Roy HarterRLH Global Services
Workshop on Understanding the Role of Severe Accident Management Guidelines
December 15-16, 2016
Accident Management Guidance for Spent Fuel Pools & Shutdown Conditions
2
IAEA SAMG-D Modules
Module 1: Fundamentals on Reactor Safety
• Basic concepts of nuclear safety.• Fundamental safety principles, defense-in-
depth.• Nature and role of procedures and guidelines.• Main elements to develop SAMG.
Module 2:Severe Accident Challenges and Strategies
• Associated radiological phenomena with severe accidents.
• Processes that challenge fission product barriers.
• Strategies to protect fission product boundaries.
Chapter 2.11 - Spent Fuel Pool damages
Module 3: Severe Accident Management Guidelines
• Description on how strategies are developed into plant specific guidelines, to most effectively manage an accidental scenario.
Chapter 3.6 - SAMG for spent fuel pool and shutdown phases
Module 4: Implementation, Requirements and Infrastructure
• Organizational measures needed to execute the SAMGs.
• Implementation of these measures in the overall plant emergency organization.
• Plant specific verification and validation
3
Main Principles:1.10 - The recommendations of this Safety Guide have been
developed primarily for accident management during at-power states, but are intended to be valid also for other modes of operation, including shutdown states.
2.16 - Severe accidents may also occur when the plant is in the shutdown state. In the severe accident management guidance, consideration should be given to any specific challenges posed by shutdown plant configurations and large scale maintenance, such as an open containment equipment hatch. The potential damage of spent fuel both in the reactor vessel and in the spent fuel pool or in storage should also be considered in the accident management guidance. As large scale maintenance is frequently carried out during planned shutdown states, the first concern of accident management guidance should be the safety of the workforce.
2.17 - Severe accident management should cover all modes of plant operation and also appropriately selected external events, such as fires, floods, seismic events and extreme weather conditions (e.g. high winds, extremely high or low temperatures, droughts) that could damage large parts of the plant. In the severe accident management guidance, consideration should be given to specific challenges posed by external events, such as loss of the power supply, loss of the control room or switchgear room and reduced access to systems and components.
IAEA Safety Guide NS-G-2.15Severe Accident Management Programmes for Nuclear Power Plants
4
Spent Fuel Pool Design
General Design Criteria (App A of 10CFR50 and RG 1.13):• The spent fuel pool structures are designed as
Seismic Category 1:• Suitable shielding for radiation protection• Appropriate containment, confinement, and
filtering systems• Residual heat removal capability that reflects
the importance to safety of decay heat• Ability to prevent significant reduction in fuel
storage coolant inventory under accident conditions
5
Cooling of Spent Fuel
6
Typical BWR Fuel Pool Cooling & Makeup System
The Fuel Pool Cooling and Cleanup System normally maintains the SFP water temperature, purity, clarity and level within limits. NOT A SAFETY RELATED SYSTEM!
7
SFP Makeup Systems
Spool Piece
SkimmerSurgeTanks
CondensateService Water
To RHR
FuelPoolCask
StorageSpent Fuel
Pool
Fuel PoolCoolingSystem
Reactor Well
Condensate & FeedwaterRHRCore SprayCRDRHRSWESWFire WaterCondensate SWWell WaterGSWSBLC
ESW HoseWell Water via ESW HoseFire Hoses on RB 855, 833, 812,786, and 757 LevelsCondensate SW HosesDemin Water Hoses
RHRSWESWFire WaterCond SWWell WaterGSW
RHR
Normal SFP Makeup Source
Alternate SFP Makeup Source
Additional SFP Makeup SourcesDuring Outages with ReactorCavity Flooded
Shield Blocks
Fuel Pool Gate
Portable Pump
8
Spent Fuel Assemblies Stored in SFP
Spent fuel assemblies have:• Radioactive fission products and actinides as a
result of critical operations• A heat source associated with the radioactive
decay of the fission products and actinides (that is, decay heat)
9
Past NRC-sponsored Analyses for Identification of SFP Severe Accidents
10
Early SFP Accident Management Guidance
• Typical SFP Accident Management Procedures– Alarm Response Procedures (Hi/Lo SFP Level)– Abnormal Operating Procedures (Loss of SFP Cooling)
