Loss Of Essential Power System
Andrija Volkanovski Jožef Stefan Institute
Jamova cesta 39 1000, Ljubljana, Slovenia Andrija.Volkanovski@ijs.si
Miguel Peinador Veira European Commission, DG JRC P.O. Box 2, NL-1755 ZG Petten, Netherlands
Miguel.Peinador-Veira @ec.europa.eu
ABSTRACT
The non-interruptible power supply system is essential for safe operation and accident mitigation and recovery at the currently operating nuclear power plants. This system provides power to essential systems during the event of loss of offsite and onsite alternate current power sources referred to as station blackout (SBO).
This paper presents the results of statistical analysis of the non-interruptible power supplies failure or deficiency events registered in two reviewed databases. The paper includes events registered in the Nuclear Regulatory Commission (NRC) Licensee Event Reports (LERs) database and the IAEA International Reporting System (IRS) database. Both databases are screened for the relevant events registered in period 2000-2015.
The identified events will be classified into the multiple categories considering: plant state, circumstances, type of events, type of equipment failed or concerned, detection of the event and direct cause. Events will be classified into single best matching category with the exception for the characteristic related to the failed or concerned equipment which can be multiple.
Main results of the analysis and observations for prevention and mitigation of the loss of essential power system will be given.
1 INTRODUCTION
The NPP electrical energy distribution system can be generally divided into offsite and on-site power system [1].
The NPP on-site power system [2] consists of electric equipment and systems of the NPP that are essential for plant operation and safety. The safety related on-site power subsystem includes the electric equipment and systems that are essential in preventing significant release of radioactive material to the environment including emergency reactor shutdown, containment isolation, reactor core cooling and containment and reactor heat removal.
Loss of alternating current (AC) to the essential and nonessential switchgear buses in a NPP (i.e., loss of off-site electric power system concurrent with turbine trip and unavailability of the on-site emergency AC power system) is referred to as a station blackout (SBO). The
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NPP should be designed to cope with the station blackout for a given time period referred to as station blackout cooping time [3].
Station blackout does not include the loss of available AC power to buses fed by station batteries through inverters or by alternate AC sources. The loss of all AC power is referred to as extended blackout.
The batteries of the power system provide direct current (DC) to the inverters that power the essential instrumentation and control (I&C) systems in the NPP. The IAEA defines I&C systems important to safety as I&C systems whose malfunction or failure could lead to radiation exposure of site personnel or members of the public [4]. Examples of I&C systems [5] important to safety include: reactor protection and control systems, systems for monitoring and controlling reactor cooling and systems for monitoring and controlling emergency power supplies. These systems are connected to non-interruptible power supplies that provide electrical power within the tolerances specified by the design basis of I&C systems.
The essential I&C is supplied with electrical power from multiple power sources through non-interruptible power supplies including station batteries in case of loss of all other alternate current power sources. The inverters have power electronics that convert the direct current from batteries to the alternating current. The inverters normally include the rectifiers that are utilized for the charging of the batteries. The inverters and batteries together are referred as the non-interruptible power supply system.
2 BACKGROUND, SCOPE OF THE STUDY & EVENTS CLASSIFICATION During the 2015 Steering Committee annual meeting of the European Clearinghouse on Operational Experience Feedback (OEF) for Nuclear Power Plants (NPP), the participating European Nuclear Regulators requested to perform a topical study on events related to failure of deficiency of the non-interruptible power supply system in the nuclear power plants.
This paper presents part of the results of the engineering and statistical analysis of relevant events identified in two operational events databases.
The paper includes events registered in the Nuclear Regulatory Commission (NRC) Licensee Event Reports (LERs) database and the IAEA International Reporting System (IRS) database. Both databases are screened for the relevant events registered in period 2000-2015.
The scope is limited to events which resulted in non-interruptible power supplies failure or deficiency regarding any of their functions (parameters of the provided electricity, capacity of the system). The scope include all components of the non-interruptible power supplies as well as busses powering essential I&C and corresponding interconnections.
The analysis included the failures of all main components of the essential power system including: batteries, chargers/rectifiers including control logic/system; direct current buses;
inverters, alternate current buses for essential I&C and failures of other systems affecting or disrupting non-interruptible power supply system.
Figure 1 features a simplified schematic design of NPP power system with main components of the non-interruptible power supplies.
The events originated in components outside the system boundaries but significantly affecting the performance of components inside the boundaries are also included in the study.
Figure 1: NPP Electrical Power System with indicated scope of study
3 RESULTS
In the US NRC LERs database was screened with utilization of database search tool and relevant keywords and their combination for period 2000-2015. The 56 identified events are selected as relevant for the analysis.
The IAEA International Reporting System for Operating Experience (IRS) database was searched to identify representative events for the same time period. Initially, there were 47 event reports identified in the first screening and 15 events were selected for detailed analysis.
