STRESS TEST METHODOLOGY FOR NUCLEAR POWER PLANTS IN THE WAKE OF THE FUKUSHIMA ACCIDENT

Transcription

1 STRESS TEST METHODOLOGY FOR NUCLEAR POWER PLANTS IN THE WAKE OF THE FUKUSHIMA ACCIDENT Frank Nuzzo IAEA Nuclear Power Engineering

2 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

3 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 10/20/2011 3

4 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]). 10/20/2011 4

5 Response to Fukushima in the US The US NRC set up a Fukushima Near-Term Task Force (NTTF) which recommended: Actions identified in SECY (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 (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 10/20/2011 5

6 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.) 10/20/2011 6

7 Response to the Fukushima accident in Europe 10/20/2011 7

8 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 10/20/2011 8

9 ENSREG Ground Level Earthquake Operators in Europe respond by addressing ENSREG spec. and WANO SOER 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 10/20/2011 9

10 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 10/20/

11 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 10/20/

12 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" 10/20/

13 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 10/20/

14 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 10/20/

15 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) 10/20/

16 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) 10/20/

17 Development of IAEA s Stress Test Specification 10/20/

18 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 10/20/

19 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 10/20/

20 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. 10/20/

21 Thank you for your attention 10/20/