• Historical industry focus was based on sequences associated with loss of Decay Heat Removal and SFP level
• PRIMARILY driven by refueling outage risk management
11
Terrorist Attacks – September 11, 2001
The 9/11 terrorist attacks brought a new focus on SFP accidents!• Zirconium fires re-introduced into the safeguards domain
• Phase 2 of NRC B.5.b response focused on SFP vulnerabilities and capabilities
12
Fukushima Daiichi Accident
Fukushima Daiichi accident brought new concerns with SFP accidents!The loss of cooling to the fuel pools for units 1, 2, 3, and 4 resulted in the pools heating up and ultimately reaching saturation or near saturation temperatures. The resultant evaporation reduced the spent fuel pool inventoriesThe loss of spent fuel pool cooling, coolant inventory, and makeup capability at the Fukushima Daiichi plant could have resulted in damage to stored spent nuclear fuel and significant radiological consequences to station personnel, the site, and the surrounding region
13
US Nuclear Industry Responses to Fukushima Daiichi Spent Fuel Pool Insights
• Key recommendations in INPO IER L1-11-2:– Verify implementation of recommendations of SOER 09-1, Shutdown
Safety, for SFP Safety Functions – When SFP Temp >200F, protect SFP heat removal and makeup systems– Identify time to 200F in event cooling is lost– Verify adequacy of SFP AOPs– Revise Station EOPs to monitor key SFP parameters during accidents
• NRC Order EA-12-051, Reliable Spent Fuel Pool Instrumentation• NRC Order EA-12-049, "Order Modifying Licenses with Regard to
Requirements for Mitigation Strategies for Beyond Design-Basis External Events“– Makeup with a Portable Injection Source Rate to exceed boil off for
design basis heat load – Hoses on deck – Connection to SFP cooling piping – Vent pathway – Spray capability (200 gpm/unit to pool)
14
Recent EPRI SFP Analyses
EPRI 1023403June 2011
Summary of EPRI Research Applicable to Nuclear Accident Scenarios
EPRI 1025058May 2012
Summary of the EPRI Early Event Analysis of the Fukushima Daiichi Spent Fuel Pools Following the March 11, 2011 Earthquake and Tsunami in Japan
EPRI 1025206August 2012
Impacts Associated with Transfer of Spent Nuclear Fuel from Spent Fuel Storage Pools to Dry Storage After Five Years of Cooling
EPRI 1025295October 2012
Severe Accident Management Guidance Technical Basis Report (Volume 1: Candidate High-Level Actions and Their Effects)
EPRI 3002000498 May 2013
Spent Fuel Pool Risk Assessment IntegrationFramework (Mark I and II BWRs) and Pilot Plant Application
EPRI 3002000499May 2013
Spent Fuel Pool Accident Characteristics
EPRI 1025750April 2013
Fukushima Technical Evaluation, Phase 1—MAAP5 Analysis
15
EPRI 3002000499 - Spent Fuel Pool Accident Characteristics (2013)
• The SFP contains a significant radioactive source term
• No hazard to the public as long as fuel is covered by water in the spent fuel pool or suitable alternative storage location
• The risk associated with the release of radioactivity from the SFPs varies with class of plant
16
Spent Fuel Pool Critical Safety Functions
Spent Fuel Pool Critical Safety Functions:• Reactivity control• Inventory control• Temperature control• Radioactivity control• Combustible gas control
Exceptions to the critical support functions compared to those applied to the RPV are the following:• No high-pressure injection is needed• No depressurization method is required• There is not a containment, only a confinement
EPRI TBR 3002000499 - Spent Fuel Pool Accident Characteristics
17
Installed Systems Available to SatisfySFP Critical Safety Functions
Reactivity Control
Inventory Control
Temperature Control
Radioactivity Control
• Fuel pool racks• Poison curtain or
poison plates• Liquid poison
solution• Borated water
from the refueling water storage tank (PWR)
• Fuel pool cooling and cleanup system
• Condensate transfer• Demineralized Water• Fire protection
system water• Refueling water
purification system• SFP cooling and
purification system• Primary water
systems• CVCS flow (PWR)• RHR fuel pool assist• ECCS (via the RPV
when fuel transfer canal is open)
• Fuel pool cooling• RHR in fuel pool
assist
• Standby gas treatment system (BWR)
• Building Ventilation and filtration system
• Secondary containment / isolation
No design features for control of combustible gas in the SFP at many plants
EPRI TBR 3002000499 - Spent Fuel Pool Accident Characteristics
