One of key tools for ensuring safety of NPP operation and its further improvement is consideration of operating experience, which includes accounting and analysis of NPP operational events, corrective measures to eliminate detected causes and prevent recurrence of events.
NPP event is one of the most important indicators of operational safety level.
During 2015, 15 events occurred at 15 power units with water-cooled water-moderated reactors (WWER) under commercial operation. Events were not registered at Chornobyl NPP units that are at the decommissioning stage.
Figure 1 presents distribution of operational events at operating Ukrainian NPPs (without considering Chornobyl NPP) during 2000-2015.
Figure 1. Number of operational events at Ukrainian NPPs in 2000-2015.
Figure 2 presents the distribution of events by NPP sites in 2000-2015.
The number of operational events in 2015 increased twice at ZNPP and KhNPP (and is the highest one at these sites for the last 5 years), at SUNPP increased thrice compared to the previous year. At RNPP in 2015, events did not occur.
Figure 2. Distribution of events by NPP sites in 2000-2015.
According to the INES, the worldwide instrument developed to inform the public on significance of nuclear and radiological events for safety, Ukraine had no events higher than “below scale/level 0” (insignificant for safety) in 2015. Figure 3 presents the distribution of events at operating Ukrainian NPPs in 2000-2015 classified according to the INES.
Events resulting in personnel overexposure or release of radioactive materials to the environment, incompliance with safe operation boundaries and conditions did not occur in 2015. In addition, events related to inoperability of systems important to safety and events resulting in fall and/or damage of fuel assemblies and fuel rods did not occur.
Depending on features and consequences, NPP operational events in 2015 included the following:
- NPP unit unloading by 25 % and more – 7 events (47%);
- reactor shutdown with scram, preventive protection, power limiter – 2 events (13 % of general event number in 2015);
- power unit disconnection from the grid by emergency automatics – 3 events (20 %);
- failures of equipment and piping important to NPP safety – 3 events (20 %).
During an NPP operational event, there is deviation from normal operation (abnormal event) that may be caused by equipment failure, external hazard, human error or procedural drawbacks.
Figure 4 presents the distribution according to systems that failed or were affected during abnormal events in 2015.
Figure 3. Distribution of event number at operating Ukrainian NPPs by the INES in 2000-2015.
Figure 4. Distribution according to systems that failed or were affected during abnormal events
In 2015, the most failures (30%) refer to auxiliary systems ensuring operability of the main systems’ equipment. In addition, significant part (25% each) includes failures of secondary process systems, turbine building, and failures of primary process, reactor building.
Investigation of 15 NPP operational events occurred in Ukraine this year registered 20 abnormal events whose 25 root causes were determined. Figure 5 presents the contribution of each root cause group to general number of events in 2000-2015.
Causes related to equipment failures (52 %) contribute the most. A part of root causes related to documentation quality increased significantly (by 3.2) compared to the last year. Two causes in 2015 were not identified under event investigation (8%) and would be determined during scheduled outage. Although causes related to personnel and the management system has decreased (from 28 % in 2014 to 24 % in 2015) this index is still high enough.
Figure 5. Distribution of root causes of abnormal events in 2000-2015.
The operator takes measures aimed at improving the quality of event investigation, adherence to unified technical policy, and improvement of operating experience feedback to correct this situation.
State Nuclear Regulatory Inspectorate of Ukraine