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
The objectives of the study were as follows:
||to determine core damage
||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
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
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).
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
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.