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Event Trees

Quantitative Risk Analysis

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Event Trees - Overview

Definitions

StepsOccurrence frequency

Mean Time between Sut!own

Mean Time "etween RunawayE#ample

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Acci!ents !o appen$

%en an acci!ent or process!eviation &i'e' an (event)* occurs in aplant+ various safety systems &botmecanical an! uman* come into playto prevent te acci!ent from

propa,atin,'Tese safety systems eiter fail orsuccee!'

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Event Trees

Event trees are use! to follow tepotential course of events as teevent moves trou, te varioussafety systems' Te probability ofsuccess or failure of eac safety

intervention is use! to !etermine teoverall probability of eac finaloutcome'

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Event Trees

An Event Tree is use! to !eterminete frequency of occurrence of

process sut!owns or runawaysystems'n!uctive approac

Specify.!entify an initiatin, even an!work forwar!'!entifies ow a failure can occur an! teprobability of occurrence

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Steps to /onstruct an Event Tree

!entify an initiatin, event ofinterest'

!entify te safety functions!esi,ne! to !eal wit te initiationfollowe! by te impact of te safety

system/onstruct te event treeDescribe te resultin, acci!ent event

sequences'

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!entify an initiatin, event

May ave been i!entifie! !urin, a0A1O2 as a potential event tatcoul! result in a!verse consequences'3sually involves a ma4or piece ofoperatin, equipment or processin,

step+ i'e' a 0A1O2 (Stu!y 5o!e)'

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!entify safety functions

6rom 2D+ process flow seet+ or proce!ures fin!wat safety systems are in place an! wat teirfunctions are'Tese can inclu!e tin,s suc as automaticcontrollers+ alarms+ sensors+ operatorintervention+ etc'On you Event Tree write across te top of te

pa,e in te sequence of te safety interventionstat lo,ically occur'7ive eac safety intervention an alpabeticletter notation'

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/onstruct te Event Tree

0ori8ontal lines are !rawn betweenfunctions tat apply

9ertical lines are !rawn at eacsafety function tat applies

Success : upwar!

6ailure : !ownwar!n!icate result of event

/ircle : acceptable result

/ross-circle : unacceptable result

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/onstruct Event Tree &cont'*

/ompute frequency of failures

λ" is te failure per !eman! or te

unavailability of safety function "

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Occurrence 6requency

6ollow process trou, wit eacstep to calculate te frequency ofeac consequence occurrin,'Typically tree final results

/ontinuous operation

Sut!own &safely*

Runaway or fail

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Mean time between Sut!own

Mean Time "etween Sut!own+ MT"Sis calculate! from;

MT"S

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E#ample : >oss of coolant to reactor

6our safetyinterventions

0i, temperature alarmOperator noticin, tei, temperature !urin,normal inspectionOperator re-establiseste coolant flowOperator performsemer,ency sut!own ofreactor

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Resultin, Event Tree Analysis

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E#ample : 2ossible outcomes

Te letterin, is use! to i!entify eacfinal outcome'

6or instance+ A"DEn!icates tat after nitiatin, event Aoccurs+ tat safety system " faile! &i,

T alarm*+ tat safety system D faile!&te operator was unable to re-start tecoolant* an! safety system E faile! &teoperator was unable to successful sut!own te reactor*'

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E#ample : Determination of MT"S

6or Mean Time "etween Sut!ownstake te reciprocal of te sum of all

sequences tat resulte! in asut!own' &E#ample ,ives =.'CC <B'Byrs*

6or Mean Time "etween Runaway !ote same tin, wit all sequencestat resulte! in a runaway' &E#ample,ives =.@'C@ < B@yrs*

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%at is wron, wit te lo,ic oftis e#ample analysisF

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%at is wron,F

f te operatorfails to notice te

i, temperatureafter te alarmsfails+ ten e.sewill never restart

te coolin,'

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n /lass E#ample

/onstruct an Event Tree an! !etermine teMT"S an! MT"R for a loss of coolant for tereactor sown in 6i,ure ==-G'

Assume loss of coolant occurs once every tree years'Alarm fails @'=? of time place! in !eman!Operator will notice i, reactor temperature out

of B timesOperator will successfully restart coolant flow B outof timesOperator successfully suts !own reactor out of =@

times

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Solution : /onstruct Event Tree

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Solution /ontinue! :Mean Time "etween Events

Mean Time Between Shutdown

1

!""u#en"e o$ Shutdown1

5.560.17982 0.000135

Mean Time Between %unawa&1

!""u#en"e o$ %unawa&

1'9.'

0.01998 0.000015 0.00025

 MTBS 

 MTBS yrs

 MTBR

 MTBR yrs

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