international atomic energy agency l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
International Atomic Energy Agency PowerPoint Presentation
Download Presentation
International Atomic Energy Agency

Loading in 2 Seconds...

play fullscreen
1 / 50

International Atomic Energy Agency - PowerPoint PPT Presentation


  • 662 Views
  • Uploaded on

International Atomic Energy Agency REGULATORY GUIDANCE ON REVIEW OF BEYOND DESIGN BASIS AND SEVERE ACCIDENTS WITHIN THE FRAMEWORK OF LICENSING Jozef Misak –SAS/NSNI

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'International Atomic Energy Agency' - niveditha


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
international atomic energy agency
International Atomic Energy Agency

REGULATORY GUIDANCE ON REVIEW OF BEYOND DESIGN BASIS AND SEVERE ACCIDENTS WITHIN THE FRAMEWORK OF LICENSING

Jozef Misak –SAS/NSNI

Workshop on “Safety Analysis Report, Safety Analysis in Licensing, including Analysis of Severe Accidents, and EOPs/SAMGs Development and Review”

23 – 26 February 2004, Islamabad, Pakistan

contents
Contents
  • Categorization of analysis and steps in analysis
  • Objectives and scope of analysis
  • Computational tools (computer codes)
  • Selection of initiating events and scenarios
  • Interpretation of results
  • Review methodology
basis
Basis
  • Each NPP should have an AMP
  • AMP includes preventive (EOP domain) and mitigative parts (SAMG domain)
  • Safety Analysis Report should include BDBA and severe accident analyses to the extent needed for AMP and emergency planning
  • Role of analytical support for AMP is crucial since often it is the only way for predicting the course of accidents
  • More precise meaning of these requirements to be specified
slide4

COVERAGE OF NPP ABNORMAL REGIMES AND ACCIDENTS BY PROCEDURES AND GUIDELINES

Incidents

Normal

operation

Design basis

accidents

Beyond design

accidents

Severe

accidents

Power

operation

EMERGENCY

OPERATING

PROCEDURES

SEVERE

ACCIDENT

GUIDELINES

ABNOR-

MAL

PROCE-

DURES

Hot

shutdown

Intermediate

shutdown

Cold

shutdown

RCS

open

slide5

Organization of activities in typical SAMG approach (Westinghouse)

Reactor trip,

ESFAS

CSF

CHALLE–

NGED

CORE

DEPLE-

TION

CORE

DEGRA-

DATION

RPV

CHALLENGE

NORMAL

OPERA -

TION

AB-

NORMAL

OPERA -

TION

EVENT

ORIEN –

TED

CONTAINMENT CHALLENGE

earlylate

CONTROL

ROOM

NOP

AOP

Actions directed by TSC

EOP/

event

EOP/

SF restor.

SA CRG 1

SA CRG 2

TECHNICAL

SUPPORT

CENTER

Information

Instructions

SAMG

TSC activation

EMERGENCY

CENTER

Communication

EMERGENCY PLAN

slide6

Steps in development and implementation of AM (EOP/SAMG)

Team formation

Selection of approach, boundary conditions

Perform DBA and BDBA analysis

Development of adequate EOPs

Perform severe accident analysis

Ensure availability of information

Select suitable AM strategies

Develop guidelines for selected strategies

Set-up staff responsibilities

Validate AM guidelines

Organize appropriate training

Incorporate new developments

slide7

AMP Developer

Analyst

Define approach

Identify

Preliminary phase

Challenges and

analysis without

vulnerabilites

operator actions

High

level strategies

Development

phase analysis for

strategies and

guidelines

Guidelines /

detailed

strategies

Implementation

Implementation

/

phase analysis

validation

Relation between AMP Developers and Supporting Analysts

general objectives of analytical support
General objectives of analytical support
  • Understanding of the behaviour of a specific plant during BDBA and severe accidents, to determine which accident phenomena are important for a specific design, and to understand and rank the challenges to FP boundaries (containment in particular)
  • Understanding the plant’s capabilities and vulnerabilities, and to provide a sound basis for subsequent investigations of preventive and mitigative AM measures
slide10