18
SFP Initiating Events
EPRI TBR 3002000499 - Spent Fuel Pool Accident Characteristics
Loss of SFP Cooling• LOOP• Seismic LOOP• Weather-related LOOP (for
example, hurricane, tornado)• Equipment Failures• Internal flood• External flood
Loss of SFP Inventory• External event causing
structural failure of the SFP• Failure of the SFP gate• Turbine missiles• Cask or heavy load drop
results in structural damage to the SFP
• Siphoning of the pool• Primary System LOCA
Core Damage Events• At power• At shutdown
19
SFP Hazard Impacts
EPRI TBR 3002000499 - Spent Fuel Pool Accident Characteristics
20
New Focus on SFP AMG
• BWR and PWR Owner’s Groups developed SFP accident management guidance in both EOPs/SAGs
• Similar efforts performed for other plant types in other countries
21
EPRI 1025295 - SAMG TBR Insights
Loss of water Fuel Clad Heatup Fuel Swell / Burst Gap Release Zirconium Oxidation
Zirconium Fire Fission Product Release
22
EPRI 1025295 - SAMG TBR Insights
SFP Candidate High Level Actions (CHLA):• Inject into the SFP • Spray the SFP
23
SFP AMG Strategies
Strategies:o Water injection & Spray capability using ALL available methods
including permanent and portable equipmento Decay heat removal using ALL available methods including permanent
and portable equipmento Radiation levels in SFP area
Use of SFP sprays o Hydrogen control
Operating HVAC Venting Refuel Floor areas Use of H2 Recombiners and Igniters
Supporting Tools & Computational Aides:o Decay heat for various SFP configurationso Boil-off rate of water in the pool and leakage rate
Identify margins to limits and timing of actionso Estimates for hydrogen and oxygen production rates o Estimate increases in dose rates corresponding to lowering SFP level
24
Decay Heat as a Function of Time for a Spent PWR Assembly
25
SFP Level as a Function of Time following a Loss of SFP Cooling
55 Hours
110 Hours
165 Hours
220 Hours
26
Time to Boil & Time to Uncover Fuel following Loss of SFP Cooling
27
Dose Rates from a Drained SFP
NRC Response Technical Manual 96 (NUREG/BR-0150) provides a basic indication of area dose rates corresponding to a typical spent fuel pool that has drained.
A ground level whole body gamma dose of ~ 100 mSv/hr (10 rem/hr) corresponds to a point 100 m from the edge of the drained pool.
28
• Reliable SFP Level Instrumentation• Hydrogen Detectors in areas near the SFP• Hydrogen Vents in buildings housing SFP• Hydrogen Igniters / Recombiners in SFP area • Connections to utilize portable equipment
Plant Modifications for SFP SAMG
29
Plant Modifications for SFP SAMG
SFP Level Instrumentation
Hydrogen Detector Hydrogen Vent
Connections to Use Portable Equipment
30
NEI 12-02: Guidance for Reliable Spent Fuel Pool Instrumentation
• Level 1 – level that is adequate to support operation of the normal fuel pool cooling system
• Level 2 – level that is adequate to provide substantial radiation shielding for a person standing on the spent fuel pool operating deck (~10 feet above fuel)
• Level 3 – level where fuel remains covered and actions to implement make-up water addition should no longer be deferred (Top of Rack)
The level breakpoints support key SAMG decision making points.
31
• SAMG entry conditions established for scenarios with and without in-vessel core damage– Entry to SAMG typically based on SFP water level or area
radiation levels• Westinghouse SFP SAMG:
– SAG: Refill the Spent Fuel Pool– SCG: Recover Spent Fuel Pool Level– SCG: Mitigate Fission Product Releases
• US BWROG SAMG– SFP level and temperature control– Secondary Containment Area Temperature, Level, and
Radiation control
Addition of SFP into SAMG
32
Example – BWR AMG Addressing SFP
33
Example – PWR AMG Addressing SFP
Accident Management Guidance for Shutdown Conditions
35
OSART Highlights related to Severe Accident Management
Trends:• The SAM strategies and arrangements are not complete or
robust enough to ensure capability to take effective countermeasures. (2/12)
• The Accident Management guidelines and procedures do not cover all operation modes or are not in place. (6/12)
• The scope of the Severe Accident Management guidance does not systematically address accidents involving multiple units. (3/12)
• The verification and validation process for AM procedures and guidelines is not comprehensively described in dedicated procedure or not applied effectively. (3/12)
35
36
Why is AMG for Shutdown Conditions Important?
