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GIF Risk and Safety Working Group (RSWG):

Safety Approach of Gen-IV systems; Application to SFR

Gian Luigi Fiorini (CEA) & Tim Leahy (INL)

RSWG Co-Chairs

Consultants’ Meeting :“IAEA Workshop on Safety Aspects of Sodium Cooled Fast Reactors”IAEA Headquarters, Vienna 23 – 25 June2010

Purpose of the RSWG (Cf. RSWG Terms of Reference)

• “The primary objective of the RSWG is to promote a

consistent approach on safety, risk and regulatory

issues between Generation IV systems.

The RSWG will advise and assist the Experts Group

and the Policy Group particularly on:

Slide 2“IAEA Workshop on Safety Aspects of Sodium Cooled Fast Reactors”; IAEA Headquarters, Vienna 23 – 25 June2010

– Gen IV safety goals and evaluation

methodologies to be considered in the design

and thus to guide R&D plans;

– Interactions with the nuclear safety regulatory

community, the IAEA and relevant

stakeholders.”

Gen IV safety Goals

• Three specific safety goals to be used to stimulate the search for innovative nuclear energy systems and to motivate and guide the R&D on Generation IV systems:

– “Generation IV nuclear energy systems operations will excel in safety and reliability.


– Generation IV nuclear energy systems will have a very low likelihood and degree of reactor core damage.

– Generation IV nuclear energy systems will eliminate the need for offsite emergency response.”

�� The RSWG has focused on defining the attributes and identifying methodological advances applicable to Gen IV systems, that might be necessary to achieve and demonstrate achievement of these goals.

Inputs from the GIF - Senior Industry Advisory Panel (SIAP) Cf. the presentation of the SIAP chairman (Peter Wakefield – British Energy) at the GIF-PG meeting in November 2006

• SIAP asked the PG to implement a specific activity :

– to move from “Present Licensing Rules” toward “New

Higher Level Principles & Objectives”,

– leaving prescriptive notions as, for example, the


– leaving prescriptive notions as, for example, the

“Containment barriers” to go toward an approach by

“Safety functions” (Control of fission products)

� I.e. there is a need to be back to the fundamentals of the nuclear

safety.

� This is why the RSWG has focused on defining technology

neutral objectives and principles, and identifying

methodological approaches which are applicable to all the Gen

IV technologies.

RSWG’s report: Basis for the Safety Approach for the Design & Assessment of Generation IV Nuclear Systems

Delivered by the OECD/NEA on November 2008

• The first major work product of the RSWG,

• It presents findings and recommendations:

– objectives,

– principles,


– principles,

– attributes,

– tools, and

– crosscut R&D

• These findings and recommendations are intended to provide designers with concepts and methods, that can help designing and assessing on an agreed basis, and guiding their R&D activities in a way that promotes the safety basis and efficient licensing of advanced nuclear technologies.

a – Basis for the suggested Generation IV Safety Philosophy

I. Opportunities exist to further improve safety record through the implementation of a safety that will be “built-in” to the fundamental design rather than “added on” to the system architecture.

II. Safety improvements should simultaneously be based on several elements: ambitious safety objectives; the search for robust safety architecture; … the requirement for the improvement of the safety demonstration’s robustness.


robust safety architecture; … the requirement for the improvement of the safety demonstration’s robustness.

III. The need for a homogeneous strategy applicable for the design and the assessment of Gen IV systems justify re-examination of the traditional safety approach.

IV. “Defence in depth” must be preserved in the design of Gen IV systems.

V. The design process should be driven by a “risk-informed” approach.

VI. For Gen IV systems, in addition to prototyping and demonstration, modelling and simulation should play a large role in the design and the assessment.

b- Basis for the Design and Assessment of innovative systems

I. The Design Basis should cover the full range of safety significant conditions.

II. Updated safety analysis methods have to be applied to examine the full range of safety-significant issues.

III. Ways for achieving the objectives and for implementing the practices for the design improvement :

• critical and systematic examination and consideration of the feedback


• critical and systematic examination and consideration of the feedback experience;

• rationalization of the design approach by the deliberate adoption of ALARP

• implementation of the concept of defence in depth in a manner that is demonstrably exhaustive, progressive, tolerant, forgiving and well-balanced.

