National Report of The Slovak Republic , september 2001


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2.1.2.4 Probabilistic safety assessment


Level 1 PSA for power operations

The level 1 PSA study for NPP V-1 was prepared in the framework of the PHARE Programme by the UK company Electrowatt Engineering Services in cooperation with RELKO and VÚJE in 1992. The objectives of the study were as follows:

  • to determine core damage frequency (CDF) based on the failure trees method and event trees method,

  • to identify initiating events and accident sequences with a predominating contribution to core damage,

  • to assess the effects of various modifications of safety systems on CDF,

  • to specify recommendations for the updating of emergency operating procedures based on predominant accident sequences.

The study assessed NPP V-1 unit 1 as a pilot unit, being on power, for the status quo of in July 1992. The study contemplated initiating events (transients and LOCA), internal fires and internal floods. In a majority of cases, own specific data for the individual intensities of component failures were used in the PSA. Published generic data were used whenever specific data were not available.

Event trees and failure trees were modeled in the Risk Spectrum software environment. A very detailed model of the unit was set up for the individual technological systems, electric switchboards and I&C systems with the corresponding controls down to the sensor and relay level. Also, this programme was used to finally quantify the model.

Results of the first Level 1 PSA study:
CDF = 1.70E-03/year

Since „small reconstruction “ of NPP V-1 was under preparation during July 1992, the obtained PSA model was used to evaluate and to supplement suggested changes into the project. Under the „small reconstruction“, improvements of I&C systems, electric and technological circuits were made. Also emergency operating procedures were supplemented by adding critical safety functions restoration guidelines.

The changes made in the configurations of both NPP V-1 units by the end of 1993, and the completion of symptoms-based emergency procedures resulted in a new model of Level 1 PSA.

Level 1 PSA study after „small reconstruction“ was drafted in December 1993. The new model incorporated, in addition to unit hardware modifications, also remedial operator actions during accidents. The modeling of operator's interventions enabled the completion of symptoms-based emergency procedures. Values obtained from a detailed analysis of the human factor reliability, using the THERP method were used for the probability analysis of operator's error.

Results of Level 1 PSA study after „small reconstruction “:
CDF = 8.9E-04/year


Based on the results obtained from PSA study some objectives were set for the NPP V-1 „gradual upgrading“ under preparation.

Preliminary suggestions for changes in unit systems were used to set up Level 1 PSA model ”Basic Engineering”. The model was subsequently quantified, and based on sensitivity analyses, the most appropriate project of change for the individual unit systems was selected.

The PSA model ”Basic Engineering” was used to determine the preliminary core damage frequency - CDF of 2.57E-04/year, without symptom-oriented emergency operating procedures.

Having completed the NPP V-1 “gradual upgrading“, a new Level 1 PSA model was developed to reflect the actual status of the unit in 2000. The Level 1 PSA study after „gradual reconstruction “ of NPP V-1 was developed by Slovak engineering and research companies in cooperation with VÚJE.

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Fig. 2.1.1 Comparison of the outcome of NPP V-1 upgrading: PSA aspect

Main results of the recent Level 1 PSA study:
CDF = 2.56E-05 /year


Table 2.1.1 documents the results of the most recent NPP V-1 PSA study from the aspect of the contributions of the individual initiating events.

Table 2.1.1

Contributions of initiating events to RC melting frequencies

Initiating event

Description

Frequency
[1/year]

% of overall CDF

LL-LOOP2,5

Large leak from loop 2 or 5

7.00E-6

23.0

ML(32-100)

Medium leak (32-100 mm)

4.30E-3

19.8

SL

Small leak (0-32 mm)

2.80E-2

15.6

LL-LOOP1,4

Large leak from loop 1 or 4

7.00E-6

12.8

SGTM

SG manifold rupture

5.00E-6

3.8

IFL

Interfacing LOCA

5.00E-4

3.5

PSL

Leak from pressurizer

6.90E-4

2.8

SGTR

SG tube rupture

8.90E-4

2.8

LL-LOOP3,6

Large leak from loop 3 or 6

7.00E-6

2.6

FIRE-TGHALL

Fire at the machine room

6.20E-2

2.4

ML(100-200)

Medium leak (100-200 mm)

1.50E-4

2.3

2TG

Trip of both TG

5.50E-1

2.3

LOP

Complete failure of house power supply

2.50E-2

2.2

SHB

Main steam header rupture

2.20E-3

1.3

 

Other events

 

2.8


Main recommendation of the most recent PSA study


To reduce CDF, the PSA study has recommended to develop and implement symptom-oriented emergency operating procedures - SBEOP. The development of SBEOP is in the course and implementation is expected by the year 2003.

Plans and ongoing activities related to NPP V-1 PSA

Based on the most recent PSA model for NPP V-1 the system of real-time risk monitoring system ”EOOS - Risk Monitor” is implemented for power operation, for which NPP Bohunice have been licensed.

Development of Level 1 PSA study for low-power and shut-down reactor (shut down PSA) is in progress.

The PSA model will have to be updated after the development of SBEOP.

The power plant has systematically used PSA as a tool for the development and setting of priorities with respect to improvements that were implemented during the upgrading programme aimed at achieving the best safety improvements possible. Moreover, the conclusions and the results of the PSA have been incorporated into the training programme for NPP V-1 operators, and selected emergency scenarios have been implemented into the new unit simulator at VÚJE.

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