The events are classified into 6 categories considering: plant state, circumstances, type of events, type of equipment failed or concerned, detection of the event and direct cause.
Each event is classified into single best matching category with the exception for the characteristic related to the “failed or concerned equipment”.
3.1 Plant State
The events counts and percentage distribution by plant status in U.S. NRC LERs and IAEA IRS database are given on Figure 2.
Figure 2: Events counts by category (U.S. NRC LERs & IAEA IRS)
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Figure 2 shows that largest count in NRC LER is registered for events during power operation that contribute 86% to the total number of events.
Figure 2 shows that largest count in IAEA IRS is registered for events during power operation contribute 73% of all registered events.
3.2 Circumstances
The distribution of the events considering the circumstances in two analysed databases (NRC-LER and IAEA-IRS) is given on Figure 3.
Figure 3: Events counts by "Circumstances" (U.S. NRC LERs & IAEA IRS) Figure 3 shows that in U.S. NRC LERs the largest number of events is registered during normal power operation with 45%. This is followed by inspection and functional test with 23% and maintenance activities with 20%.
For events registered in the IAEA IRS database, as shown in Figure 3, the largest number of events is registered during normal power operation mode with a share of 40%.
3.3 Type of event
Events identified in U.S. NRC LERs classified by their type and modes are given in Figure 4.
Figure 4: Distribution of "Type of Events" (U.S. NRC LERs & IAEA IRS) Figure 4 shows that the largest number of events is registered for "Partial loss of power"
events with comparable number identified for the "Degraded state" events.
Figure 4 shows that the largest number of events is registered for "Partial loss of power"
events. Smaller number of events is registered for the "Total loss of power" and "Degraded state" events.
3.4 Type of equipment failed or concerned
The classified events in the U.S. NRC LERs database considering the "Type of equipment failed" are given on Figure 5.
Figure 5: LOOP events counts by "Type of equipment failed" (U.S. NRC LERs &
IAEA IRS)
Figure 5 shows that the largest number of events in the U.S. NRC LERs database is registered for the inverter and batteries failures.
The distribution of events in IAEA IRS is given in Figure 5, showing that the largest number of events is registered for the inverter and rectifier failures.
3.5 Detection of failure
Figure 6 show that largest numbers of events in U.S. NRC LERs are identified by the fault report in the control room followed by the events registered during maintenance activities and test. Figure 6 show that largest numbers of events in IAEA IRS are identified by the fault report in the control room.
Figure 6: Events counts by "Detection of failure" (U.S. NRC LERs and IAEA IRS)
3.6 Direct cause
The number of the events registered in U.S. NRC LERs by the direct causes are given on Figure 7.
Figure 7: LOOP events counts by "Direct causes" (U.S. NRC LERs and IAEA IRS)
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Figure 7 shows that the largest number of events in U.S. NRC LERs is resulting from electrical deficiency. The similar result is obtained for the IAEA IRS "Direct causes" given in Figure 7.
4 CONCLUSIONS
The results of statistical analysis of non-interruptible power supplies failure registered in two reviewed database, NRC LER and IAEA IRS, are presented. Both databases are screened for the relevant events registered in period 2000-2015. The identified events are classified into the predefined categories considering: plant state, circumstances, type of events, type of equipment failed or concerned, detection of the event and direct cause.
Obtained results show that considering circumstances largest number of events is registered for during normal power operation in both databases. Largest number of events is registered for "Partial loss of power" events in both databases with comparable number of events identified for the "Degraded state" in the NRC LER. Largest number of events resulted from inverter and batteries failures in NRC LER and inverter and rectifier failures in IAEA IRS. Most of the events registered in both databases are identified by the fault report in the control room. Electrical deficiency is identified as major direct cause for the events in the analysed databases.
The number of the events of failures of essential power system is mot negligible and requires further and detail analysis. Observations and lessons learned from those studies can be beneficial for the improvement of the reliability of the essential power system and consequently the nuclear safety.
ACKNOWLEDGMENTS
This research is supported partly by the European Union (EU) Framework Programme for Research and Innovation Horizon 2020 (research project 2014-PTT-F-40-000-2735) and partly by the Slovenian Research Agency (research program P2-0026).
REFERENCES
[1] IAEA, 2012, "Electric grid reliability and interface with nuclear power plants,"
International Atomic Energy Agency, Vienna.
[2] Volkanovski, A., 2013, "On-site power system reliability of a nuclear power plant after the earthquake," Kerntechnik, 78(2), pp. 99-112.
[3] NRC, U. S., 1988, "Station Blackout," Regulatory Guide 1.155Washington.
[4] IAEA, 2016, "Design of Instrumentation and Control Systems for Nuclear Power Plants ", IAEA.
[5] IAEA, 2016, "Design of Instrumentation and Control Systems for Nuclear Power Plants,"
International Atomic Energy Agency.