vaalputs post closure radiological safety assessment (pcrsa)
TRANSCRIPT
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2007 Vaalputs Post Closure Radiological Safety Assessment (PCRSA)
J.J. van Blerk, M.W. Kozak and J.F. Beyleveld, A.C. Carolissen
International Workshop on A Common Framework for the Safety of Radioactive Waste Management and Disposal
2-6 July 2007 Cape Town South Africa
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Vaalputs
Springbok
Vioolsdrif
Siyanda District Municipality
Karoo District Municipality
Frances Baard District
Municipality
Namakwa District Municipality
Vaalputs Site
Designated facility for the disposal of LILW in South AfricaOwned and operated by Necsa since 1986Authorisation for the disposal of LILW generated at the KNPS
Polokwane
Bloemfontein
Durban
East London
Cape Town
Johannesburg
Kimberley
Maseru
MmabathoNelspruit
Pietermaritzburg
Port Elizabeth
Pretoria
Springbok
Ulundi
NORTHERN CAPE
WESTERN CAPE
EASTERN CAPE
FREE STATE
LESOTHO
KWAZULU NATAL
NORTH WEST
MPUMALANGA
GAUTENG
LIMPOPO
Atlantic Ocean Indian Ocean
Orange
Vaal
Caled
on
Tugela
Orange
Vaal
Vaalputs
Pelindaba
Koeberg
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Regulatory Authorization Review
Changes to Regulatory Framework National Radioactive Waste Management Policy and Strategy
(2005) Safety standards (2006)
Disposal of a national inventory of radioactive waste KNPS Reconsideration of nuclear power as an option
Second PWR PBMR
Necsa historical and future waste Safari-1 reactor (1965) Nuclear fuel production facilities (1970-1998) Decommissioning of facilities at the Pelindaba site
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Purpose of the 2007 Vaalputs PCRSA Assess the post-closure radiological safety of the Vaalputs site for a
best estimate national inventory of radioactive waste
Assess if current disposal concept of near-surface earth trenches are sufficient to ensure long-term safety for the national inventory
Derive nuclide specific activity limits for the disposal of LILW at the Vaalputs site
Provide insight with respect to qualitative waste acceptance criterianecessary to ensure long-term safety
Identify where further data or information would be most helpful to improve the safety case
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Independent Safety Assessment
Parallel assessment in support of the Necsa assessment Increase credibility in the 2007 Vaalputs PCRSA Enhance confidence in the long-term safety of Vaalputs
Common safety assessment methodology Consistent assessment context, system description and
exposure scenarios Independent model development process
Necsa assessment more conservative Consistent assumptions and parameter values for consistent conceptual
and mathematical models
Performed by Monitor Scientific LLC (Denver, USA)
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Assessment Context
Consistent with ICRP standards and recommendations Dose constraint of 0.25 mSv per year (SA safety standards)
Target audience NNR identified as the primary audience Waste generators, state departments, statutory consultees, non-nuclear and
scientific communities, and environmental concern groups Operational period of 50 years
2036 the starting point for calculations (all disposals completed) Institutional control period of 300 years
No credit for controls after 2336 Considered 10,000 years as the period of regulatory concern
Analyses carried out to 100,000 years
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Vaalputs System Near surface trenches
Use standardized containers Semi-arid environment
MAP of 74 mm per annum 129 mm between 1986 to 2005 30 mm min; 305 mm max
Temperature Mean daily maximum: 34.8C Mean daily minimum: 0.7C
Sparsely populated (52 people are currently based in the area)
Farming community (sheep and game farming)
Rain main source of drinking water
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Vaalputs System Near surface trenches
Use standardized containers Semi-arid environment
MAP of 74 mm per annum 129 mm between 1986 to 2005 30 mm min; 305 mm max
Temperature Mean daily maximum: 34.8C Mean daily minimum: 0.7C
Sparsely populated (52 people are currently based in the area)
Farming community (sheep and game farming)
Rain main source of drinking water
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Vaalputs System Near surface trenches
