stress test methodology - international … event: flooding ... seismic exceeding dbe + flooding...

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”

10/20/2011 2 Frank Nuzzo - IAEA - NPNE

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


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]).


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


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.)


Response to the Fukushima accident

in Europe

10/20/2011 Frank Nuzzo - IAEA - NPNE 7

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


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


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


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


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"


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


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


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)


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)


Development of IAEA’s

Stress Test Specification


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


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


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.


Thank you for your attention


stress test methodology - international … event: flooding ... seismic exceeding dbe + flooding...

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