8/16/2019 Event Trees (1)
1/23
Event Trees
Quantitative Risk Analysis
8/16/2019 Event Trees (1)
2/23
Event Trees - Overview
Definitions
StepsOccurrence frequency
Mean Time between Sut!own
Mean Time "etween RunawayE#ample
8/16/2019 Event Trees (1)
3/23
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!'
8/16/2019 Event Trees (1)
4/23
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'
8/16/2019 Event Trees (1)
5/23
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
8/16/2019 Event Trees (1)
6/23
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'
8/16/2019 Event Trees (1)
7/23
!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)'
8/16/2019 Event Trees (1)
8/23
!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'
8/16/2019 Event Trees (1)
9/23
/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
8/16/2019 Event Trees (1)
10/23
/onstruct Event Tree &cont'*
/ompute frequency of failures
λ" is te failure per !eman! or te
unavailability of safety function "
8/16/2019 Event Trees (1)
11/23
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
8/16/2019 Event Trees (1)
12/23
Mean time between Sut!own
Mean Time "etween Sut!own+ MT"Sis calculate! from;
MT"S
8/16/2019 Event Trees (1)
13/23
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
8/16/2019 Event Trees (1)
14/23
8/16/2019 Event Trees (1)
15/23
Resultin, Event Tree Analysis
8/16/2019 Event Trees (1)
16/23
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*'
8/16/2019 Event Trees (1)
17/23
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*
8/16/2019 Event Trees (1)
18/23
%at is wron, wit te lo,ic oftis e#ample analysisF
8/16/2019 Event Trees (1)
19/23
%at is wron,F
f te operatorfails to notice te
i, temperatureafter te alarmsfails+ ten e.sewill never restart
te coolin,'
8/16/2019 Event Trees (1)
20/23
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
8/16/2019 Event Trees (1)
21/23
Solution : /onstruct Event Tree
8/16/2019 Event Trees (1)
22/23
8/16/2019 Event Trees (1)
23/23
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
=∑
= =+
=∑
= =
+ +