Vaalputs Post Closure Radiological Safety Assessment (PCRSA)

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

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

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

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

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)

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

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

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

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

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)

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

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

Assessment Results







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

Assessment Results







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

Assessment Results







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%)

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

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

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

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

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

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

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

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

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

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

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

Thank You for Your Attention!

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

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Vaalputs Post Closure Radiological Safety Assessment (PCRSA)