Focus on scenarios in the safety case context

Safety case for Loviisa LILW repository 2018

Olli Nummi, Fortum Power and Heat Oy, Loviisa NPP

Nuclear Science and Technology Symposium 2019 (SYP2019), October 31th 2019

Safety case

• “Documentation for demonstrating

compliance with the long-term safety

requirements” (STUK 2018a)

• Scope defined in international guidance

(e.g. IAEA 2011, 2012), STUK’s

requirements (STUK 2018a,b)

• Safety case methodologies by Posiva

(2012) & SKB (2015) were followed

2

Analysis of releases and doses

Safety assessmentSafety assessment

Safety caseSafety case

Main report

Description of the disposal system and design basis

Activity inventory

Terrain and ecosystems modelling

Surface and near-surface hydrology

modelling

Groundwater flow modelling

Gas generation and transport

Performance assessment and formulation of scenarios

Background reports

Analysis of releases and doses

Safety assessment

Safety case

Main report

Description of the disposal system and design basis

Activity inventory

Terrain and ecosystems modelling

Surface and near-surface hydrology

modelling

Groundwater flow modelling

Gas generation and transport

Performance assessment and formulation of scenarios

Background reports

Fig

ure

: N

um

mi 2

01

9a

Disposal site

3

Pic

ture

: F

ort

um

Po

we

r a

nd

He

at O

y

Waste caverns

4

Fig

ure

s: F

ort

um

Po

we

r a

nd

He

at

Oy

Activity in comparison to spent nuclear fuel

5

2,2

E+7

1,2

E+7

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

LILW SNFcanister

Act

ivit

y (G

Bq

)

Total

5,6

E+0

4

3,2

E+0

2

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

LILW SNFcanister

Act

ivit

y (G

Bq

)

C-14

4,7

E+

06

4,4

E+

08

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

1E+9

LILW SNFcanister

Do

se

(S

v)

Dose potential

Fig

ure

s: F

ort

um

Po

we

r a

nd

He

at

Oy

Safety functions

6

Limitgroundwater

flow

Limit and retard release

Limit andretard transport

Protectmechanically

Limitgroundwater flow

Reduce likelihood of inadvertent

human intrusion

Containwaste

Reduce natural and human impacts,

and likelihood of inadvertent human intrusion

(Waste caverns)

Protect chemically and mechanically

SURFACE ENVIRONMENT

BEDROCK

CLO

SUR

E

LOW LEVEL WASTE PACKAGES

WASTE CAVERNS

INTERMEDIATE LEVEL WASTE PACKAGES

WASTE CAVERNS

CONCRETE BARRIERS

Fig

ure

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01

9a

Concept of scenario

7

• Uncertainty in future evolution is managed by scenarios

• Scenario describes a potential evolution of the entire disposal system during the assessment

period (100,000 years) associated with fulfilment of or deviation from safety functions and

performance targets

– Scenarios are based on performance assessment results and uncertainties therein

• Principles adopted:

– Plausibility

– Consistency

– Small number & distinctness

– Transparency & traceability

Phases of scenario formulation (after Kosow and Gaßner2008)

• Phase 1: Scenario field identification

– What do we want study?

– What is included / excluded?

• Phase 2: Key factor identification

– What are the factors driving the system evolution

– Key characteristics of barriers providing safety functions identified with uncertainty

• Phase 3: Key factor analysis

– Define states (evolutions) for the key factors

8

Key factors and key factor states

9

Key factor Key factor states

Inte

rme

dia

te le

vel

was

te p

acka

ges

Concrete container evolution Reference evolution Accelerated concrete degradation

Mechanical damage

Areas with zero wall thickness in reactor pressure vessels and steam generators

Reference evolution Initial defect in welds

Reactor pressure vessel and steam generator wall thicknesses

Reference evolution Early loss of alkaline conditions and microbiological corrosion

Initial defect and microbiological corrosion

Early loss of alkaline conditions, initial defect and microbiological corrosion

Co

ncr

ete

bar

rie

rs Concrete barrier evolution Reference evolution Accelerated concrete degradation

Mechanical damage

Was

te

cave

rns Groundwater flow through the waste

cavernsReference evolution No plugs

Clo

sure Hydraulic conductivity of the concrete

plugsReference evolution Gap between

concrete and rockMechanical damage

Fig

ure

: N

um

mi 2

01

9b

Example of evolution – reactor pressure vessel corrosion time

10

1E+03

1E+04

1E+05

1E+06

1E+07

Reference evolution Acceleratedconcrete

degradation

Microbiologicalcorrosion

Microbiologicalcorrosion &accelerated

concretedegradation

Co

rro

sio

n t

ime

(a)

