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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
: N
um
mi 2
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
11
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
12
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
: N
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
16
• 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.
17
Thank you!
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