Use standardized containers Semi-arid environment
MAP of 74 mm per annum 129 mm between 1986 to 2005 30 mm min; 305 mm max
Temperature Mean daily maximum: 34.8C Mean daily minimum: 0.7C
Sparsely populated (52 people are currently based in the area)
Farming community (sheep and game farming)
Rain main source of drinking water
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Vaalputs System Underlain by unconsolidated
sand, calcrete, greywacke, clay, granite and gneiss Bedrock extensively folded,
thrusted and fractured Underlying aquifer
Situated in weathered and hard granitic rock
Piezometric surface at 50 to 60 m Very flat groundwater gradient
Unsaturated zone Soil moisture increases in top
4 m after precipitation event Upward movement induced by
evapotranspiration is limited to 1 m below surface
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Scenario Development
Four natural exposure and two human intrusion scenarios Nominal Scenario
Judged to be a reasonable future behaviour of the facility Late Subsidence Scenario
Represent the uncertainty about the degradation of waste container and materials in the LLW trenches
Climate Change Scenario Seismic Scenario Drilling Intruder Scenario
Assess the exposure of a driller to borehole cuttings brought to the surface during a drilling intrusion event
Post-Intrusion Resident Scenario Farmer builds a house on top of the disposal trenches, receive exposure from
the borehole cuttings, and uses the borehole for farming purposes
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Model Development
Compartmental modelling approach Amber (Necsa assessment) Ecolego (Supporting assessment) Near field was compartmentalised according to 5 waste types
Necsa unstabilized LLW Necsa stabilized LLW Necsa stabilised ILW NPS unstabilized LLW NPS stabilized ILW
Contribution of certain compartments was excluded Grounds of uncertainty (e.g. the saturated zone) Indications that the nominal fractions of activity accumulating in a compartment
would be limited (e.g. upward pathway due to evapotranspiration)
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Model Development
Advective transport throughunsaturated zone
Saturated Zone
Backw
ard dispersive transportbetw
een compartm
ents
Surface Soils
Cover
Necsa UnstabilizedLLW
Necsa StabilizedLLW
Necsa StabilizedILW
NPS UnstabilizedLLW
NPS StabilizedILW
Unsaturated zone divided
into multiple compartments
Forw
ard
disp
ersi
ve t
rans
port
betw
een
com
part
men
ts
Advectivetransport
Advectivetransport
UpwardAdvectivetransport
Erosion
Borehole concentration=
(Transfer rate into aquifer)/(Pumping rate)
Advective transport
Cap
Backfill
Waste
Atmosphere
Upper Soils(Drilling
Residue)
DrillingCrew
Elsewhere
Dilution
Dilution
Exhume
Deposition
Inhalation(dust)
Externalirradiation
Dispersion
IngestionExternal
irradiation
ErosionLeaching
ExcretionDrilling
Re-suspension
A
B
C
D
E
F
G
1 2 3 4 5 76
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Assessment Results
Natural exposure scenarios Peak dose at 10,000 years: below 10-2 mSv/y
Below 10-1 mSv/y at all times Dominant nuclides
I-129, Tc-99 and Np-237 Dominant pathway
Water consumption Egg/mutton consumption
Probabilistic analysis Deterministic analyses
represent 95th percentileof uncertainty range
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+03 1.0E+04 1.0E+05
Time (Years)
Tota
l Dos
e (m
Sv
y-1)
Ac_227
Am_241
Am_242m
Am_243
C_14
Cd_113m
Cl_36
Cm_243
Cm_244
Cm_245
Co_60
Cs_135
Cs_137
H_3
I_129
Ni_63
Np_237
Pa_231
Pb_210
Pu_238
Pu_239
Pu_240
Pu_241
Pu_242
Ra_226
Se_79
Sm_151
Sn_126
Sr_90
Tc_99
Th_229
Th_230
Th_232
U_232
U_233
U_234
U_235
U_236
U_238
Cm_246
Total Dose
Advective Release : Nominal Scenario
I-129
Np-237
Tc-99
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Assessment Results
Natural exposure scenarios Peak dose at 10,000 years: below 10-2 mSv/y
Below 10-1 mSv/y at all times Dominant nuclides
I-129, Tc-99 and Np-237 Dominant pathway
Water consumption Egg/mutton consumption
Probabilistic analysis Deterministic analyses
represent 95th percentileof uncertainty range
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+03 1.0E+04 1.0E+05
Time (Years)
Tota
l Dos
e (m
Sv
y-1)
Ac_227Am_241Am_242mAm_243C_14Cd_113mCl_36Cm_243Cm_244Cm_245Co_60Cs_135Cs_137H_3I_129Ni_63Np_237Pa_231Pb_210Pu_238Pu_239Pu_240Pu_241Pu_242Ra_226Se_79Sm_151Sn_126Sr_90Tc_99Th_229Th_230Th_232U_232U_233U_234U_235U_236U_238Cm_246Total Dose
