Grid Stability and Safety Issues Associated With Nuclear Power Plants

1. Grid Stability and Safety Issues Associated With Nuclear Power Plants Dr. John H. Bickel, Ph.D. Second Workshop on International Grid Interconnection in Northeast Asia Shenzhen, China May 7, 2001 Evergreen Safety & Reliability Technologies, LLC

2. Items to Be Discussed: • What are NPP Safety Issues with Electric Power Grid Scenarios? • How do US and International Nuclear Safety Standards address Electric Power Grid Reliability concerns? • What Scenarios should be Studied in a Grid Evaluation of LWR Project in DPRK? Evergreen Safety & Reliability Technologies, LLC

3. NPP Safety Issues Evergreen Safety & Reliability Technologies, LLC

4. What are NPP Safety Issues with Electric Power Grid ? • Load Rejection, Loss of External Load. • Degraded Voltage / Frequency. • Loss of Offsite Power due to External Grid Disturbance. • NPP trip causing cascading grid collapse and Loss of Offsite Power to NPP. Evergreen Safety & Reliability Technologies, LLC

5. Safety Issue: NPP Heat Persists a Very Long Time 3 . 1 10 Decay Heat in Megawatts Q ( t ) 100 10 3 4 . . 1 10 100 1 10 1 10 t Time in Seconds Evergreen Safety & Reliability Technologies, LLC

6. Safety requires more than Automatic Shutdown. • Electric power needed indefinitely to remove fission product decay heat. • Electric power is needed for instrumentation, control, and monitoring systems in control room. • Battery power for instruments typically available for ~ 4 hours (without recharging). • Electric power needed for HVAC systems used for assuring operable environments for equipment and personnel. Evergreen Safety & Reliability Technologies, LLC

7. Load Rejection / Loss of Load • Typically caused by sudden disconnection of major load Evergreen Safety & Reliability Technologies, LLC

8. Load Rejection / Loss of Load • Load Rejection results in sudden (step) decrease in electrical and reactor demand. • Turbine/generator shaft speeds up • Reactor controls respond by inserting control rods to balance power output with demand. • If imbalance too large or controls inadequate, leads to reactor/turbine trip. • Supply to remaining loads lost. Evergreen Safety & Reliability Technologies, LLC

9. Degraded Voltage: • A drop in grid voltage to levels which do not support equipment operation. • Degraded voltages of concern are plant specific (typically <85%nom). Evergreen Safety & Reliability Technologies, LLC

10. Degraded Voltage: Large Pump Motor Minimum Starting Voltage Requirements vs. Frequency Evergreen Safety & Reliability Technologies, LLC

11. Degraded Voltage: • Large motor minimum starting voltages are typically 85% Nominal. • Lower voltages cause over-current and opens protective fusing or breakers. Thus: pumps can not be started, running pumps trip • Degraded voltage to running pump motors will cause over-current and over-heating. • Nuclear power plants are provided with logic to sense degraded voltage condition and trip reactor. Safety analyses presume back-up power sources have correct voltage and frequency for long term decay heat removal. Evergreen Safety & Reliability Technologies, LLC

12. Degraded Voltage: • A window exists for allowable in-plant voltages. • Protective system voltage setpoints must be: • Sufficient to protect safety related cooling pump motors during starting transients, >85% • Not too high - so that protection settings do not result in premature unit trip and disconnection from grid, typically < 93% Evergreen Safety & Reliability Technologies, LLC

13. Degraded Frequency: • Grid frequencies which result in abnormal AC motor operation Evergreen Safety & Reliability Technologies, LLC

14. Degraded Frequency Concern: • In a narrow range: reactor coolant flow ƒ (grid frequency). • Reduced grid frequency reduces coolant flow. • NPPs provided with logic to sense severe degraded frequency, trip electrical buses, thus tripping reactor. • Safety analyses presume back-up power sources have correct voltage and frequency for long term decay heat removal. Evergreen Safety & Reliability Technologies, LLC

15. Loss of Offsite Power Concern: • Loss of Offsite Power causes numerous reactor protective trips (turbine/generator trip, low coolant flow, loss of feed water flow, etc.). • In-plant electrical loads must then be temporarily powered by batteries and stand-by diesels until offsite power restored. • Diesels typically fail to start or run ~ 1% of time. • Diesels are not as reliable as offsite grid. Evergreen Safety & Reliability Technologies, LLC

16. Example Trip of Large NPP: Evergreen Safety & Reliability Technologies, LLC

17. NPP Trip Causing Loss of Offsite Power: • Following any plant trip, safety analysis presumes back-up power sources will be available at correct voltage and frequency. • If nuclear power plant “carrying” a disproportionate amount of system load suddenly trips… collapse of the grid is likely. • Two incoming circuits to NPP are not independent in this specific case. • Outcome similar to Loss of Offsite Power but is caused by NPP itself. Evergreen Safety & Reliability Technologies, LLC

