The role of I&C systems in power uprating projects

1. The role of I&C systems in power uprating projects

2. Why to uprate? • Market changes induce the claim to operate the plants in an ever increasing efficiency. • Efficiency can be increased either by a better utilization of existing capacities or by increasing the capacities. • The utilities are aiming for additional production through the better utilization of available assets. • Gaining public acceptance to increasing existing nuclear power plant capacity is significantly easier than that to constructing a new NPP.

3. Conditions enabling power uprating • The average age of the nuclear units operating for the time being is above 20 years. • The units were designed in the mid-seventies. • Today, there is a more accurate knowledge on the behavior of structural materials and integrated effects of external and internal factors exerted on the components. • Today demands affecting components during transients can be defined more exactly, uncertainties of calculations can be reduced, and, as a result, the conservatism applied in the original design can be reduced. • Today more accurate and reliable control and assessment methods are available (accuracy of measurements, reduction of detection thresholds, etc) • Knowledge related to nuclear fuel and core thermal hydraulics also had a considerable development. Fuel utilization arose to one and a half times of that in the eighties.

4. Definition of power uprate • The process of increasing the maximum licensed power level at which a commercial nuclear power plant may operate is called a power uprate. (Definition from the U.S. NRC) • Types of power uprates • measurement uncertainty recapture power uprates • stretch power uprates • extended power uprates

5. Basic ways of power uprating Damage limit, or ultimate capability H4 Safety limit H3 Margin for RPS initiation Limit value for initiating RPS H2 RPS uncertainty Operating limit (in the design documents and the Tech. Spec.) H1 Normal operation Operational margin • Reducing uncertainty • Improving efficiency • Increasing thermal power

6. Measurement uncertainty recapture uprates • The reactor thermal power is validated by the nuclear steam supply system energy balance calculation. • The reliability of this calculation depends primarily on the accuracy of feedwater flow, temperature, and pressure measurements. • Because the measuring instruments have measurement uncertainties, margins are included to ensure the reactor core thermal power does not exceed safe operating levels. • 10 CFR, Part 50, Appendix K (1973), required licensees to assume a 2.0 percent measurement uncertainty for the reactor thermal power. • The current rule (2000) allows licensees to justify a smaller margin for power measurement uncertainty when more accurate instrumentation is used to calculate the reactor thermal power.

7. Measurement uncertainty recapture uprates 2. • Measurement uncertainty recapture (MUR) power uprates are achieved by implementing enhanced techniques, such as the improved performance of plant equipment both on the primary and secondary side, protection and monitoring system, operator performance, etc. These uprates are less than 2% measured in electrical output power. • The use of state-of-the-art feedwater flow measurement devices that reduce the degree of uncertainty associated with feedwater flow measurement can be an example.

8. Measurement uncertainty recapture example

9. Stretch power uprates • Uprates are typically up to 7-percent and are within the design capacity of the plant. The actual value for percentage increase in power a plant can achieve and stay within the stretch power uprate category is plant-specific and depends on the operating margins included in the design of a particular plant. • Stretch power uprates usually involve changes to instrumentation setpoints, but do not involve major plant modifications. This is especially true for boiling-water reactor (BWR) plants. • In some limited cases where plant equipment was operated near capacity prior to the power uprate, more substantial changes may be required.

10. Extended power uprates • Extended power uprates are greater than stretch power uprates and are usually limited by critical reactor components such as the reactor vessel, pressurizer, primary heat transport systems, piping etc., or secondary components such as the turbine or main generator. To cope with these limitations, extended uprates usually require significant modifications to major balance-of-plant equipment such as the high-pressure turbines, condensate pumps and motors, main generators, and/or transformers. Extended power uprates have been approved for increases as high as 20 percent.