Categories of plant specific analysis in support of AMP

  • preliminary analysis that are needed for evaluating basic strategies of EOPs and SAM guidelines
  • procedure and guideline development analysis that are needed for confirmation of strategies and set-point calculations
  • verification analyses for procedures and guidelines
preliminary phase analysis purposes
Preliminary phase analysis - purposes
  • Understand plant response to BDBA/severe accidents
  • Identify nature and importance of challenges to FP boundaries
  • Identify timing of challenges
  • Identify symptoms (plant parameters indicative of challenge)

A good PSA Level 1+2 often contain adequate severe accident analyses for these needs. If there is no PSA, then new analyses are necessary

development phase analysis purposes
Development phase analysis - purposes
  • Evaluate systems capabilities
  • Confirm choice of symptoms and strategies
  • Support set-point calculations
  • Support development of computational aids (for SAM)
  • Investigate potential plant modifications
implementation phase analysis purposes
Implementation phase analysis - purposes
  • Demonstrate capabilities and choice of strategies
  • Optimize strategies
  • Pre-analysis of validation and exercise scenarios
  • Support to training
comparison of important phenomena before and after core damage
BEFORE (EOPs)

pressure waves inside RPV

pressurized thermal shock

mechanical impact of escaping coolant

reaction forces on components

direct releases due to containment by-pass

containment pressurization

limited radioactivity releases from the containment or due to containment by-pass

AFTER (SAMGs)

direct containment heating due to high pressure expulsion of the corium

hydrogen or carbon monoxide combustion (deflagration /detonation), local or global

core-concrete interactions (containment foundation melt-through)

long term containment pressurisation (decomposition of concrete, steam)

major radioactivity releases

Comparison of important phenomena before and after core damage
comparison of important phenomena before and after core damage15
BEFORE (EOPs)

power excursions or recriticality

boiling crisis

overheating, damage of cladding

limited exothermic Zr-water reaction, limited H2 production

primary/secondary system pressurization

AFTER (SAMGs)

formation of eutectics

melting of the cladding, fuel and core materials

downward relocation of the corium

massive Zr-water reaction and production of hydrogen

interaction of corium with residual water, potential steam explosions

heating of the RPV by corium

Comparison of important phenomena before and after core damage
slide16

Challenges to barriers resulting from severe accidents

  • cladding damage;excessive heat-up in combination with pressure difference acting on cladding leads to loss of cladding integrity with gap release
  • fuel melting and core degradation;FPs accumulated in the fuel matrix are released
  • fuel-coolant interaction in the RPV;steam explosion with potential generation of missiles and additional dynamic loading of the reactor coolant system (RCS)
  • high-pressure melt ejection (HPME) from the reactor vessel;with direct containment heating leading to rapid increase of containment temperature and pressure
slide17

Challenges to barriers resulting from severe accidents

  • slow RPV melt through;with a possibility of ex-vessel steam explosion, generation of missiles, dynamic loads of the containment and ex-vessel containment phenomena
  • hydrogen combustion;(deflagration/detonation) leading to fast loading with possible early containment failure
  • containment overpressurization;due to generation of steam or non-condensable gases from decomposition of the containment concrete and combustion of combustible gases
  • core-concrete interaction;possible loss of containment integrity due to basemat melt-through
  • containment by-pass;e.g. steam generator (SG) tube rupture or damage or interface systems and direct release of reactor coolant and FPs to outside containment
two categories of analytical support
Two categories of analytical support
  • Level of understanding of phenomena is different for preventive (EOP) and mitigative (SAM) domain
  • Due to different level of understanding the phenomena and different objectives, analytical support should be separated into support of EOPs and support of SAMGs
  • Because of the need to analyze integrity of barriers, not only system TH codes, but also other detailed codes (e.g. coolant mixing, structural analysis) should be used
slide19

Computer

codes for

analytical

support of AM

Prevention

Mitigation

(EOP domain)

(SAM domain)

System BE

Special codes for

analysis of indi-

vidual phenomena

(mixing, PTS)

Integral fast run.