IV. Attention should emphasize the treatment of the severe plant conditions

V. The achievement of the safety demonstration’s robustness rests on the capacity of the designer and the developer to be exhaustive in the recognition of risks stemming from phenomena considered for the design

VI. Practical instruments & tools are suggested to be used by the designers to support the design activity as well as the assessment activities.

Pre-Conceptual Design Conceptual Design Final Design Licensing and Operation

Generation IVIntegrated Safety Assessment Methodology (ISAM)

PIRT

• Identify important phenomena

• Characterize state of knowledge

Qualitative Safety Requirements/Characteristic Review (QSR)

Primarily

Quantitative

Primarily

QualitativeFormulation →→→→ Refinement of Safety Requirements and Criteria


• Characterize state of knowledge

Deterministic and Phenomenological Analysis (DPA)

Probabilistic Safety Assessment (PSA)

• Demonstrate conformance with design intent and assumptions

• Characterize response in event sequences resulting from postulated initiating events

• Establish margins to limits, success criteria for SSCs in PRA, and consequences

OPT

• List provisions that assure

implementation of DiD

• DiD level → safety function →

challenge/mechanism → provisions

• Provides integrated understanding of risk and safety issues

• Allows assessment of risk implications of design variations

• In principle, allows comparison to technology neutral risk metrics

Qualitative Safety Features Review (QSR)• The Qualitative Safety Features Review is a “Checklist” based on

principle of defense in depth

• QSR provides a systematic means of ensuring and documenting that the evolving Generation IV system’s design incorporates the desirable safety-related attributes and characteristics that are identified and discussed in the RSWG’s report

• Using a structured template, the QSR :

– Provides a useful preparatory step to shape designers’


– Provides a useful preparatory step to shape designers’ approaches to their work to help ensure that safety truly is “built-in, not added-onto” since the early phases of the design of Generation IV systems.

– Helps designer qualitatively assess safety-related strengths and weaknesses of the evolving design

– From earliest phases of design, helps influence design selecting or discarding options among the provision important to safety

N.B. : A systematic crosscut comparison has been done between the QSR recommendations and the set of INPRO Basic Principles/ User Requirements. The analysis showed that improvements were required on both sides : RSWG QSR & INPRO (done for the RSWG QSR)

Example of application to the concept with the stratified REDAN : Comparison with the EFR concept


Phenomena Identification and Ranking Technique (PIRT)

• Applied initially in pre-conceptual design phase, and iteratively thereafter

• Used as an early “screen” to identify, categorize, and characterize phenomena and issues that are potentially important to risk and safety

• Can be focused on general issues, or on specific design questions, phenomenology, and temporal frames as needed

• PIRT can be used, e.g., to:


• PIRT can be used, e.g., to:

1) prioritize confirmatory research activities to address the safety-significant issues,

2) inform decisions regarding the development of independent and confirmatory analytical tools for safety analysis,

3) assist in defining test data needs for the validation and verification of analytical tools and codes,

4) provide insights for the review of safety analysis and supporting data bases, and

5) form input to PSA, and helps identify areas in which additional research is needed.

N.B. : PIRT relies heavily on expert elicitation

Gaps identification > Needs for further R&D effort

(4) Fully known; small

uncertainty

ILMH

Rank of PhenomenonAdequacy of knowledge

Knowledge Base Gap Determination

(4) Fully known; small

uncertainty

ILMH

Rank of PhenomenonAdequacy of knowledge

Knowledge Base Gap Determination


GAPGAPGAP(1) Very limited

knowledge; uncertainty

cannot be characterized

GAPGAP(2) Partially known; large

uncertainty

(3) Known; moderate

uncertainty

GAPGAPGAP(1) Very limited

knowledge; uncertainty

cannot be characterized

GAPGAP(2) Partially known; large

uncertainty

(3) Known; moderate

uncertainty

Objective Provision Tree (OPT)

• OPT is a practical tool for identifying and documenting provisions for accident prevention, control, and mitigation

• Applied iteratively from late pre-conceptual design stage through conceptual design

• Focuses on ensuring and documenting “lines of protection” in response to safety-significant phenomena, e.g. identified in PIRT

• Useful in structuring thinking about sequence phenomenology,


• Useful in structuring thinking about sequence phenomenology, success criteria, etc.