11. Power uprates and I&C • Necessary modifications in the instrumentation and control systems in relation to power upratings are usually not very substantial. The following preconditions must be fulfilled in the frame of I&C: • sufficient measurement ranges • sufficient accuracy of process parameter measurements • sufficient calculation algorithms to indicate credible reactor thermal power • sufficient possibilities for the adaptation of new limit values in the Reactor Protection System, limitation systems and control systems

12. Typical examples of I&C changes • Modification of specific control systems to enable operation under different conditions. • Inclusion of additional process sensors • Replacement of sensors by ones with improved accuracy • Optimised calculation of the measurement uncertainties permitting a reduction in the margin applied to the measurement of reactor thermal power. • Modification of the reactor protection system setpoints • Changes in the appropriate HSIs to accurately assess the current state of the plant • Changes in alarm setpoints • Changes in the instrument calibration procedures • Adjustment of the plant computer and safety parameter display system • Development of additional instrument validation processes

13. Power Uprating inthe Paks NPP

14. Early activities (uprate from 440 to 465 MW) • High Pressure Turbine • Blades of all stages of the High Pressure Rotor – the 1st stage excepted – were exchanged. • From the diaphragms in the HP housing, stages No. 5 and 6 were exchanged. As for the diaphragms of stages No. 2, 3 and 4, only the projections above the bandage were replaced. • Final (end) - and diaphragm sealing were exchanged from flat springs to spiral ones. • Low Pressure Turbine  • Blades of rotor stages No. 1, 2, 3 and 4 were exchanged. • From diaphragms of LP housing, those of stages No. 1, 2, 3 and 4 were exchanged. • Having the steam separator exchanged, steam intake of the LP housing was modernized. • Final (end) - and diaphragm sealing were exchanged from flat springs to spiral ones. • Took place from 1997 to 2001

15. The replacement of the turbine rotor

16. Modifications for the new, 8% power uprating • New fuel • Primary circuit • Core monitoring (VERONA) • Secondary circuit • Electrical systems • I&C systems • No feedwater flow measurement problems

17. Systems, components The essence of the modification The reasons for the modification The specific modifications Fuel development Increased lattice pitch, Hafnium inserts in the upper part of control fuel assemblies. It is required for utilizing the contingencies, to attain the 108% reactor power, Primary circuit Improvement of the primary circuit pressure control, It is required for utilizing the contingencies and safe operation at the enhanced power level, Increasing the primary coolant circulation in Unit 2, It is required for utilizing the contingencies and safe operation at the enhanced power level, Modification of Hydro-accumulator parameters It is required for the safe operation at the enhanced power level. Modifying the lowest boric acid concentration in the emergency systems to 13,5 g/kg, Required for the safe operation

18. Core surveillance, VERONA Implementation of a new system and new reactor-physics calculations. Required for reducing the margins, and safe operation at the enhanced power level, The specific modifications (cont’d) Turbine, secondary circuit Exchange of turbine nozzle ring, Provides support for improving the efficiency and operation at the enhanced power level. Modification of turbine control, Electrical systems Improving the Generator Cooling, Provides support for the operation at the enhanced power level. Exchange of encased bus bars in Units 1 and 2, Reduction of the asymmetric load on the 6 kV household supply, Instrumentation and control systems Modification of Reactor Protection System setpoints, Needed for the safe operation at the enhanced power level. Modification of set values of control and protection systems and interlocks.

19. The new encased bus bar for Units 1 and 2

20. The new encased bus bar for Units 1 and 2

21. Implementation of power uprating in Paks • The first modified fuel was loaded in 2005 in Unit 4 (one third of the core). In 2006, when the Unit 4 reactor contained two loads of the modified fuel, the 108% power could be attained. • The stepwise increase of power, however, required a test run at about 104% for several months; thus, the further increase up to 108% took about four months from the unit restart, and was reached on 28 September, 2006. • As for Units 1-3, operation of the fuel assemblies that are suspected to contain deposits will be terminated probably in 2006 in Unit 1, and in 2008 in Unit 2. Power uprating in a core loaded with fuel assemblies with deposits is not considered.