Detailed codes

Detailed codes

Special codes

Ther.-hydraulic

codes for analys.

for analysis

for analysis

for analysis of

codes

both in- and ex-

of in-vessel

of ex-vessel

individual

vessel phenom.

phenomena

phenomena

phenomena

RELAP5/MOD3

MAAP 4

SCDAP/RELAP5

CONTAIN

ATHLET

MELCOR 1.8.

ATHLET-CD

COCOSYS

CATHARE V1

ASTEC

ICARE/CATHARE

GOTHIC

examples symptoms and am entry point
Examples: symptoms and AM entry point
  • SG water level
  • RCS or secondary system pressure
  • Core exit temperature
  • Containment water level
  • Containment pressure
  • Containment hydrogen concentration
  • Site radioactivity releases
slide22

Examples: preventive strategies for PWRs

  • Manual restoration of` systems aimed at restoration of efficient heat removal
  • Primary circuit feed and bleed to restore primary side cooling
  • Secondary circuit feed and bleed to restore secondary side cooling
  • Primary circuit depressurization to allow for injection from low o pressure water sources
  • Secondary circuit depressurization to allow feeding the stem generators (SGs);
  • Restart of the main circulation pumps to transport residual coolant to the core
basic features of eop supporting analysis
Basic features of EOP supporting analysis
  • Analysis to be done using an up-to-date best estimate approach, including BE codes and models, BE data and BE assumptions
  • Considerations of the most probable response of both plant systems and operators need to form the base case of the analysis; reasonable agreement of calculations with reality is most important, while the speed of calculations is less important
  • If the strategy applied allows the operator to choose among various systems which have similar safety functions, analysis considering various possibilities and combinations of such systems needs to be performed
basic features of eop supporting analysis24
Basic features of EOP supporting analysis
  • If the value of certain parameters can affect the necessary actions significantly, sensitivity studies need to be performed
  • The available plant instrumentation needs to be modelled in order to confirm that the event can be diagnosed and to check the steps in the procedure
  • The performance of systems (e.g. instrumentation) not included in the model, but potentially affecting the course of accident, needs to be considered; in some cases, this can be accounted for by changing the input sequence of events in the model or assuming a range of variation for relevant parameter
  • QA including independent assessment of the analysis results should be arranged
analytical support for critical safety functions restoration examples
Analytical support for critical safety functions restoration - examples
  • Subcriticality:
    • ATWS - capability to borate the RCS under high pressure conditions with existing boration systems
    • FW flow control and its impact on reactor power (due to coolant temperature reactivity feedback)
    • recriticality aspects - loss of shutdown margin due to RCS cooldown (resulting from excessive secondary side heat removal) or RCS boron dilution
  • Core cooling
    • RCS depressurisation strategies based on heat removal into the secondary side (i.e. dumping of steam from SGs to available sinks - turbine condenser, atmosphere) and primary side (i.e. pressurizer SVs and RVs) for different RCS cooling rates (depending on the severity of challenge to the core cooling CSF)
    • effectiveness of the RCPs restart for delay of core degradation;
    • FRGs entry conditions/set-points (i.e. 650°C).
analytical support for critical safety functions restoration examples26
Analytical support for critical safety functions restoration - examples
  • Heat removal:
    • Entry condition/set-point for application of the feed&bleed strategy (SG level, RCS temperature, etc.)
    • Time window for successful feed&bleed for plant specific feeding and bleeding capabilities (i.e. coolant delivery versus RCS relief capacities)
    • Strategy how to reduce number of HP SI pumps/open PRZR SVs in order to depressurise the RCS while maintaining adequate cooldown rate (avoiding PTS) and transfer from HP SI to LP SI pumps
    • Ultimate RCS temperature achievable by feed&bleed strategy - possibility to transfer to RHR system for stable long term residual heat removal
    • Thermal stress analysis/assessment for recovery of FW into a hot SG, limits
  • Reactor Pressure Vessel (RPV) integrity
    • PTS
    • cold overpressurisation
analytical support for critical safety functions restoration examples27
Analytical support for critical safety functions restoration - examples
  • Containment integrity:
    • containment pressure response to pipeline breaks
    • strategies for operator control of containment pressure by operation of sprays
    • containment underpressure concern
  • Station blackout:
    • effectiveness of RCS depressurisation by heat removal into the secondary side (dumping of SG steam – entry condition)
    • effectiveness of RCS depressurisation by the pressurizer SVs/RV - entry condition
    • shutdown margin provided by HA injection
    • preferred sequencing of depressurisation steps (primary versus secondary depressurisation)
slide28

Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

slide29

Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

slide30

Some Important Issues in BDBA Preventive AM Typically Requiring Thermal Hydraulic Analysis (examples)

slide31

Examples of analyses for development of preventive

(Critical Safety Function restoration part) and mitigative AM

slide32

Examples of analyses for development of preventive

(Critical Safety Function restoration part) and mitigative AM (cont’d)

specific tasks for am supporting analysis
Specific tasks for AM supporting analysis
  • choice of key symptoms; confirmation of choice of symptoms for long-term processes
  • specification of set-points to initiate and to exit a strategy
  • prioritisation and optimisation of strategies; positive effects and possible negative effects of the strategy
  • evaluation of capability of systems to perform intended functions; expected trends in the accident progression (projections of the timing)
  • conditions for leaving SAM domain
  • specifications of environmental conditions for operation of instrumentation and NPP systems; recommendations for equipment/instrumentation upgrades
  • computational aid development
slide34

Examples: mitigative strategies for PWRs

  • Coolant injection to the degraded core (from any source)
  • Primary circuit depressurization to prevent HPME
  • External RPV cooling to avoid ex-vessel effects
  • Operation of hydrogen recombiners/igniters
  • Secondary circuit feeding to protect SG tube integrity and scrubbing of radioactive releases
  • Spraying of the containment to remove FPs from containment atmosphere, to reduce the containment pressure
  • Containment filtered venting to protect containment integrity
  • Operation of containment fan coolers
  • Containment injection to submerge RPV and to cool ex-vessel core debris
  • Containment inertization
slide35

Examples: contradictory effects for mitigative AM actions

  • Water added to the core cools the core but at the same time it may increase production of hydrogen from cladding-steam reaction
  • Spraying of the containment reduces the containment pressure but may increase volumetric concentration of hydrogen resulting in concentration inside the flammability limit
  • Injecting water into the SGs contributes to removal of the residual heat but may lead to damage of SG tubes and to the containment by-pass

Uncertainty in the NPP response to mitigation measures is the main reason why guidelines are used in the severe accident domain instead of structured EOPs

level of understanding of phenomena for in vessel analysis
Level of understanding of phenomena for in-vessel analysis
  • Well understood phenomena
    • Majority of phenomena in early phase of core degradation (boil-off, recriticality, reflooding before significant oxidation, cladding balooning, dissolution of fuel and other materials, …)
  • Low level of knowledge of phenomena
    • Hydrogen production during flooding of degraded core
    • Recriticality of degraded core
    • Steam flow through the degraded core
    • Formation of debris be
    • Formation of molten pool, formation of crust, its stability, break-through
    • Molten core relocation
level of understanding of phenomena for ex vessel analysis
Level of understanding of phenomena for ex-vessel analysis
  • Well understood phenomena
    • Both local and global containment pressurization
  • Low level of knowledge of phenomena
    • Long lasting processes, including late phase of in-vessel phenomena as a boundary condition
    • Natural convention in the containment
    • Heat exchange with structures
    • Temperature stratification (typically underpredicted)
    • Hydrogen distribution
    • Material interactions, mainly molten corium concrete interaction
most important aspects in modelling
IN-VESSEL :

Natural circulation in RPV, RCS

Radiation heat transfer

Oxidation during heat up and quenching

Formation of eutectics, dissolution of UO2

Formation of flow blockages in the core, core coolability

Melt interactions with supports

Melt fragmentation coolant

Coolability of the reactor lower head

EX-VESSEL :

Prediction of gas flows (steam, non-condensable gases)