• It can be useful :

– In helping to focus and structure the analyst’s identification and understanding of possible initiators and mechanisms of abnormal conditions, accident phenomenology, success criteria, and related issues.

– In helping to identify, motivate and document the right requirements for the design of the implemented “lines of protection”

– To inform the design process and to help structure inputs that will eventually make their way into the PSA.

OPT for safety function 2 (core heat removal) at level 3 of DiD Ex. of JSFR

Level of Defense

Objective and Barriers

Safety function (SF)

Challenge

Level 3

Control of accidents within the design basis

Core heat removal

Acceptance criteria: adequate cooling of the fuel, vessel internals, vessel and reactor cavity by active/passive systems, via heat transfer to ultimate

heat sinks, ensuring core geometry, and reactor vessel integrity

Degraded or disruption of


Challenge

Mechanism

Provisions

Degraded or disruption of

heat transfer path

Long-term loss of

forced convection

in the 1ry circuit

Loss of ultimate

heat sink (e.g.,

2ry circuit, water

/steam system)

Partial loss of DHRS

functionality (e.g.,

DHRS leakage)

Leakage of coolant

in the 1ry circuit

(pipe break)

Automatic actuation

of DHRS (natural

convection and

battery-operated

air-cooler dampers)

Functional

redundancy of

DHRS

Adequate

margin to fuel

failure temp.

Heat transfer by

passive measure

(DHRS) (natural

convection and

battery-operated air-cooler dampers)

Insufficient provisions

at level 1 and 2

Layout of piping (high

position to maintain

reactor level)

Localization and isolation of

leaking Na (GV & double

wall piping)

Short-term loss of

forced convection

in the 1ry circuit

Rapid reactor

shutdown

Secure flow coast

down of 1ry circuit

Rapid reactor

shutdown

Deterministic and Phenomenological Analyses (DPA)

• Classical deterministic and phenomenological analyses

constitute a vital part of the overall Generation IV ISAM and are

essential to prove the effectiveness of design provisions

• These analyses will be used as needed to understand safety

issues that must guide concept and design development, and


issues that must guide concept and design development, and

will form inputs into the PSA.

• DPA will be used from the late portion of the pre-conceptual

design phase through ultimate licensing and regulatory

processes of the Generation IV system.

Probabilistic Safety Analysis (PSA)

• PSA serves as the focal point of the ISAM

• While they have inherent value on their own, other elements of the ISAM serve largely to support the PSA

• The PSA is a widely recognized methodology for assessing risk and safety issues


risk and safety issues

• Worldwide, PSA is increasingly an expected part of the licensing and regulatory process

• By integrating PSA into the earliest practical stages of the design process, designs can be more fully informed by insights and findings developed in the PSA

• The PSA is facilitated and informed by other elements of the ISAM

– Examples of preliminary ISAM applications to

innovative SFR:

� JSFR : PIRT, OPT, DPA and PSARef. : Ryodai NAKAI (JAEA/RSWG)

Practical Application to Gen IV System JSFR case; PIRT, OPT, DPA and PSA

RSWG Meeeting - Petten – April 2010


� French option for the internal structures, the « Stratified

Redan » : QSR , PIRTG.L. Fiorini (CEA/RSWG)

The Qualitative Safety Review (ISAM/QSR) & the Phenomena Identification Ranking Table (ISAM/PIRT)

RSWG Meeeting - Petten – April 2010

Summary of PIRT preliminary application

Identified the key phenomena to be considered in evaluating the

effectiveness of the SASS upon the ULOF accident.