Phase 4: Scenario formulation

• Combination of key factor states into scenarios

– 3×2×4×3×2×3 = 432 possible combinations

• Morphological analysis

– Consider dependencies between key factor states and scenario distinctness

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Hydraulic conductivity of the concrete plugs

Groundwater flow through the waste caverns

Concrete barrier evolution

Concrete container evolution

Areas with zero wall thickness in reactor pressure vessels and steam generators

Reactor pressure vessel and steam generator wall thicknesses

Reference evolution Reference evolution Reference evolution Reference evolution Reference evolution Reference evolution

Gap between concrete and rock

No plugsAccelerated concrete degradation

Accelerated concrete degradation

Initial defect in welds

Early loss of alkaline conditions and microbiological corrosion

Mechanical damage Mechanical damage Mechanical damageInitial defect and microbiological corrosion

Early loss of alkaline conditions, initial defect and microbiological corrosion F

igu

re: N

um

mi 2

01

9b

Phase 5: Scenario transfer

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Scenario

Variant scenario: Declined concrete performance

Postulated initial state deviations

Initial defect in welds

Concrete quality deviations

Disturbance scenario: Large earthquake

Variant scenario: Initial defect in welds

Base scenario

High hydraulic conductivity of plugs

Microbiological corrosion

Reinforcement bar corrosion

Mechnical damage of plugs, concrete barriers and concretecontainers

Alternative evolutions

Fig

ure

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um

mi 2

01

9b

Scenarios and calculation cases

13

Fig

ure

: Ja

nsso

n e

t a

l. 2

01

9

Resulting dose rates in each scenario

14

Base scenarioInitial defect in

welds

Acceleratedconcrete

degradationLarge earthquake

5th percentile 4,1E-03 4,2E-03 4,0E-03 4,4E-03

95th percentile 5,0E-02 4,9E-02 5,5E-02 5,2E-02

Median 1,3E-02 1,4E-02 1,4E-02 1,5E-02

1,0E-03

1,0E-02

1,0E-01

Max

imu

m d

ose

rat

e (m

Sv/a

)Maximum dose rates

Fig

ure

: Ja

nsso

n e

t a

l. 2

01

9

Resulting dose rates in each scenario

15

Basescenario

Initial defectin welds

Initial defectin weldscrushedconcrete

Acceleratedconcrete

degradation

Largeearthquake

5th percentile 1,1E-02 1,7E-02 1,4E-02 7,5E-02 1,2E-02

95th percentile 3,6E-01 1,4E+00 5,3E-01 6,0E-01 4,4E-01

Median 5,3E-02 1,0E-01 8,6E-02 2,3E-01 7,2E-02

1,0E-03

1,0E-02

1,0E-01

1,0E+00

1,0E+01

No

rmal

ized

rel

ease

rat

e

Maximum normalized release rates

Fig

ure

: Ja

nsso

n e

t a

l. 2

01

9

Literature

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• IAEA (2011), Disposal of Radioactive Waste, IAEA SafetyStandards Series No. SSR-5, Vienna, Austria.

• IAEA (2012), The Safety Case and Safety Assessment for the Disposal of Radioactive Waste, IAEA Safety StandardsSeries No. SSG23, Vienna, Austria.

• Jansson et al. 2019. Safety case for Loviisa LILW repository 2018 – Analysis of releases and doses. Fortum Power and Heat Oy, Espoo.

• Kosow, H. & Gaßner, R. 2008. Methods of future and scenario analysis: overview, assessment, and selection criteria. DIE Research Project "Development Policy: Questions for the Future". Deutsches Institut für Entwicklungspolitik (DIE), Bonn, Germany. (Studies / Deutsches Institut für Entwicklungspolitik; 39). ISBN 978-3-88985-375-2.

• Nummi 2019a. Safety case for Loviisa LILW repository 2018 – Main report. Fortum Power and Heat Oy, Espoo.

• Nummi 2019b. Safety case for Loviisa LILW repository 2018 – Performance assessment and formulation of scenarios. Fortum Power and Heat Oy, Espoo.

• Posiva 2012. Safety Case for the Disposal of Spent Nuclear Fuel at Olkiluoto - Synthesis 2012. Posiva report 2012-12 Posiva Oy, Eurajoki, Finland. ISBN 978-951-652-193-3.

• SKB 2015. Safety analysis for SFR Long-term safety –Main report for the safety assessment SR-PSU. SKB report TR-14-01. Svensk Kärnbränslehantering AB (SKB), Stockholm, Sweden.

• STUK 2018a. Radiation and Nuclear Safety Authority Regulation on the Safety of Disposal of Nuclear Waste. Regulation STUK Y/4/2018.

• STUK 2018b, Disposal of nuclear waste, Guide YVL D.5.

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Thank you!