elbaz - irsn approach to safety of snf storage pools after fukushima
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Wednesday, 21.03.2012, Spent Fuel SessionTRANSCRIPT
IRSN approach of the safety of the
spent nuclear fuel storage pools after
Fukushima accident
Virginie ELBAZ
Institute of Radioprotection and
[N]uclear Safety
Fontenay aux Roses - France
International Experts’ Meeting on Reactor and Spent Fuel Safety in the Light
of the Accident at the Fukushima Daiichi Nuclear Power Plant
Vienna March 19 – 22, 2012
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
IAEA Meeting - Vienna 21st March 2012 – Virginie ELBAZ 2 2
Contents
1. Introduction
2. French approach to the stress test
3. Spent fuel storage pools
4. Safety assessment on spent fuel storage pools
5. IRSN assessment approach and outcomes
6. R&D program on Spent Fuel Pool accidents at IRSN
7. Conclusion
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▌The complementary safety assessment CSA called stress test consists in reassessing the safety margins of the nuclear installations for extreme natural events (earthquake, flood) and total loss of the safety system (loss of power supply, loss of cooling)
After the Fukushima accident in March 11th in 2011, a review of the
safety of the facilities has been undertaken at the European level
based on European specification produced by WENRA and approved
by ENSREG
1. Introduction
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Background : Periodic safety review of a facility
Process defined
by the
French Law
PSR
≈Every 10 years
Safety reassessment Ageing and conformity examination
Avoid discrepancy between the facility’s
state and new safety approaches,
practices and regulations,
Assess the robustness in the actual safety
requirement
Facilitate the on-going improvement of
the facilities' safety and their operations
2. French approach to the stress test
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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▌Updating of safety guide (definition of earthquake, seismic design…)
▌Taking into account the future operation context: lifetime
activity evolution
equipment characteristic evolution (ageing)
▌Operating feedback - events related to similar facilities: criticality accident of Tokaï-Mura in 1999
flooding at Le Blayais NPP in 1999
Periodic Safety Review
2. French approach to the stress test
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Stress Test
Specific
examination of
the conformity with
the design
requirements
Extreme natural events
taken into account
in the safety
requirement
Robustness
beyond the safety
design requirement
(severe accident)
2. French approach to the stress test
Identification of a list of Systems Structures and Components
(SSC) need to be qualified for hazard levels higher than those
considered in the existing safety framework
For extreme situations, this « Hardened Safety Core » allows to
bring back the plants in a safe state
In addition
to the PSR
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Loss of Power sources
Flood
Loss of all the sources of cooling
Earthquake
Deterministic approach
Loss of supply even back-up,
and ultimate back-up
IRSN assessment available on the website: www.irsn.fr
Beyond the design basis
usually taken
in French assessment
+
Combination of scenarios
2. French approach to the stress test
Assessment of succession of events even in situation of
degraded site and isolated from outside
Potential induced events (fire, explosion, industrial site around…)
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Nuclear Fuel cycle
Pool
Pool
3. Spent fuel storage pools
▌ In France, NPP fuels are stored in pools located in the nuclear power plants of EDF and before treatment in pools of the reprocessing plants of AREVA La Hague
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▌EDF PWR reactors pools
▌AREVA La Hague pools
▌EDF Superphenix pool
Spent fuel storage facilities in France
Under water storage
Dry storage ▌CASCAD, CEA (French Alternative
Energies and Atomic Energy) Commission) Cadarache
3. Spent fuel storage pools
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EDF PWR REACTOR STORAGE POOLS
▌ Fresh nuclear fuel used in PWR is manufactured with uranium, slightly enriched (4.5 %) with fissile 235U isotope and MOX
▌ Locations between 380 and 630 of fuel elements according to PWR plants
▌Time of decay before going to AREVA La