JSFR Self Actuated Shutdown System ( SASS)


effectiveness of the SASS upon the ULOF accident.

The comparison of PIRT application results between the two different

time points showed that the knowledge level of the key phenomena

has been improved through the various experimental studies for the

SASS R&D.

PIRT can be helpful to identify needs for a key experimental study if it

is conducted before addressing a new R&D issue.

Design improvement: Non-safety-related AC blowers to enhance PRACS and DRACS capability

PRACSDRACS

Steam

PRACS

Steam

PRACSDRACS

Steam

PRACS

Steam MM MM MM


PRACS: Primary Reactor Auxiliary Cooling System

DRACS: Direct Reactor Auxiliary Cooling System

PHTS: Primary Heat Transport System

SHTS: Secondary Heat Transport System

Steam

Generator

Feedwater

Steam

Generator

Feedwater

PHTSSHTS SHTS

Steam

Generator

Feedwater

Steam

Generator

Feedwater

PHTSSHTSSHTS SHTSSHTS

� Following the method described in the draft RSWG/ISAM report,

applicability of PIRT, OPT, DPA and PSA to the JSFR system was

preliminarily examined.

� It is useful to show the adequacy of safety-related design/R&D

activities of JSFR.

• PIRT: confirmed the appropriateness for key R&D studies

• OPT: organized structure of safety-related provisions based

Summary for the JSFR


• OPT: organized structure of safety-related provisions based

on the DiD philosophy

• DPA: provided key information for the success criteria to be

defined in the PSA model

• PSA: assessed the level of safety quantitatively and provide

useful information for the system design improvement

The SFR concept with the stratified REDAN (CEA/DEN)

Redan Phénix

Piéce compliquée

Nombreuses traversées

Redan Phénix

Piéce compliquée

Nombreuses traversées

EFR type

Phenix typeSPX1 type


Pieds assemblage

Centre

coeur

sommier

Platelage

PEM

IHX

EMP for

the IHX

EPM

for the core

Two plates to

organize and

keep the

sodium

stratification

Decay heat removal

IHX

BCC

Hot

collector

Cold

collector

Input Output of

secondary sodium

Reactor roof

Thermocouple

poole

Pieds assemblage

Centre

coeur

sommier

Platelage

PEM

IHX

EMP for

the IHX

EPM

for the core

Two plates to

organize and

keep the

sodium

stratification

Decay heat removal

IHX

BCC

Hot

collector

Cold

collector

Input Output of

secondary sodium

Reactor roof

Thermocouple

poole

Stratified REDAN

Nombreuses traverséesNombreuses traversées

From the ISAM/QSR analysis :Examples of difficulties that seem significant

• The thermo hydraulic behaviour of the primary circuit will be more

complex due to the needed specific pumps regulation to guarantee the

stable stratification within the internal volume of the REDAN

• The large REDAN structures are exposed to the risk of vibrations. The

design of these structures has to address the risk of vibrations for all

the plausible conditions, operating, abnormal and accidental.


the plausible conditions, operating, abnormal and accidental.

• Specific exhaustive instrumentation is requested to minimize and

master these uncertainties.

• The concept should likely show a greater sensitivity to perturbations.

On the other side the reversible transition for the primary “forced >

natural > forced”, once proved, will significantly alleviate this

drawback.

• Etc.

ISAM/PIRT– Preliminary Conclusions

The nominal operational conditions

• The tools for mechanical / thermo hydraulic calculations are adequate to describe the stratified REDAN.

The abrupt rundown for the pumps which are located on the primary heat exchanger

• The calculations to evaluate the loads on structures, and the consequent behaviour, will be done with a good degree of confidence.

Earthquake


Earthquake

• For the collector’s hydraulic simulation which defines the fluid structure interaction; large uncertainties are expected and, despite mechanical computing tools with high performances, it will be difficult to provide accurate results.

Primary pumps rundown

• The induced phenomenon is not primarily important for the core structures while it is expected of medium / high importance for the REDAN’s structures.

• This can be considered as a “gap” and could justify specific R&D efforts to help the design stage and to validate the final solution.

