Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

This third pilot study has further confirmed the

importance of including EOC contributions in the

plant risk characterization. The three plant-specific

studies also underscore the feasibility of a systematic

treatment of EOC: the studies identified

a limited number of EOC situations (about 5–10

scenario-specific EOCs), with a risk significance

comparable to that of the most important errors of

omission (Podofillini, Dang, 2012a). The identified

EOC situations possibly suggest safety-enhancing

improvements (generally related to procedural

guidance) aimed at decreasing the opportunities

for committing these errors. The search process,

based on the systematic screening of potential

errors characterized by low risk-importance and

plausibility provides further confirmation of the

plant technical and administrative protections.

Quantification of EOCs

2. Documentation of revised CESA-Q quantification

approach.

The overall goal of this subproject is to revise the

CESA-Q guidance [3] to increase traceability as

recommended by the assessment of the CESA-Q

analyses in the International HRA Empirical Study

and to extend the method’s scope. The CESA-Q

quantification process is being revised to incorporate

a Bayesian Belief Network model, which will

yield the probability estimates as a direct function

of the CESA-Q factor evaluations. This approach

(first developed during 2011) was further refined

and documented in draft form. In order to subject

the technical core of the revised method to a peer

review, an article was prepared and submitted to a

journal (Podofillini, Dang, 2012d).

As noted above, the current interpolation-based approach

to quantification in CESA-Q was applied for

the quantification of the EOCs identified in the EOC

Pilot Study III work from this year. While the resulting

probabilities are expected to be plausible to the

plant subject matter experts, these analyses confirm

the difficulties of applying the current CESA-Q

method and highlight the subjective elements of

the quantification.

In addition, the results of the current EOC study

and the previous two pilot studies were reviewed to

derive insights on the type of EOCs generally of concern

(Podofillini, Dang, 2012b), resulting in a taxonomy

of generic EOCs. Table 2 shows that EOCs affecting

a safety injection function, for example, may

result from a diverse set of actions. The taxonomy

provides the spectrum of EOCs that CESA-Q should

address (and therefore that the associated guidance

should be able to treat). Furthermore, during the

identification of EOCs, it is also useful as a checklist

for verifying the completeness of the EOC search.

Table 1:

Pilot study III: overview

of the EOCs carried

forward for detailed

analysis

EOC event Scenario EOC identifier

Inappropriate alignment of Cooling Loss of Auxiliary Cooling Water System CWS.EOC1

Water System (CWS) for Shutdown (ACWS), with subsequent operator failure to

and Torus Cool-ing System (STCS)

cooling

perform early actions in response to ACWS

(Fire Water alignment to Control Rod Drive

system and Control Air system)

Termination of both Core Spray

System (CSS) and Al-ternate Low

Pressure Spray (ALPS) injection

Medium Loss of Coolant Accident (LOCA) CSS&ALPS.EOC1

Automatic Depressurizatoin System

(ADS) blocking

RCIC termination

Medium LOCA

Loss of feedwater; condenser vacuum; Loss

of Turbine Building Intermediate Cooling

Water (TBICW); either with subsequent

RCIC failure to start or run

Loss of feedwater; condenser vacuum; Loss

of TBICW; Loss of ACWS

ADS.EOC1

ADS.EOC2

RCIC.EOC1

Table 2:

Taxonomy of generic

EOCs of concern for

CESA-Q guidance

development

Function

Injection

Generic EOC of concern

Function initiation

Function termination

Inhibition of automatic actuation

Injection flow diversion – injection to different location then required

Suction flow diversion – alignment to source with limited/no inventory

Suction flow diversion – alignment of source to different system

Support systems disablement – component cooling termination/diversion

Support systems disablement – AC and DC Power disconnection

Support systems disablement – pressurized air termination/diversion

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ENSI Erfahrungs- und Forschungsbericht 2012

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