Hague ≈ 2-3 years for UOX fuel
▌ Stainless steel liner with reinforced concrete walls
▌The pool is situated in a specific building (BK), near the reactor building
3. Spent fuel storage pools
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▌The maximum thermal power is between 8 and 14 MW
▌Water capacity between 1150 and 1900 m3
▌The pool cooling system is provided by an external system to the pool (PTR) and earthquake proof
▌The cooling system has 2 redundant loops (each one consists of pump and heat exchanger)
▌The water is drawn from the pipe located about 4 m below the surface of the pool
Cooling
system
EDF PWR REACTOR STORAGE POOLS
3. Spent fuel storage pools
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▌Pools built between 1976 and 1988
▌500 and 1000 PWR baskets per pool
C. Cieutat – Copyright AREVA
Transport
BK La
Hague
C. Cieutat – Copyright AREVA
3. Spent fuel storage pools
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fuel cladding
basket
water
▌Volume of water is between 10 000 and 15 000 m3 (10 times more than NPP pools)
▌Height of the water: 9 m (twice the height of a basket)
▌Maximal Thermal Power authorized is between 8 to 16 MW per pool (similarly to NPP pools)
C. Cieutat – Copyright AREVA
▌PWR design basket with initial 235U fuel enrichment of 4,5% and MOX
▌Main assemblies come from EDF power plants
AREVA LA HAGUE SPENT FUEL POOLS
3. Spent fuel storage pools
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▌The reinforced concrete basin rests on a slab, independent of any adjacent structure, on neoprene bearings pads, and permits free thermal expansion.
▌Pool water is permanently cooled and purified with cooling exchanger C. Cieutat – Copyright AREVA
AREVA LA HAGUE SPENT FUEL POOLS
3. Spent fuel storage pools
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Diagram of cooling water system of storage spent fuel pools
UNDER WATER STORAGE – AREVA LA HAGUE SPENT FUEL POOLS
POOL
Outdoor installation
Air cooling tower
Indoor installation
3. Spent fuel storage pools
Water temperature 35°C
Fuel Assembly
Thermal exchanger
slab
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Damage the fuel cladding integrity
Safety of
spent fuel pools Pool water must be continuously
cooled to remove the heat
produced by spent fuel
assemblies
ACCIDENT
Keeping water level
Release of radioactive materials
to environment
Leak in the pool
Loss of the water
evaporation
draining
4. Safety assessment on spent fuel storage pools
SAFETY REQUIREMENT
•Robustness of the civil engineering and of the cooling
system
•Monitoring system
•Redundancy of equipment
•Backup generators/supply
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Monitor the situations of pools (water level, temperature of water) in case of a severe accident (degraded site)
Lots of questions involved the spent fuels storage pools during Fukushima accident
5. IRSN assessment approach and outcomes
Water Supply taking into account the increase of the radiation
Prevent the loss of the water level
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Consequences
of initiating
events
Total loss of
cooling
Total loss of
power supplies
Dewatering of pool
- Leakage
- Drainage through a pipe
Failure of the
instrumentation
and control system
5. IRSN assessment approach and outcomes
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Parameter of the
spent fuel storage
Initiating
events
Identification
of scenario of accident
- Robustness
- Conformity with
design requirement
Water level
Temperature of Water
Redundancy of equipment
Crisis
organization
Defence in depth
D
E
S
I
G
N
H
A
R
D
C
O
R
E
Current water supply - make-up
means
- Fire network
- Demineralized water
- Safety tanks
Instrumentation/measure Prevention Emergency plan
TO BE DEFINED
IRSN approach
- Strengthten make-up water (pre-assembly pipe) in
any condition (earthquake, high level of radiation…)
-Study more accident scenarios
-Identification of operational and accessible devices
set up in any conditions even for extreme situations
5. IRSN assessment approach and outcoming
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IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Definition of the “HARDENED SAFETY CORE”
5. IRSN assessment approach and outcomes for NPP pools
Means of crisis to be investigated
- Accessibility to actions means in case of a high level of radioactivity due to sky
effect
-Hydrogen production ?