Hydrogen production, distribution, deflagration

Melt release

Corium-concrete interactions, effect of water injection, coolability

Relocation, spreading of melt

Most important aspects in modelling
slide39

Example for severe accident phenomena significant for potential challenges to fission product boundaries

slide41

Identification of Suitable Accident Sequences for the Investigation of Hydrogen Deflagration Challenges

slide42

Components of safety analysis for AM

Selection of analytical tools

Collection of information needed for analysis

Selection of accident sequences

Analysis without operator action

Evaluation of capabilities and

limitations of existing equipment

Identification and analysis of preventive measures

Identification and analysis of mitigative measures

Consideration of uncertainties

regulatory review of safety analysis for am issues to be reviewed
Regulatory review of safety analysis for AM – issues to be reviewed

Selection of analytical tools

  • Select computer code reasonably covering the most important phenomena (MAAP, MELCOR, ASTEC
  • Consider how specific design features will be addressed by the code
  • Using code documentation, check validity of models and correlations for a given application
  • Estimate main uncertainties related to use of the code for a specific purpose
  • Specify other computational/experimental sources to complement the code calculation
regulatory review of safety analysis for am issues to be reviewed cont d
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Information needed for analysis

  • Collect plant specific data with clear references to data sources; attention to data with largest effect on results
  • Provide for verification of the data
  • Develop plant database and engineering handbook
  • Verify engineering handbook
regulatory review of safety analysis for am issues to be reviewed cont d45
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Selection of accident sequences

  • Select list of possible plant specific damage states (based on PSA or any other source of information - design specifications, operational experience, accident precursors, design specific experimental results, severe accident research, information from similar plants)
  • Specify groups of sequences to be analyzed (typically LB LOCA +LOECC, SB LOCA +LOECC, station blackout, loss of feedwater)
  • Select representative sequences for analysis (typically several tens of sequences)
regulatory review of safety analysis for am issues to be reviewed cont d46
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Analysis without operator action

  • Define important phenomena to be modeled
  • Select reasonable number of representative scenarios
  • Perform analyses of scenarios without any accident management action
  • Determine challenges to fission product boundaries, their timing and expected mode of a failure
  • Summarize results of analysis in a practicable format
regulatory review of safety analysis for am issues to be reviewed cont d47
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Identification and analysis of preventive measures

  • Specify possible preventive measures
  • Select symptoms to be used to initiate preventive strategy (core exit temperature, SG level, containment pressure, radioactivity level, …)
  • Define proper timing (time window) to initiate the action
  • Confirm effectiveness of individual preventive measures
  • Define order of priority for different preventive actions (design specific)
  • Consider uncertainties in specifying preventive measures
  • Prepare complete and concise report with results of analysis (large number of calculations, user-friendly format)
regulatory review of safety analysis for am issues to be reviewed cont d48
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Identification and analysis of mitigative measures (similar as for preventive measures)

  • Specify possible mitigative measures
  • Select symptoms to be used to initiate preventive strategy (containment pressure, hydrogen concentration, ra- level, …)
  • Define proper timing (time window) to initiate the action
  • Confirm effectiveness of individual mitigative measures
  • Define order of priority for different mitigative actions (design specific)
  • Consider uncertainties in specifying mitigative measures
  • Prepare complete and concise report with results of analysis (large number of calculations, user-friendly format)
regulatory review of safety analysis for am issues to be reviewed cont d49
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Evaluation of capabilities and limitations of existing instrumentation and control

  • Confirm availability of measurements for selected symptoms
  • Specify environmental conditions for operation of I&C
  • Evaluate reliability of measurements, also outside their design range
  • Specify alternate measurements
  • Develop computational aids to replace/complement measurements
regulatory review of safety analysis for am issues to be reviewed cont d50
Regulatory review of safety analysis for AM – issues to be reviewed (cont’d)

Evaluation of capabilities and limitations of existing equipment

  • Specify environmental conditions for operation of the equipment
  • Evaluate functioning of the equipment, also outside its design range
  • Evaluate availability of power/media supply for operation of equipment
  • Define priorities in operation of different kinds of equipment
  • Specify alternatives for equipment and evaluate their performance