Conclusions

• The ISAM methodology is being – punctually - applied to the SFR

concept with stratified REDAN

• The exercise is focusing - as far as feasible due to the preliminary

status of the concept - :

– on the QSR to compare the concept with the EFR design,


– on the PIRT for some peculiar characteristic (such as the

stratification within the REDAN)

• The exercise could continue :

– With complementary PIRT analysis (other plant

conditions/scenarios)

– With the OPT to focus on possible drawbacks (?) and possibilities

for specific phenomena / mechanisms related to the design (?).

• Back up Slides


A Viable Assessment Methodology Must Fulfill Multiple Purposes

• Commensurate with design maturity, yields a complete and detailed understanding of relevant risk and safety issues

• Within a given concept or design:

– guides the design process based on a detailed understanding of risk and safety

– helps identify areas for additional research and data


– helps identify areas for additional research and datacollection

• Promotes understanding of differences between concepts and designs based on risk and safety issues

• Allow evaluation of a concept or design relative to various safety metrics or “figures of merit”

• Support licensing and regulatory processes

Desirable Characteristics of an Assessment Methodology

• Consists of, or is largely based on, existing tools that are widely accepted for their validity. Minimizes need for development of new techniques.

• Practical and flexible - allows for graded approach to technical issues of varying complexity and importance. Offers analysis tools tailored to appropriate stage of design


appropriate stage of design

• Identifies vulnerabilities and relative contributions to risk

• Helps identify areas for additional research, data collection, etc

• Supports integration of multidisciplinary inputs

• Combines probabilistic and deterministic perspectives

• Consistent with RSWG safety philosophy, PRPP methodology, and other relevant work (e.g. IAEA Safety Standards, INSAG, INPRO, NUREG-1860, TECDOC-1570, etc)

ISAM is integrative

• From earliest phases, developers must strive to reduce or eliminate

vulnerabilities. Beginning in the “pre-conceptual” development phase,

ISAM provides the systematic means to identify vulnerabilities and

their magnitudes

• The ISAM framework integrates multidisciplinary inputs, both

qualitative and quantitative as well as probabilistic and deterministic; it

can reflect a range of uncertainties inherent in complex technological


can reflect a range of uncertainties inherent in complex technological

systems

• ISAM can be used to assess effectiveness of design provisions - safety

and cost optimization

• As design matures, ISAM is iteratively updated in a way that both

reflects and guides (“drives”) the evolving design

• Methodologically consistent with the notion that, in Gen IV systems,

safety must be “built-in, not added-on”

N.B. : RSWG needs to provide guidance and examples of how to apply ISAM

Status of Methodology Development

• Analytical elements of integrated methodology are well defined

• Roles and purposes of each element have been defined

• Relationships and interfaces between elements are conceptually

defined - more work needed

• Limited scale “trial” of the methodological elements has been


• Limited scale “trial” of the methodological elements has been

completed

• Meeting with System Steering Committees in April 2010 to

obtain feedback

• Initial draft report describing the methodology has been

completed

RSWG : Future activities

• Draft Methodology Overview document to be published in 2010

• Future work may focus on identifying and developing appropriate risk metrics, development of guidance regarding applications of analysis results, and preparation of documentation:

– To further develop and finalize the definition of the safety principles and the safety objectives introduced within the report;

– To identify the crosscut R&D necessary for their adoption and application;

• Significant interaction with System Steering Committees begun with a workshop in April 2010


workshop in April 2010– Open invitation to all RSWG working meetings

– Invite review and comment regarding all aspects of methodology and its application

– Seeking feedback on ways in which RSWG can best support activities of the SSCs

– To help the SSC in the identification and the implementation of the specific R&D effort needed for the development of the different systems .

• Moving forwards from here, the RSWG needs to widen the scope of its reflection to consider parts of the nuclear system other than the reactor (e.g. fuel cycle installations).

