Frank Nuzzo IAEA Nuclear Power
Engineering [email protected]
STRESS TEST METHODOLOGY FOR NUCLEAR POWER PLANTS
IN THE WAKE OF THE FUKUSHIMA ACCIDENT
ENSREG High Level Requirements
Following the extreme events in Fukushima, most countries
are conducting a safety review of the robustness of their NPPs
The US, Russia, China, Korea, Japan have undertaken
rigorous reviews
International / National organizations (IAEA, WANO, INPO)
are preparing or have issued guidelines and specifications to
deal with extreme events
In Europe the European Nuclear Safety Regulators Group
(ENSREG) have issued a specification for a stress test of their
facilities ”The European Declaration on the post-Fukushima
stress test”
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Example of an “extreme event”
Initiating event: An unforeseen initiating external event such as an
earthquake of a magnitude larger than the plant design and
licensing basis, inducing the destruction of all infrastructure around
the plant and the collapse of a dam upstream of the NPP which
floods the region and de facto isolates the NPP.
Consequences on the plant: The NPP may experience this
external event as both a beyond design basis earthquake
combined with a flood exceeding the design basis flood, which
together may induce cliff-edge effects (sub-events) such as:
Prolonged loss of AC = prolonged Station Black Out (SBO) or
Prolonged loss of UHS or
Prolonged loss of both services extending well beyond the mission time
of the safety batteries and of all FW & reserve water inventories
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Japan’s stress test & safety review
Japan is conducting impact analyses of various
earthquakes/tsunami combinations with
prolonged SBO + loss of UHS as a result.
The method involves 3 steps:
1. Identification of components susceptible to
damage for impacts exceeding design basis
2. Identification of accident sequences for
hazards/magnitudes exceeding design basis,
assuming loss of safety functions and of
cliff-edge effects inducing loss of
components needed to mitigate the accident.
3. Identification of measures to prevent fuel
damage (hardware and Severe Accident
Management Guidelines [SAMG]).
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Response to Fukushima in the US
The US NRC set up a Fukushima Near-Term Task Force (NTTF) which recommended: Actions identified in SECY-11-0124 (LONGER-TERM REVIEW OF LESSONS LEARNED)
Seismic and flood hazard re-evaluations
Seismic and flood walk downs
Station blackout (SBO) regulatory actions
Equipment covered under Title 10 CFR 50.54 (hh) - aircraft threat
Reliable hardened vents for Mark I and Mark II containments and two additional items:
The inclusion of Mark II containments in the staff’s recommendation for reliable hardened vent
The implementation of spent fuel pool (SFP) instrumentation
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Response to Fukushima in the US
The US industry formed “The Fukushima Response Steering Committee” [Electric Power Research Institute (EPRI) Institute for Nuclear Power Operations (INPO) and Nuclear Energy Institute (NEI)] to coordinate the industry’s overall response to the accident
INPO and NEI (response to national/international emergencies)
INPO/WANO to collect lessons learned
INPO + NEI - Engage an Expert advisory Group (EAG) to interface with WANO, IAEA to develop a detail scope from the lessons learned
NEI-INPO’s Expert Advisory Group EAG recommended: Prolonged SBO (Coping duration, MCR evacuation & accident command & control)
External / Internal flood (assess margins, develop mitigation strategy, SAMG expansion to combined flood, loss of power & loss of UHS)
Coastal plants protection (tsunami/hurricane/tornado) recommends a study on frequency/magnitude + consequences
Seismic protection: Adequacy of ground acceleration spectra; analysis of seismic induced SBO and flood
Hydrogen: Reactor, RB and Spent Fuel Pool (SFP) expanded protection
SFP: Fuel Storage vulnerabilities - Defences enhancement - Study adoption of safety shutdown rules (protected trains, make up cooling, backup power etc.)