33 percent of a station’s overall risk is associated with shutdown periods
Plant configurations are different Many systems / components are out of service Small amount of coolant during certain phases Some automatic features not available Many activities take place Many personnel, unfamiliar with the plant are
working during the outage
Events initiated from cold shutdown or refuel conditions, security events, and widespread catastrophic failures of major plant structures were not specifically considered in the original development of EOPs/SAGs.
37
Operating Verses Shutdown Conditions
38
Variability in Outage Conditions - BWR
Common SDC
Switchyard Work for NERC relays and Hiawatha Line
Plant Startup to Generator on-line
FP Gates installed
Lowered Inventory
Lowered Inventory
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed Thu Fri Sat Sun Mon
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
OPDRVS, RECIRC
LPRM
s CRDMs
Class 1 leak TestRX/Cavity
Water Level
FS # 1 60 hours
Normal Rx Level
Torus Water Level
A RHR Work
B SDC in service
1T035 -SBDGs inop but available
First Fuel Shuffle
Second Fuel
Shuffle
A EDG work
A CS Work
“B” LO
OP LO
CA test
All RHR not available for injection
B EDG work
FS # 272 hours
125 VDC Div 2 work
125 VDC Div 1 Work
Generator O
ffline to Cold S/D
1 32 4 5 6 7 98 10Risk Sectors
11
Friction Testing
Scram time testing
FP Gatesremoved
Rx level at flange
TensionRPV Head
12
B RHR Work
Secondary Containment Work
RWCU work
B CS
“A” LOO
P LOC
A test
A SDC in service B SDC
39
Variability in Outage Conditions - PWR
U # 1 9 8 R F OU # 1 9 8 R F OU # 1 9 8 R F O
RCSINVENTORY
U # 1 9 8 R F O U # 1 9 8 R F O U # 1 9 8 R F O
MODE6
DRAFTPoint Beach U1R32 OUTAGE SCHEDULE (Spring 2010)Point Beach U1R32 OUTAGE SCHEDULE (Spring 2010)
Flange 70%
COOLINGPATH
COOLINGPATH
ELECTRICALPATH
ELECTRICALPATH
RCS & ECCSRCS & ECCS
MODE4
MODE3
Normal Pzr Lvl
TURBINE GENERATOR
TURBINE GENERATOR
EPU &PROJECTS
EPU &PROJECTS
Approximate Duration = 35 DaysApproximate Duration = 35 Days
MODE4
MODEMODE33
MODE5
Date: Feb 06, 2010Date: Feb 06, 2010
SYNC TOGRID
MODE2
DefueledWindow
DefueledWindow
3/28Sun28
3/27Sat27
3/26Fri26
3/25Thu25
3/24Wed24
3/23Tue23
3/22Mon22
3/21Sun21
3/20Sat20
3/19Fri19
3/18Thu18
3/17Wed17
3/16Tue16
3/13Sat13
3/12Fri12
3/11Thu11
3/10Wed10
3/9Tue
9
3/8Mon
8
3/7Sun
7
3/6Sat6
3/5Fri5
3/4Thu
4
3/3Wed
3
3/2 Tue
2
3/1Mon
1
3/14Sun14
3/15Mon15
3/29Mon29
4/4Sun35
4/3Sat34
4/2Fri33
4/1Thu32
4/10Sat41
3/31Wed31
3/30Tue30
4/7Wed38
4/8Thu39
4/9Fri40
4/6Tue37
4/5Mon36
4/11Sun42
MODE5
Pzr Solid
19 Hours off line to mode 5
Flange 70%ORT 3
CORERELOAD
COMPLETE
CORERELOADSTART
COREOFFLOAD
COMPLETE
COREOFFLOAD
START
3/28Sun28
3/27Sat27
3/26Fri26
3/25Thu25
3/24Wed24
3/23Tue23
3/22Mon22
3/21Sun21
3/20Sat20
3/19Fri19
3/18Thu18
3/17Wed17
3/16Tue16
3/13Sat13
3/12Fri12
3/11Thu11
3/10Wed10
3/9Tue
9
3/8Mon
8
3/7Sun
7
3/6Sat6
3/5Fri5
3/4Thu
4
3/3Wed
3
3.2 Tue
2
3/1Mon
1
3/14Sun14
3/15Mon15
3/29Mon29
4/4Sun35
4/3Sat34
4/2Fri33
4/1Thu32
4/10Sat41
3/31Wed31
3/30Tue30
4/7Wed38
4/8Thu39
4/9Fri40
4/6Tue37
4/5Mon36
4/11Sun42
SWITCHYARD WORK WINDOW #1
BUS SECT 2 ATC WORK
X-01 MAIN TRANSFORMER WORK
B RHR AND RECOVERY
COMMON RHR
A RHR AND RECOVERY
MAIN TURBINE AND AUXILIARIES
RWST COMMON SUCT LINE
OPEN GENOPEN GENBKRBKR
BUS SECTION 1 ATC WORK
X-03 WORK
EPU AUXILIARY FEEDWATER MODIFICATION
B-04 WORK
A-06 WORKA-01 ENERGIZED WORK
B-41 WORK
B-42 WORK
A-06 ENERGIZED WORK
Y05
A-01 WORK
A05 ENERGIZED WORK
B-03 ENERGIZED WORK
Y104
ALTERNATE SOURCE TERM
GL08-01 GAS VOID
SUMP B
HEAD UNDRESS
HEAD LIFT
REMOVE U/I REMOVE CORE BARREL INSTALL U/I
HEAD SET10 YR ISI
REPLACE CORE BARREL RCS FILL&VENT
70% in
Pzr
Cavity Flooded 10 Yr ISICavity Flooded
A-02 ENERGIZED WORK
