25 x 10-12 4.82 x 10-11 7.27 x 10-11
Co60 --- --- --- --- --- ---
Post-Closure Safety Assessment of a
NSDF: A case study
Confidence building
Radionuclide concentrations in groundwater and corresponding
radiological doses to the critical group as a function of different
geo-environmental parameters such as distribution co-efficient,
aquifer thickness (d) and groundwater velocity (U).
Sensitivity Analysis
Parameter Range
Groundwater velocity (m/d) 0.1-1.5
Distribution coefficient
(ml/g)
Sr90 140-300
Cs137 600-1000
Aquifer thickness (m) 2-8
Post-Closure Safety Assessment of a
NSDF: A case study
Variation of 90Sr peak concentration and corresponding peak concentration
arrival time as a function of groundwater velocity and distribution co-efficient
at a distance of 200m from boundary of the facility
Post-Closure Safety Assessment of a
NSDF: A case study
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.61E-24
1E-20
1E-16
1E-12
1E-8
1E-4
Kd (ml/g)
140
150
160
200
250
300
Pe
ak d
ose
arr
iva
l tim
e (
y)
90S
r P
ea
k d
ose
ra
te (
Sv/y
)
Groundwater velocity (m/d)
200
400
600
800
1000
1200
1400
1600
Peak dose arrival time
Peak dose rate
Variation of Sr90 peak dose rate and corresponding peak dose arrival time as
a function of groundwater velocity and distribution coefficient at a distance of
200m from boundary of the facility