National Report of The Slovak Republic , september 2001

Predchádzajúca kapitolaPredchádzajúca stranaOBSAHNasledujúca stranaNasledujúca kapitola Probabilistic safety assessment

PSA Level 1 for power operation

The first Level 1 PSA study for NPP V-2 was drafted in the framework of the SAR after 10 years of operation, by the Slovak companies RELKO Bratislava and VÚJE in 1994.

The objectives of the study were as follows:

to determine core damage frequency (CDF),
to identify initiating events and accident sequences with dominant contribution to core damage.

The study evaluated NPP V-2 unit 3 as a pilot unit on power, for the condition as of September, 1993. The events considered in the study included initiating events (transients and LOCA), internal fires and internal floods. In a majority of cases, the PSA used own specific data for the individual intensities of component failures; published generic data were used whenever specific data were not available.

The project was developed in two stages. During stage 1, the dominate contribution of electric supplies of category 2 safety systems of uninterruptible supplies toward CDF was identified. Following a change of the project that removed the shortcoming identified, an updated PSA model was developed for unit 3 as of May 1995.

Results of Level 1 PSA study for unit 3 as of 1995:
CDF = 6.41E-04 / year

Conclusions and recommendations of Level 1 PSA study:

initiating events including break of main steamline, main steam header or feed water header in the machine room at the elevation +14,7m have a dominant contribution on CDF ( 41%),
operator errors to take remedial recovery actions during accidents are among the major factors contributing towards CDF (82.7%),
symptoms-oriented emergency procedures for operating staff have to be developed,
independent SG feed water lines have to be installed from the emergency feed water system.

The study completed in 1995 was evaluated by the IPERS Mission organized by IAEA, which highlighted the high level and confirmed the correctness of the PSA study that was conducted according to the internationally recognized methods.

PSA Level 1 model was updated to the condition of NPP V-2 unit 3 as of July 1997, after having taken some recommended measures in the area of electric supplies to distribution switchboards, valves in treated water system, and supplements to emergency operating procedures of most important corrective actions.

The updated model was modified in cooperation with the US company SAIC to make it fit for the use in real-time risk monitoring system.

Results of PSA following the introduction of some recommended for changes:
CDF = 1.33E-04 / year

The study identified an important contribution of clogging of strainers at the emergency core cooling system pumps suction toward CDF.

After the symptom-oriented emergency operating procedures have been put into operation at the NPP V-2 unit control room in 1999 and after the strainers at the emergency core cooling system pump inlets at SG box had been replaced, also Level 1 PSA study was updated for the condition of the NPP V-2 unit 3 as of 1 January, 2000.

Results after the implementation of recommended changes in 2000:
CDF = 7.36E-05 /year

The results of the updated study confirmed that the implementation of the new generation of emergency procedures has been associated with a reduction of core damage frequency by 35.4%. With the symptom-oriented emergency procedures in place, the unit meets the ÚJD criterion concerning CDF (Fig. 2.2.1).

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Fig. 2.2.1 Comparison of the results of the various PSA studies for NPP V-2

Low-power and shutdown Level 1 PSA

Level 1 low-power and shutdown PSA study (SPSA) was completed in 1999. The project was financed by the European Commission under PHARE programme, and was developed by an international consortium of organizations comprising ENCONET, WESTINGHOUSE ES, KEMA, VÚJE, RELKO, ÚJV Řež. The study was considered as a pilot one for Soviet WWER-440/V-213 projects, and presently represents the most comprehensive probabilistic risk assessment for shutdown WWER-440 reactors. The completed study was evaluated in September 1999 by the IAEA IPERS Mission, and confirmed the correctness of the evaluation procedures used.

Results and conclusions of Level 1 SPSA study:
CDF = 6.44E-04 / year

The frequency of core and fuel damage is comparable with CDF for power operation. To reduce CDF, an extension of operating procedures for normal and emergency operation of shutdown unit has been recommended. In 2000, the study was updated due to changes performed in NPP V-2 installation.

Real-time risk monitoring at NPP V-2

EOOS Riskmonitor was implemented on NPP V-2 within 1997 to 1999, to enable real-time risk monitoring. The project was initiated in cooperation of SE-EBO and ÚJD in 1996. The development of the risk monitor was performed in cooperation with companies SAIC and RELKO, in accordance with the NRC method NUREG/CR 5925. In January 2000, EOOS Riskmonitor was implemented at NPP V-2, and is now available at the unit control room. Shutdown Riskmonitor has been in trial operation at the nuclear safety department, and its use is expected mainly for planning and coordination of works during outages of NPP V-2 units.

PSA Level 2

PSA Level 2 for power operation and shutdown was completed in March 2001 at the reference NPP V-2 unit 3. The study was developed by the Austrian company ENCONET in cooperation with the Slovak companies VÚJE and RELKO Bratislava and foreign partners. The results of the study are currently being verified.

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