Solid
BUS SECT 2 ATC WORK
SWITCHYARD WORK WINDOW #2 SWITCHYARD WORK WINDOW #3
SWITCHYARD STOP WORK WINDOW #1 SWITCHYARD STOP WORK WINDOW #2 SWITCHYARD STOP WORK WINDOW #3
SWITCHYARD WORK WINDOW #4
BUS SECT 2 ATC WORK
A-04 ENERERGIGED WORK
D-04 WORK
A RHR HX LICENSE RENEWAL INSPECTION
Solid
40
• Identified Five Key Safety Functions during shutdown conditions
1. Decay Heat Removal Capability
2. Inventory Control3. Electrical Power
Availability4. Reactivity Control5. Containment - Primary /
Secondary• Required procedures to be
developed for the loss of each Safety Function
NUMARC 91-06 – Industry Guidelines for Shutdown Safety (1991)
41
• AMG should be established to address loss of RCS inventory during shutdown conditions. The guidance should consider the following:
– identifying the potential source and magnitude of the loss – providing sufficient makeup capability– coping with high radiation levels in containment
• AMG should prioritize available alternate cooling methods (e.g., gravity feed and bleed, low pressure pump feed and bleed, high pressure pump feed and bleed, etc.) for conditions that are planned for the outage
• AMG should be established to address loss of power during shutdown conditions
• AMG should include guidance to maintain adequate shutdown margin in the RCS and spent fuel pool
• Containment closure - hatches (equipment and personnel) and other penetrations that communicate with the containment atmosphere should either be closed or capable of being closed prior to core boiling following a loss of DHR and should be addressed in procedures
NUMARC 91-06 Accident Management Guidance Expectations
42
Key Insights• Outage planning is crucial to safety during
shutdown conditions since it establishes the level of mitigation equipment available
• Well-trained and well-equipped plant operators play a very significant role in accident mitigation for shutdown events
• PWR accident sequences involving loss of RHR during operation with a reduced inventory (e.g., mid-loop operation) are dominant contributors to the core-damage frequency
• Extended loss of decay heat removal capability in PWRs can lead to a LOCA caused by failure of temporary pressure boundaries in the RCS or rupture of RHR system piping. In either case, the containment may be open and ECCS recirculation capability may not be available.
• Passive methods of decay heat removal can be very effective in delaying or preventing a severe accident in a PWR
• All PWR and Mark III BWR primary containments are capable of providing significant protection under severe core-damage conditions, provided that the containment is closed or can be closed quickly
• Mark I and II BWR secondary containments offer little protection, but this is offset by a significantly lower likelihood of core damage in BWRs
NUREG-1449 – Safety During Shutdown and Low Power Operation (1993)
43
Key Operations Recommendations• Shift manager maintains overall
responsibility for control of the key safety functions
• Consider standby equipment as “available” only when it can be made operational by automatic or simple operator actions
• Minimize the time at lowered inventory• Protect systems and equipment that
operate to support a key shutdown safety function
• Verify that AOPs/EOPs for mitigating challenges to shutdown safety such as a loss of shutdown or spent fuel pool cooling can be performed as written based on the outage system/equipment configurations
• Develop contingency plans when equipment required by the procedures will not be available
• Provide training on procedures for events such as loss of inventory, shutdown cooling, fuel pool cooling, containment integrity, and off-site power.