Schematic view of JSFR

Steam

Generator

Secondary

Pump

Large-scale loop-type SFR

Major design specifications

Power output1500MWe /

3570MWt

Number of loops in PHTS 2

Major design specifications

Power output1500MWe /

3570MWt

Number of loops in PHTS 2


Reactor

Vessel

IHX

Primary Pump

Primary coolant temperature 550℃ / 395℃

Primary coolant mass flow rate 1.8 ×104 kg/s

Secondary coolant temperature 520℃ / 335℃

Main steam temperature and pressure

497℃ / 19.2MPa

Primary coolant temperature 550℃ / 395℃

Primary coolant mass flow rate 1.8 ×104 kg/s

Secondary coolant temperature 520℃ / 335℃

Main steam temperature and pressure

497℃ / 19.2MPa

Outline of the Curie point electromagnet type of Self-Actuated Shutdown System (SASS)

Core outlet coolant temperature rise

Sensing alloy temperature

reaching the Curie point


Passive de-latch due to

decreasing magnetic force

Passive insertion of

the rod by gravity

Ref.

. Ichimiya et al., “A Next Generation Sodium-Cooled Fast Reactor Concept and its R&D Program,” Nucl. Eng. Technol., 39(3), 171-186(2007).

Preliminary PIRT application result by two assessors A & B

System Component Phenomena/Characteristics/State variables

IR KL1 KL2

A B A B A B

BRSS SASS SASS actuation temperature H H 1 2 3 4

Reactor

Upper core region

around SASS

Coolant transport delay time from core outlet to around SASS H H 3 2 3 3

Time constant of temperature response delay from coolant around SASS to SASS

deviceM M 1 2 3 3

Reactor core

Core outlet temperature of the coolant that flows to around SASS H H 3 3 3 3

Doppler reactivity M M 4 4 4 4

Fuel temperature reactivity L M 4 3 4 3

Fuel cladding temperature reactivity M M 4 4 4 4

Coolant temperature reactivity H H 4 4 4 4

Coolant flow rate halving time H H 4 4 4 4

Power distribution M M 4 4 4 4

System Component Phenomena/Characteristics/State variables

IR KL1 KL2

A B A B A B

BRSS SASS SASS actuation temperature H H 1 2 3 4

Reactor

Upper core region

around SASS

Coolant transport delay time from core outlet to around SASS H H 3 2 3 3

Time constant of temperature response delay from coolant around SASS to SASS

deviceM M 1 2 3 3

Reactor core

Core outlet temperature of the coolant that flows to around SASS H H 3 3 3 3

Doppler reactivity M M 4 4 4 4

Fuel temperature reactivity L M 4 3 4 3

Fuel cladding temperature reactivity M M 4 4 4 4

Coolant temperature reactivity H H 4 4 4 4

Coolant flow rate halving time H H 4 4 4 4

Power distribution M M 4 4 4 4

Improved

knowledg

e level


Reactor core Power distribution M M 4 4 4 4

Flow rate distribution among core assemblies M M 4 4 4 4

Coolant temperature at the core inlet and outlet L L 4 4 4 4

Fuel pin gap heat transfer coefficient M M 4 3 4 3

Fuel pellet thermal conductivity I I 4 4 4 4

Thermal material property of fuel cladding and coolant I I 4 4 4 4

RPCS Temperature I&C Coolant temperature to be used reactor power control M L 4 4 4 4

PHTS

Pump Pump rotating inertia M M 4 4 4 4

- Pressure loss in the reactor and PHTS M M 4 4 4 4

Reactor core Power distribution M M 4 4 4 4

Flow rate distribution among core assemblies M M 4 4 4 4

Coolant temperature at the core inlet and outlet L L 4 4 4 4

Fuel pin gap heat transfer coefficient M M 4 3 4 3

Fuel pellet thermal conductivity I I 4 4 4 4

Thermal material property of fuel cladding and coolant I I 4 4 4 4

RPCS Temperature I&C Coolant temperature to be used reactor power control M L 4 4 4 4

PHTS

Pump Pump rotating inertia M M 4 4 4 4

- Pressure loss in the reactor and PHTS M M 4 4 4 4

IR: Importance ranking

KL1: Knowledge level before starting SASS

R&D

KL2: Knowledge level at present

BRSS: Backup Reactor Shutdown

System

RPCS: Reactor Power Control System


Summary of DPA results and interpretation to PSA input

Seq.