Advective Release : Late Subsidence Scenario
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Assessment Results
Natural exposure scenarios Peak dose at 10,000 years: below 10-2 mSv/y
Below 10-1 mSv/y at all times Dominant nuclides
I-129, Tc-99 and Np-237 Dominant pathway
Water consumption Egg/mutton consumption
Probabilistic analysis Deterministic analyses
represent 95th percentileof uncertainty range
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+03 1.0E+04 1.0E+05
Time (Years)
Tota
l Dos
e (m
Sv
y-1)
Ac_227
Am_241
Am_242m
Am_243
C_14
Cd_113m
Cl_36
Cm_243
Cm_244
Cm_245
Co_60
Cs_135
Cs_137
H_3
I_129
Ni_63
Np_237
Pa_231
Pb_210
Pu_238
Pu_239
Pu_240
Pu_241
Pu_242
Ra_226
Se_79
Sm_151
Sn_126
Sr_90
Tc_99
Th_229
Th_230
Th_232
U_232
U_233
U_234
U_235
U_236
U_238
Cm_246
Total Dose
Advective Release : Climate Change Scenario
Pa-231
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Assessment Results
Natural exposure scenarios Peak dose at 10,000 years: below 10-2 mSv/y
Below 10-1 mSv/y at all times Dominant nuclides
I-129, Tc-99 and Np-237 Dominant pathway
Water consumption Egg/mutton consumption
Probabilistic analysis Deterministic analyses
represent 95th percentileof uncertainty range
Diffusive Release : Nominal Scenario
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1000 10000 100000
Time (y)
Dose (mSv/y)
I-129 (mean)I-129 (50%)I-129 (5%)I-129 (95%)Tc-99 (mean)Tc-99 (50%)Tc-99 (5%)Tc-99 (95%)
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Assessment Results
Driller Intruder Scenario Single drilling event in single waste category Peak dose below 1 mSv/y at all times (0.5 mSv at 10,000 years) Highest doses from
Necsa stabilized LLW U-238 and U-234
No mass transferassumed
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+02 1.0E+03 1.0E+04 1.0E+05
Time (Years)
Tota
l Do
se (
mS
v y-
1)
NPS Unstab LLW
Necsa Stab LLW
Necsa Unstab LLW
Necsa Stab ILW
NPS Stab ILW
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Assessment Results
Post-Intrusion Residence Scenario Companion scenario for drilling intrusion scenario Farmer builds house and is exposed under nominal conditions Radon dose dominates Peak dose at 10,000
years is 1 mSv/y Below 10 mSv/y at all
times No mass transfer
assumed
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
100 1,000 10,000 100,000
Time of Intrusion (Years)
Tota
l Do
se (
mS
v y-
1)
OutdoorIndoorTotal (Rn-222)NominalTotal Dose
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Barrier Neutralization and Sensitivity Analysis Issues unimportant from a long-term safety perspective
Elaborate cap design Thickness important
Concrete container lifetime Important in the broader context of radioactive waste management
Unsaturated zone dispersivity Structural features in the unsaturated zone could alter this conclusion
Horizontal dimensions of the waste trenches Vertical dimensions important
Trench layout assumed for the disposal of LILW at the Vaalputs site
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Barrier Neutralization and Sensitivity Analysis Issues important from a long-term safety perspective
Refinement of the national inventory estimate Characteristics Extrapolation to future NPS Scaling factors used for radionuclide estimates
Improved recharge estimates and near surface hydrological processes Nominal and alternative scenarios
Nature of the underlying aquifer Associated groundwater flow regime
Near field and geosphere sorption properties Iodine, technetium, neptunium, carbon, uranium, and uranium decay
progeny
-
Barrier Neutralization and Sensitivity Analysis Issues important from a long-term safety perspective
Improve knowledge of human behavioral patterns Construction habits, eating habits, animal husbandry patterns, water use
patterns Biotic characteristics broadly relate to an up and out transport
pathway Insect excavation, animal excavation, rooting depths of local plants, and
root uptake and foliar shedding of eucalyptus trees Chemical nature of the waste
Different chemical forms of U in the waste and the uncertainty of the associated mass that may form corrosive agents
Waste form characteristics Waste form evolution, effect of waste form moisture content
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Question
Why does one want to go through an exercise of this nature?