INPO SOER 09-01 – Shutdown Safety
44
EPRI 1025295 - SAMG TBR InsightsEPRI TBR Revised after the Fukushima Daiichi Accident:• Fukushima Daiichi Units 1, 2, and 3 were
operating at full power at the time of the earthquake, and Units 4, 5, and 6 were in a shutdown state
• The loss of cooling to units in a shutdown state highlighted the need for additional considerations relating to the challenges in deploying particular management strategies, which is a necessary component of the decision-making process
• With respect to external events, the consideration of the capability to cope with a severe accident progressing from the complete range of operational configurations is a key means of ensuring robustness of site SAMG implementations
• The effects of the various CHLAs on plant conditions during other modes of operation, and particularly during shutdown, depends on the configuration of the plant when the CHLA is implemented
45
EPRI SAMG TBR – Operational Configurations Verses Defined Modes (PWR)
Mode 1 - power is >5% full powerMode 2 - startup mode, power <5%full powerMode 3 - hot standby, reactor critical & Tave >[330]°FMode 4 - hot shutdown, reactor critical and RCS Tave is between [330]°F and [200]°FMode 5 - cold shutdown, reactor is critical and RCS Tave is less than [200]°FMode 6 - refueling mode. Transition between 5 to 6 when RPV head bolts are less than fully tensioned
46
EPRI SAMG TBR – Operational Configurations Verses Defined Modes (BWR)
Mode 1 - encompasses all operational states associated with power operation.
Mode 2 – reactor startup mode
Mode 3 - hot shutdown operational states
Mode 4 - contains all the operational states associated with cold shutdown conditions
Mode 5 - encompasses the operational states associated with reactor refueling
47
EPRI SAMG TBR – CHLAs
Many CHLAs are not applicable, viable, or are diminished during Shutdown Conditions
48
Shutdown SAMG – Scope of Applicability
49
Shutdown SAMG - Development Process
Review the existing shutdown PSAs to gain insights with regard to:• dominant accident sequences and initiators,• vulnerable plant states,• time to boiling, time to core damage, and time
to containment failure,• consequences of core damage, and• the symptoms of severe accident phenomena.
50
Shutdown SAMG - Development Process
Review the existing emergency operating procedures. The objective of this review is to identify:
• changes required to EOPs to accommodate shutdown conditions
• core damage diagnostic criteria for shutdown reactor states
• conditions for entry into SAMG for accident sequences not covered by Shutdown EOPs
• Transitions from Shutdown EOPs to SAMG.
51
Shutdown SAMG Considerations
• Traditional methods for determining core damage may not be applicable (RPV water level and Core Exit Thermocouples). Consider indirect indications:– Containment radiation– Containment hydrogen– Ex-core neutron flux
• Challenges to core damage diagnostics and accident mitigation actions:– Whether the RPV head has been removed or not– Potential loss of automatic start or isolation functions– Potential openings in the RCS– Status of the RCS injection and containment spray systems– Viability of instrument indications
• Criteria and priorities for the use of available equipment and systems (use for reactor, containment, or SFP)
52
Sample PWR Diagnostic Flow Chart
NEW
ShutdownSAMG
EXISTING
At-powerSAMG
53
PWR Shutdown SAMG - Elements
May be integrated with
SACRG-1,2
SACRG-3 & 4 when RCS T < 180°C
or RHRS operated
One more
SAG-8
54
Example - Shutdown DFC Guidance
55
Example - Shutdown AMG Guidance
56
Example – Areva Operating Strategies for Severe Accidents (OSSA)
57
CONCLUSIONS
• SAMG must be modified and extended to effectively cover ALL plant operating states. Most common gap found during OSART inspections.
• Plant-specific Shutdown Accident Analyses is a key prerequisite to the success of Shutdown SAMG development.
• Procedures for Accident Management during shutdown (EOPs, SAMGs) represent cost effective measures to improve shutdown safety.
• During shutdown modes, several conditions are favourable with respect to core quenching by alternate Accident Management measures such as mobile equipment. – Long time windows
– Core degradations starts considerably later after fuel uncovery
58
CONCLUSIONS
• Shutdown risk with respect to large early releases (LERF) is dominated by scenarios with failure to reclose the containment equipment hatches or airlocks
• To support validation, Operator and TSC training, upgrade to Full Scope Simulators is recommended to support high fidelity simulation of Shutdown states, including:– low reactor inventory states,
– open reactor and open containment states,
– refuelling operations, and
– Spent Fuel Pool accidents.
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