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The European Nuclear Safety Regulatory Group
(ENSREG) Stress Test
ENSREG requires a review of 3 targeted areas: Assessment of Safety Margins:
• Analysis of extreme natural events relevant to the nuclear site,
• Analysis of the adequacy of such selection,
• Capability of the plant to cope with these extreme events
• Determination of safety margins from such events,
• Cliff-edge effects and escalation to loss of safety functions & core
damage
Assessment of the consequences of loss of safety functions from
IE (SBO, Loss of UHS, Cliff edge effects and their combination).
• Barriers to prevent releases and measures to mitigate the
consequences
Assessment of the Plant’s Severe Accident Management
capability and ability to preserve containment integrity
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ENSREG – Ground Level Earthquake
Operators in Europe respond by addressing ENSREG spec.
and WANO SOER 2011-2
External Events – Earthquake as initiating event
DBE in terms of peak ground acceleration (PGA) and
reasons for the choice: Horizontal xxx g / vertical xxx g
Method used to evaluate DBE and validity of data in time
Seismic Methodology, i.e. the methodology is based on:
The maximum possible earthquake in 1000 years,
Data from the national historical records
Whether the national geological & seismic-tectonic institutes or
equivalent update this data and how often.
Every 10 years at PSR review time, is PGA reassessed?
Conclusions on the current adequacy of the design basis
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ENSREG : Earthquake - Plant Response
Seismic regulations; sizing codes & standards
Compliance with current licensing basis (periodic inspections, maintenance, testing...)
Key SSCs needed to achieve safe shutdown
Functional requirements during seismic events
Independent / voluntary compliance checks
Deviations and consequences to safety
Main operating provisions to prevent core damage (plant features, procedures, mobile equipment…)
Indirect failures: “non-seismic-SSCs” collapsing and damaging essential SSCs (2-1, pipe breaks, leaks…)
Planning & remediation
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ENSREG - Evaluation seismic margin limits
Evaluation of seismic limit beyond which loss of
safety functions & core damage are unavoidable
Cliff-edge effects (subsequent loss of other services)
Identification of vulnerabilities
Provisions or mods to increase plant robustness
Confinement: Range of seismic severity the plant can
withstand without losing confinement integrity
Independent / voluntary Compliance checks
Planning & remediation
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ENSREG: Flood – Design Basis
Initiating Event: Flooding (regardless of origin) + severe weather conditions
Design Basis for flooding (DBF):
Define methodology to evaluate DBF (Return periods, past events, flood sources, margins, data validity in time) & reasons for choice
Plant compliance.- Processes to ensure compliance (periodic maintenance, inspections, testing ) - Processes to ensure off-site equipment can reach the plant
Known deviations, safety consequences, Remedial actions
Voluntary compliance checks already performed following the Fukushima accident
"Provisions to protect against DBF:- Key SSCs to survive the flood (water intake, EPS, etc.) - Platform levels, dikes, surveillance / monitoring- Emergency Operating Procedures - Mobile Equipment- Mitigating equipment - Coincident effects: Severe weather conditions, loss of external power supply; plant isolation, curtailed accessibility"
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ENSREG: Flood – Safety Margins
Max. Level of flooding the plant can withstand without fuel damage. Indicate: Protective measures against it
Cliff-edge effects - Buildings & equipment that will be flooded first
Provisions to prevent cliff edge effects or increase plant robustness (mods., procedure changes. organisational changes)
Evaluation of margins:
Max. flood level the plant can withstand without fuel damage, indicate: Protective measures against it
Buildings & equipment that will be flooded first
Cliff-edge effects
Provisions to prevent cliff edge effects or increase plant robustness (mods., procedure changes. organisational changes)
Key SSCs to survive the flood (water intake, EPS, etc.) Platform levels, dykes, surveillance / monitoring
Emergency Operating Procedures
Mobile Equipment- Mitigating equipment
Coincident effects: Severe weather conditions, poor plant access & isolation
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ENSREG: Flood – Plant Robustness
Loss of offsite electrical power – Indicate: Internal back up sources and their mission times
Provisions needed to prolong mission time
Mods to design, procedures, organisation to increase robustness & reduce reliance on off-site power
Loss of Off-site + on-site Power (SBO) – Indicate: Battery Capacity, mission time & design provisions
Time under SBO before fuel degrades
Actions to prevent fuel damage (equipment on-site, off-site, from another reactor, near-by power stations. Check alignment time and resources required
Identify cliff edge effects and provisions to prevent them & increase plant robustness
Loss of Ultimate Heat Sinks due to flooding: Describe system and indicate provisions to prevent Loss of UHS.