to establish confidence that the basic principles of radioactive waste management, namely to protect human health and the environment at all times, are adhered to
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Confidence The NEA (NEA, 1999) defines confidence as
to have reached a positive judgement that a given set of conclusions are well-supported
The NCRP definition for a post-closure safety assessment (NCRP, 2005) emphasises reasonable assurance of compliance
It is neither possible nor desirable to argue absolute assurance
What one really wants to achieve is to reach defensible decisions on the extent to which the disposal system may comply with the regulatory criteria
Both technical and non-technical arguments may be required for this purpose
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Confidence Building
Process internal and external to the safety assessment process Internal confidence
Confidence the people performing the safety assessment has in their results
Proving that the analysis and the results are accurate, and Uncertainties are clearly identified and minimized where possible
External confidence Building confidence in the regulatory body and in the public Providing an acceptable level of proof that the safety assessment is
suitable for the purpose of making or supporting a decision
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Confidence in the Safety Assessment Used an internationally recognised, systematic and structured
safety assessment methodology
Used site-specific data as far as possible, complemented with justified literature values Selection of parameter values were conservatively biased
Presented analyses results and finding in an accurate, traceable and transparent manner
Clearly identified and minimised uncertainties where possible
Parallel assessments produced consistent and complementary results over a wide spectrum of assessment conditions
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Confidence in the Disposal System
A robust disposal system can be described as a system that continues to perform its expected global safety function, no matter what kind of reasonable perturbation may occur
Integrating the concepts of robustness into the disposal system or components of the system, lead to an increase in the confidence of the disposal system
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Confidence in the Disposal System
Factors demonstrated to contribute to the intrinsic robustness of the Vaalputs disposal system Comprehensive site selection process (1979-1982) Remoteness of the site Environmental site characteristics Limited contribution of disposal system components
Trench Cover (cap) Concrete containers Trench layout Trench horizontal dimensions
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Conclusions
Given the assessment results and the conservative nature of the assessment, the assessment concluded that most new data collection activities (with a few key exceptions) would be expected to lead to improved system performance
The assessment concluded that the likelihood is high for post-closure safety at Vaalputs to be demonstrated successfully for the disposal of a national inventory of LILW
It was concluded that, given the assumptions and conditions imbedded in the assessment, the use of near surface disposal trenches is effective and sufficient for the disposal of the national inventory of LILW
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Thank You for Your Attention!
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National Inventory
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
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.00E+13
1.00E+14
1.00E+15
1.00E+16
1.00E+17
Am-2
41Am
-242
m
Am-2
43C
-14
Cd-
113m
Cl-3
6
Cm
-243
Cm
-244
Cm
-245
Cm
-246
Co-
60
Cs-
135
Cs-
137
Eu-1
54 H-3
Ho-
166m
I-129
Ni-6
3N
p-23
7
Pu-2
38Pu
-239
Pu-2
40
Pu-2
41Pu
-242
Se-7
9Sm
-151
Sn-1
21m
Sn-1
26
Sr-9
0Tc
-99
Th-2
30U
-232
U-2
33U
-234
U-2
35
U-2
36U
-238
Zr-9
3Tr
u
Act
ivity
(Bq)
Chart1
5550000000
4180000
23700000
260000000000000
5550000
51700
1860000000
685000000
349000
110000
222000000000000
275000
137000000000000
284000000
5.46E16
3810
123000000
14500000
35800000
2280000000
494000000
371000000
11000000000
1790000
313000
175000000
336000
638000
620000000000
29500000000
8.61
21000
19.8
1680000000000
112000000000
168000
499000000000
13600000
17700000000
Activity (Bq)
Sheet1
RadionuclideTotal LILW
Am-2415.55E+09
Am-242m4.18E+06
Am-2432.37E+07
C-142.60E+14
Cd-113m5.55E+06
Cl-365.17E+04
Cm-2431.86E+09
Cm-2446.85E+08
Cm-2453.49E+05
Cm-2461.10E+05
Co-602.22E+14
Cs-1352.75E+05
Cs-1371.37E+14
Eu-1542.84E+08
H-35.46E+16
Ho-166m3.81E+03
I-1291.23E+08
Ni-631.45E+07
Np-2373.58E+07
Pu-2382.28E+09
Pu-2394.94E+08
Pu-2403.71E+08
Pu-2411.10E+10
Pu-2421.79E+06
Se-793.13E+05
Sm-1511.75E+08
Sn-121m3.36E+05
Sn-1266.38E+05
Sr-906.20E+11
Tc-992.95E+10
Th-2308.61E+00
U-2322.10E+04
U-2331.98E+01
U-2341.68E+12
U-2351.12E+11
U-2361.68E+05
U-2384.99E+11
Zr-931.36E+07
Tru1.77E+10
Total5.52E+16
Sheet2
Sheet3
2007 Vaalputs Post Closure Radiological Safety Assessment (PCRSA)Vaalputs SiteRegulatory Authorization ReviewPurpose of the 2007 Vaalputs PCRSAIndependent Safety AssessmentAssessment ContextVaalputs SystemVaalputs SystemVaalputs SystemVaalputs SystemScenario DevelopmentModel DevelopmentModel DevelopmentAssessment ResultsAssessment ResultsAssessment ResultsAssessment ResultsAssessment ResultsAssessment ResultsBarrier Neutralization and Sensitivity AnalysisBarrier Neutralization and Sensitivity AnalysisBarrier Neutralization and Sensitivity AnalysisQuestionConfidenceConfidence BuildingConfidence in the Safety AssessmentConfidence in the Disposal SystemConfidence in the Disposal SystemConclusionsThank You for Your Attention!National Inventory