22. Actual state Value expected at 108% power level Main parameter changes after power uprating Reactor Thermal Power 1375 MW 1485 MW Primary circulation 39400-40300 m3/h 40300-41000 m3/h Cold leg temperature 265,0-265,5 °C 266,0-266,5 °C Hot leg temperature 295,2-295,7 °C 298,4-299,5 °C Shut-down boric acid concentration 12,0 g/kg 13,5 g/kg Fresh steam pressure 43,15 bar 43,15 bar Fresh steam mass flow 1350 t/h 1467 t/h Fresh steam temperature 254,9 °C 254,9 °C Moisture content ~0,3 % * ~0,5 % *

23. Primary circuit pressure control improvement

24. Physical parameters limiting thermal power • Maximum allowable temperature at the core sub-channel outlets: 325 oC • Corresponding primary circuit pressure: 120,57 bar

25. The saturation temperature and pressure bar 0C

26. The saturation temperature and pressure 2,3% (7.5 0C) uncertainty band bar Real operating temperature 0C

27. Characteristic of the old pressure controller

28. Primary circuit pressure at a regular weekend

29. Physical parameters limiting thermal power • As a result of power increase, the core outlet temperature increases, nearly proportionally with the power increase, thus it gets nearer to the saturation temperature. • The primary circuit pressure control system must ensure a finer maintenance of the primary circuit pressure, the margin of saturation shall be kept at the required level. • The operating pressure must be maintained at a stable 123.0 bar with an accuracy of +/- 0.25 bar.

30. The saturation temperature and pressure 2,3% (7.5 0C) uncertainty band bar Real operating temperature 0C

31. The saturation temperature and pressure 2% (6.5 0C) uncertainty band bar Real operating temperature 0C

32. Static characteristic of the new controller

33. The structure of the new system Profibus coupler Main PLC (Simatic S7) Pressurizer vessel FieldPLC-1 (WAGO 750) Injection valves-1 Pressure transmitter (Rosemount 3051) FieldPLC-2 (WAGO 750) Injection valves-2 FieldPLC-3 (WAGO 750) Solid state switch Electricalheaters Network switch (Hirschman) Process computer-1 Processcomputer-2 Supervisor Notebook

34. The main controller

35. Electrical heater solid state controller

36. The injection valve controller

37. IAEA TECDOC on“The Role of I&C Systemsin Power Uprating Projectsin NPPs”

38. IAEA TECDOC

39. TECDOC contents • Introduction to power uprating • Limits, margins and their relevance to I&C • Calculation of thermal power • Impact of power uprating on plant I&C • Human and training aspects • Regulatory aspects • I&C implementation guidelines for power uprating • I&C benefits and lessons learned from power uprating • Key recommendations • References • Glossary • Country reports

40. Key recommendations • It is important to fully understand the safety and technical bases for the claimed margins and limits. • It is important to fully evaluate the areas of potential measurement uncertainty. • Power uprates could potentially lead to various unwanted effects. It may be necessary to add new instrumentation to ensure that the operating conditions at the higher power level are adequately monitored and controlled. • A power uprating could provide the opportunity for a wider modernisation of the plant I&C systems. • A comprehensive analysis should be undertaken covering all aspects of plant behaviour in all operational modes to provide input for the modified I&C design. • It is important to consider the changed (possibly more severe) operating conditions for I&C equipment, qualification, etc.

41. Key recommendations (cont’d) • Particular attention should be paid to the design of the HSI modifications (if any), and of integration of this with the existing HSI, to ensure that operating staff performance is enhanced rather than degraded. • In terms of the licensing application for a power uprate project, it should be noted that the Regulatory Authority will require the licensing submission to positively demonstrate that the existing safety level has been maintained or preferably increased, including all the I&C aspects and consequences of it. • Experience feedback from past power uprate projects has shown that some plants have incurred serious problems with their implementation (e.g. inadvertent violation of licensed power limits), due to instrumentation issues. Lessons learned from other PU projects should be considered.

42. Thank you for your attention! Any questions?