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.