18. US Experience on this Issue Limited to Proposed Puerto Rico NPP in 1960’s • All other US NPPs are sited on very large interconnected stable grids • Problem: NPP sited on North Coast of island of Puerto Rico – small isolated grid • US regulators found it necessary to require additional features to assure availability of emergency AC power • Features could include: additional onsite power sources, additional dedicated offsite sources. Evergreen Safety & Reliability Technologies, LLC

19. Regulatory Standards: • US NRC Regulations • IEEE Standards • IAEA Safety Standards Evergreen Safety & Reliability Technologies, LLC

20. US and International Standards Address Grid Reliability: • NRC Regs. on Electric Power supply found in Title 10, Code of Federal Regulations, Appendix A, GDC 17. • Industrial Standards such: IEEE Std 308-1980, IEEE Std 741-1990… provide further technical guidance acceptable to US NRC. • Similar requirements found in IAEA Safety Guide No. 50-SG-D7 (Emergency Power Systems at Nuclear Power Plants). Evergreen Safety & Reliability Technologies, LLC

21. GDC 17 Requires: • “An onsite and offsite electric power system shall be provided to permit functioning of structures, systems, and components important to safety.” • “.. each system shall provide sufficient capacity and capability to assure: • SAFDLs and reactor coolant pressure boundary not exceeded.. • Core is cooled and containment integrity and other vital functions maintained in event of postulated accidents.” Evergreen Safety & Reliability Technologies, LLC

22. “onsite electric power supplies, including the batteries, and onsite electric distribution system, shall have sufficient independence, redundancy, and testability to perform their safety functions assuming a single failure.” Evergreen Safety & Reliability Technologies, LLC

23. “Electric power from transmission network to onsite electric distribution system shall be supplied by 2 physically independent circuits (not necessarily on separate right-of-ways) designed and located to minimize likelihood of their simultaneous failure..” • “A switchyard common to both circuits is acceptable.” Evergreen Safety & Reliability Technologies, LLC

24. Example NPP Interface With Grid: Evergreen Safety & Reliability Technologies, LLC

25. “Each of these circuits shall be designed to be available in sufficient time following a loss of all onsite alternating current power and the other offsite alternating current power circuit…” • “One of these circuits shall be designed to be available within a few seconds following a LOCA…” Evergreen Safety & Reliability Technologies, LLC

26. “Provisions shall be included to minimize probability of loosing electric power from any remaining supplies as a result of, or coincident with loss of power generated by nuclear power unit, loss of power from transmission network, or loss of power from the onsite electric power supplies. Evergreen Safety & Reliability Technologies, LLC

27. NPP – Electrical Grid Interface Evergreen Safety & Reliability Technologies, LLC

28. Key Grid Assumptions for NPP Design: • NPPs assumed to be “Base Load Units.” • Grid is assumed to to supply stable source of start-up AC power for all major plant loads. • Starting transients of largest pump motors will not droop grid voltage/frequency to levels where automatic trips occur. • NPPs assumed not to load any faster than 5% /minute within limited range, due to nuclear fuel metallurgical limits. • Increased generation during grid emergency can only be supplied by fast responding units (e.g. Hydroelectric Dams, Gas Turbines) - not NPPs. Evergreen Safety & Reliability Technologies, LLC

29. Key Grid Assumptions Used in NPP Design: • NPPs assumed to unload at 5-10% /minute without tripping. • Any Demand / Generation mismatch resulting in frequency decay must be accommodated by. • Voltage reductions (~ 5%) coordinated by Regional Dispatch Center. • Automatic “Load Shedding” if frequency, voltage decay becomes worse. Evergreen Safety & Reliability Technologies, LLC

30. Automatic Loading at 5% / minute: Evergreen Safety & Reliability Technologies, LLC

31. Load Rejection / Loss of Load: • Most NPP designs capable of controlled runback (without trip) following 40% Load Rejection via Fast Turbine Control Valving, Steam Bypass, and Automatic Reactor Control Rod Insertion. • When Load Reject capability is exceeded, NPPs will trip – and will not be able to return to service for many hours. • CE System 80 design is capable of fast controlled runback to house load following 50% loss of Main Feedwater, a Full Load Rejection, or temporary Loss of Load. Evergreen Safety & Reliability Technologies, LLC

32. Power Output Using RPCS: Evergreen Safety & Reliability Technologies, LLC

33. Issues Which Should be Studied in a Grid Simulation for LWR Project in DPRK: • During startup conditions (DPRK grid supplying house loads): Can grid support start of largest single of pump motor without causing voltage / frequency drop that will separate NPP from grid? • If Not: What nature of additional supply would be necessary via an interconnection? Evergreen Safety & Reliability Technologies, LLC

34. Issues Which Should be Studied in a Grid Simulation for LWR Project in DPRK: • During LWR operation: At what output levels does NPP trip result in voltage / frequency drop that will result in need to isolate NPP from grid? • If a problem: What nature of additional supply would be necessary via an interconnection? Evergreen Safety & Reliability Technologies, LLC