No.Accident sequence

Core integrity assigned by

considering DPA results

1/RS*/ANC*/DNC

(Successful DBA scenario)OK

(1)

2/RS*/ANC*DNC

(Passive cooling by using PRACS-A alone)Damage

(2)

3/RS*ANC*/DNC

(Passive cooling by using DRACS alone)Damage

(2)


(1) Confirmed the accident consequences of Seq. 1 based on the DPA results.

(2) Regarded conservatively the accident consequences of Seq. 2 and 3 as

damage by considering uncertainty.

� It is a future work to implement sensitivity analyses to establish margins to

limits and to cover imprecision in actual parameters at the design stage.

(Passive cooling by using DRACS alone)

4/RS*ANC*DNC

(Loss of all heat sink)Damage

Outline of decay heat removal system (DHRS)

PRACS

Steam

DRACS

Steam

PRACS

Steam

Steam

Generator

PRACS

Steam

DRACS

Steam

PRACS

Steam

Steam

Generator


PRACS: Primary Reactor Auxiliary Cooling System

DRACS: Direct Reactor Auxiliary Cooling System


SHTS: Secondary Heat Transport System

Steam

Generator

Feedwater

Generator

Feedwater

PHTSSHTS SHTS

Steam

Generator

Feedwater

Generator

Feedwater

PHTSSHTSSHTS SHTSSHTS

Typical event tree model in the level-1 PSA

Loss of

circulation

capability in

PRACS-B

Reactor

SCRAM

Passive

cooling by

using

PRACS-A *

Passive

cooling by

using

DRACS *

IC07-B RS ANC DNC

Seq

.

No.

Accident sequenceCore

integrity

/RS*/ANC*/DNC


Success ↑

Failure ↓

1/RS*/ANC*/DNC

(Successful DBA scenario)OK

2/RS*/ANC*DNC

(Passive cooling by using PRACS-A alone)Damage

3/RS*ANC*/DNC

(Passive cooling by using DRACS alone)Damage

4/RS*ANC*DNC

(Loss of all heat sink)Damage

This sequence is developed in

detail in other event trees5 - -

Seq. 1

30%Seq. 4

15%

Seq. 5

5%

Seq. 6

1%

The others

1%

PSA result: Major contributors to PLOHS frequency broken down by combination of loss of mitigation systems

PLOHS

9x10-

Seq. 1 Loss of passive cooling function in 2 loops &

failure to start AC blower in the other loop

Seq. 2 Loss of all electric power & human error in

manual damper operation

Seq. 3 Loss of passive cooling function in 3 loops

(after 24h)

Seq. 4 Common cause failure of PRACS dampers &

failure to start AC blower in DRACS



Seq. 2

25%

Seq. 3

23%

9x10-

9/ry


loss of active cooling function in DRACS AC

Seq. 6 Na leakage in one loop of DHRS & DHRS

actuation signal failure in one loop & human

error in manual damper operation & failure to

start AC blower in the other loop

*; ordered by contribution

PSA combined with DPA showed quantitatively that introduction of AC

blowers in both PRACS and DRACS can improve reliability of decay

heat removal significantly.

Example of application to the concept with the stratified REDAN : Comparison with the EFR concept


ISAM/QSR – Preliminary Conclusions

• The key objective for the QSR is the identification of safety related

recommendations or foreseen characteristics helpful for a standard

qualitative safety assessment.

• The exercise on the Stratified Redan shows that the tool is capable to

help the designer to qualitatively assess the design options identifying

strong characteristics or safety vulnerabilities.


strong characteristics or safety vulnerabilities.

• This is obviously one step of an iterative process where the designer is

invited to focus his attention on the identified vulnerabilities to look for

solution’s improvements or alternative solutions.

Highly important and only partially known

> Needs for specific developments

PIRT for the Earthquake


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