Means of actions based on hardware and operations
-Isolating or minimizing of leaks or breaks consequences (anti-siphon devices,
new isolating devices)
-Water supplies (strengthen the design of water supplies in any condition)
-Instrumentation (monitor the water level on the full range of the pool height)
-Possibility of restarting a cooling train
Prevention
-Robustness of the design (transfer tube)
-In-service inspections
-Compliance to the current requirements
-Important For Safety (IFS) components and systems availability
Sources: I. MIRAMON and L. GILLOTEAU IRSN
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R&D program on Spent Fuel Pool
accidents at IRSN
IRSN undertook R&D actions since several years concerning the issue of spent
fuel pool accidents, especially concerning the behaviour of clad material in air
Some studies and exploratory calculations have been done with the
ICARE/CATHARE code (ASTEC Module), for very specific air undercooling
scenarios, mainly focused on the thermo-mechanical behaviour of fuel
assemblies.
The MOZART experimental program was launched (2005-2009) to address the
phenomenology of zircaloy nitriding and oxidation in air.
Development of calculation code
6. R&D program on Spent Fuel Pool accidents at IRSN
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Devoted to the study of ignition conditions and thermal
runaway propagation in Air on BWR and PWR fuel assembly
mock-ups in pool storage conditions
IRSN is fully involved in the OCDE Spent Fuel Pool program (US-NRC) at Sandia
National Laboratory (2009-2013)
Validation of ASTEC code for PWR geometry
Rupture of cladding
New ARAMIS R&D programme on spent fuel pool accident
(2012-2016)
6. R&D program on Spent Fuel Pool accidents at IRSN
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Recommendations leaded by IRSN
Complementary attempt must be provided by the operators in
order to ensure the spent fuel pools (“hard core”) in a safe state
for scenarios beyond the safety requirements
Capacity to restore water in any conditions (i.e degraded
situation, after an earthquake) in order to increase the
robustness of the facility
Conclusion
7. Conclusion
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Thank you for your attention…
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Safety investigations dealing with postulated initiators (seism, flood) :
Overheating of the storage pool of spent fuel until boiling
Decay of water inventory of the pool by steaming (or due to a leak or a
piping break in case of an accidental draining)
Risk of degradation of the radiological conditions (1 meter above a fuel
assembly is necessary to guaranty good radiological conditions)
Dewatering of fuel assembly being handled leading to a racing of
zirconium oxidation reaction
Boiling of the stored fuel assemblies leading to :
• a risk of restarting the chain reaction (criticality), and then
• a risk of important radiolysis of water leading to a hydrogen
accumulation
Dewatering of the stored fuel assembly in the bottom of the pool
leading to a severe accident
IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Definition of safety criteria : in order to avoid worst events
Safety assessment performed during PSRs
No dewatering of fuel assembly, even partially (in order to prevent
from damaging the fuel cladding which causes a severe accident) :
Assemblies in stored position
an assembly being handled
Safety assessment performed during CSAs (to be investigated)
No localized boiling in the storage area of spent fuel assemblies (in
order to prevent criticality risk leading to a risk of radiolysis of water
and then, a risk of hydrogen accumulation)
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IRSN approach of the safety of the spent nuclear fuel storage pools after Fukushima accident –
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Definition of functional criteria in order to obtain safety assessments
outcomes leading to the implementation of hardware modifications
fundamental design principles
A leak or break in any system connected to the pools should not cause direct dewatering of stored spent fuel assemblies, even if no isolating action is launched.
If a drainage occurs via a piping connected to the pools, it must be possible either to isolate the drainage process before direct dewatering of an assembly being handled or to put the spent fuel assembly in safe position before its dewatering.
When drainage causes loss of pool cooling, an emergency water supply should prevent stored fuel assemblies from being dewatered later