Indicate time to fuel damage and provisions to prevent it.
On-site & off-site equipment to prevent meltdown, check alignment time & resources required
Cliff edge effects and provisions to prevent them or to increase plant robustness
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ENSREG: Flood – Cliff-edge Effects
Consider Loss of Primary + alternate ultimate Heat sinks:
Indicate time to fuel damage and provisions to avoid it.
On-site equipment to prevent fuel degradation and off-site equipment,
time to have these systems operational and resources required
Cliff edge effects and provisions to prevent them or to increase plant
robustness (Mods to hardware, procedures, organisation)
Loss of Primary UHS + SBO due to extreme flood – Indicate:
Time before severe fuel damage and provisions to prevent it.
On-site equipment to prevent fuel degradation and off-site equipment
and time to have these systems operational and resources required.
Cliff edge effects and provisions to prevent them or to increase plant
robustness (Mods to hardware, procedures, organisation)
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ENSREG: DBE + Flood
Seismic exceeding DBE + Flooding exceeding DBF or
seismically induced floods (uptream dam rupture)
Identify severe damage to structures outside or
inside the plant (dams, dikes, Bldgs, etc.)
Vulnerabilities, failure modes that can lead to
unsafe conditions
Cliff edge effects (Discontinuity effects, electric
encroaching, non-qualified inventory reserves etc.)
Additional features (mods capable of overcoming
cliff-edge effects and increase plant robustness)
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IAEA Stress Test Philosophy - Preview
Starts with the IAEA Safety Standards
(Even those who have not followed them in the past)
Assesses whether the SSCs which remain available
following an extreme event are sufficient to fulfil their
fundamental safety functions:
Criticality control
Residual heat removal
Confinement of radioactive material, which means:
Heat removal from containment
Containment overpressure prevention (steam & non-condensibles)
Prevention of containment bypass through interfacing systems
Achievement and preservation of containment isolation
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IAEA Stress Test Objectives - Preview
Specific objectives of the assessment are: Identify all credible limiting extreme events + associated
accident scenarios (leading to core damage) = initiating events accompanied by component failures
Identify possible technical measures that could be implemented to prevent core damage.
For cases where no reasonable measures could be suggested, perform a bounding assessment of the frequency of these extreme events (is ν = xxx acceptable?)
For extreme events of a magnitude lower than the limiting extreme event, evaluate the adequacy of the current safety margins and defence-in-depth
Identify practical measures that could be implemented to reduce plant vulnerability, if found necessary
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IAEA Stress Test Methodology Preview (under development)
The IAEA is developing a 5 stage methodology in cooperation with the member states and partnering organizations: Examination of extreme accident scenarios & cliff-edge effects
disruption of supporting functions (AC, DC, ESW, etc.) and event combinations leading to core damage
Examination of accident progression after core damage and severe accident management programmes (SAMP)
Examination of other sources of radioactivity such as spent fuel pool (SPF), radioactive waste treatment facilities, etc. also possibly leading to nuclear accident scenarios
Interactions between plant units (at multi-unit sites) and scenarios involving simultaneous containment failures
Most efficient measures for prevention / mitigation: i.e. provisions of sufficient barriers (redundancy, diversity, spatial separation), use of mobile equipment etc.
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