Improve Equipment Reliability and Performance by Implementing a Digital Turbine Control System Upgrade - MTA-EN-005

Administrative Items
Date	04/23/21
Functional Area Where Benefits Will Be Realized	Operations Maintenance Engineering
Reference Implementation Guidance	Turbine Overspeed Trip Modernization (EPRI 1013461) Handbook for Evaluating Critical Digital Equipment and Systems (EPRI 1011710) Cyber Security Technical Assessment Methodology: Risk Informed Exploit Sequence Identification and Mitigation, Revision 1 (EPRI 3002012752)
Industry SME	Matt Gibson – EPRI Contact: nuclearplantmod@epri.com
Previous Implementation	Digital turbine control upgrades have been implemented by numerous plants in the US nuclear fleet. Contact EPRI for additional information on previous implementation.
Implementation Enablers	N/A
SWEEP Score	Cost – Level 1 – Implementation cost is greater than $5 million per year. Savings – Level 2 – Savings are expected to be $1 million to $5 million per year. Payback – Level 1 – Payback period depends on avoided plant down time. However, based on the costs associated with a digital TCS upgrade, payback period is expected to be greater than five years. Licensing Readiness – Level 3 – This technology has been successfully implemented at nuclear sites. However, plants may have to implement licensing document changes. Technology Readiness – Level 3 – This technology has been successfully implemented for TCS at numerous nuclear sites and has extensive installations at fossil power stations. Implementation proficiency – Level 2 – A systematic engineering approach should be taken when implementing a digital turbine control system upgrade and requires site-specific experience with digital technologies.
Applicability	All reactor types All geographic regions
Keywords	Turbine control system; reduced maintenance; instrumentation and control; digital upgrade; replacement modification
Business Case Analysis Cross-Reference	N/A

Description

Analog turbine control systems (TCSs) that were part of the original design of many operating nuclear plants may now have aging components with reduced reliability that are difficult to repair or replace. Original Equipment Manufacturers (OEMs) may have even discontinued support for these components. Furthermore, these aging analog components can be single‑point vulnerabilities (SPVs) that cause unplanned shutdowns and operational inefficiencies from difficulties in using the analog interface. A digital upgrade to the TCS will address the obsolescence, reliability, and maintenance issues that affect analog TCSs.

Digital upgrades to the TCS include replacement of analog components with modern components that are supported by OEMs. These digital components are less susceptible to environmental conditions that can affect system performance. Additionally, digital upgrades can eliminate SPVs through system fault tolerance and component redundancy. As an example of system fault tolerance, triple modular redundancy (TMR) uses a triplicated architecture within the controlling platform (e.g., Programmable Logic Controller) from input, control processor, to output. Synchronizing of, and voting by, the triplicated processors supports continued control and protection functionality in the presence of failed or faulted system components. Component redundancy of field instruments provides for “majority‑voting” (e.g., “two‑out‑of‑four”) of critical parameters. Failure of any one component (e.g., pressure transmitter) in a redundant arrangement would not result in a spurious trip as the failed component/signal is voted out of the logical protective function. Thus, implementing a digital upgrade can reduce SPVs and potentially decrease the occurrence of unplanned shutdowns and down‑powers.

Additionally, digital upgrades to the TCS can improve operator efficiencies through enhanced digital interfaces. Analog systems are manipulated and interpreted through a hard‑panel interface with various switches, buttons, meters, and indicators; in an analog system, these components must be used or read manually in the performance of turbine operation and testing, such as quarterly turbine‑valve movement tests and monthly trip tests. A digital interface allows for the automation of several tests and reduces human error that may result from using analog components. Digital platforms also provide interface or integration with plant performance computers and asset management systems allowing comprehensive system monitoring, trending, and diagnostics.

Benefits

Benefits Estimate

Level 2 – Savings are expected to be $1 million to $5 million per year, for replacement of an aging analog TCS. These savings include a reduction in lost revenue related to unplanned shutdowns/delays and down‑powers, reduced maintenance resulting from removal of obsolete analog equipment, and improved system performance. Extent of savings depends on system reliability prior to upgrade and is therefore plant‑specific.

Benefits Description

• Improves plant reliability and potentially reduces the occurrence of unplanned shutdowns and down‑powers by reducing SPVs; reduced shutdowns allow for decreased plant downtime and unit capacity factor improvements.
• Reduced operations and maintenance costs from increased component reliability and elimination of analog system components; digital components are less susceptible to environmental conditions that can affect system performance.
• Improved operator efficiency through the automation of select operations and periodic tests.

Costs and Schedule

Cost

Level 1 – Implementation costs are expected to be greater than $5 million, and potentially significantly greater than $5 million. As an example, one utility that upgraded turbine control systems for its main turbine and two main feed pumps reported total costs for the set of three upgrades in the range of $60 million to $80 million. Costs include the capital expenditure for the upgrade as well as training associated with operation and maintenance of the new system.

Schedule

Greater than three years, including all phases from planning to system commissioning. Up to two years may be needed to train all maintenance personnel, facility operators and system engineers, and to complete procedure and user manual revisions.

Scope Context

Per unit.

This MTA focuses on digital upgrades to the TCS; other systems that may be upgraded at the same time as the TCS are not in this MTA’s scope.

Risks

• Digital TCS systems can exhibit subtle and unexpected behaviors and failure modes, which can result in more severe and difficult‑to‑predict consequences compared to the predecessor analog systems. This risk can be mitigated in part by employing a “critical digital review” (CDR) technique (described in EPRI 1011710) that examines the design of the digital system and provides assurance that the digital system will function as needed and expected.
• Implementing a TCS digital upgrade may require changes to the Final Safety Analysis Report (FSAR). Plants should review their FSAR and other licensing documents to determine whether such changes are needed.
• A TCS digital upgrade may introduce cyber security vulnerabilities. For example, an attacker could potentially gain access to the TCS, manipulate turbine valves, and damage equipment. This risk can be mitigated by following the appropriate cyber guidelines, such as those presented in EPRI 3002012752.