Erfahrungs- und Forschungsbericht 2012 - Ensi

Erfahrungs- und Forschungsbericht 2012 - Ensi

conclusions from the method assessments through

additional cases and the use of a full-scope training

simulator at a U.S. nuclear power plant.

The main 2012 tasks were to obtain the overall

findings on HRA methods in light of the results of

both studies and the formulation of recommendations

for HRA guidance and practice. These recommendations

identify specific shortcomings of the

existing guidance and some of the ways in which

guidance could be improved to increase the validity

and consistency of HRA analyses.

A parallel use of the crew performance data collected

in the HRA Empirical Studies is to derive

scenario-specific insights for HRA practitioners

(analysts) to take into consideration in predictive

analyses. This task was postponed to 2013.

4. Development of guidelines for data collection

in simulators.

Simulator studies designed to collect data for HRA

are one of the primary means to obtain qualitative

and quantitative data on the crew response to the

abnormal and emergency scenarios modeled in

PSAs. The 2012 objective was to survey the state

of HRA data, reviewing the experience from past

HRA data collection efforts, and to identify the key

topics to be addressed by the guideline. The survey

(literature review) and guideline will be completed

in 2013.

Work carried out

and results obtained

EOC plant-specific pilot study III

1. Identification of EOCs, qualitative analysis and


The inclusion of Errors of Commission (EOCs) extends

the scope of state-of-the-art PSA. EOCs refer

to PSA Human Failure Events (HFEs) modeling

the performance of actions that aggravates an accident

scenario. They can be contrasted to HFEs

where a required action is not performed, and on

which state-of-the-art PSA typically focuses. Pilot

study III is the third plant-specific, industrial-scale

application of the Commission Errors Search and

Assessment (CESA) method, developed at PSI for

identification and assessment of EOCs. The study

follows two earlier EOC studies with CESA for

Swiss plants [1, 2]. The pilot characterization of

these studies relates to the need for further development

in the method for the estimation of the

EOC probabilities as well as the need for understanding

the role of these errors in the plant risk

profile. The scope of all three studies are EOCs in

scenarios initiated by internal initiating events during

full-power operation.

The work in 2012 started with the identification

of candidate EOC events (generic aggravating

actions). These were then subjected to screening

analysis based on contextual factors, such as

availability of multiple and diverse indications, that

suggest that the performance of the inappropriate

action has negligible probability. The identification

process addressed the plant procedures central in

the response to important accidents. Six candidate

EOC events were identified (e.g. termination of

high pressure injection, blocking of the automatic

depressurization function); among these, four

were carried forward for detailed analysis after the

screening process.

The next step has been to identify the most riskimportant

accident scenarios in which these inappropriate

actions might be performed, based on

examining opportunities for the candidate EOC

events in the accident sequences of the baseline

PSA (i.e. the PSA without inclusion of EOCs). Six

scenario-specific EOC situations were then identified

for detailed analysis, as shown in Table 1. Each

EOC scenario was analyzed in detail in terms of the

applicable procedural guidance as well as of the

expected operator behaviors in these situations.

Observations of crew response as well as interviews

with plant personnel (the plant PSA team,

operators and trainers) were at the basis of the

detailed analysis. To estimate the probability of the

EOCs, CESA’s quantification module, the CESA-Q

method (see corresponding subproject) was used,

except for one case that could be quantified based

on an HRA analysis from the plant’s PSA.

A preliminary review by the plant confirmed the

credibility of the results. These suggest that the

most important EOC (CWS.EOC1 in Table 1, connected

with a misalignment of the cooling water

system during scenarios initiated by the failure of

the auxiliary cooling water system) would contribute

to an increase in the core damage frequency

of about 5% (for internal initiating events at full

power). This contribution is comparable to that

of the most important errors of omission, typically

considered in the PSA. The contribution of the

other EOCs to the risk profile was found to be limited,

thus highlighting the defense against EOCs

provided by the plant technical and administrative

protections. The detailed review of the analysis results

and study report by the plant is planned for

early 2013.

ENSI Erfahrungs- und Forschungsbericht 2012 185

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