By Brian Wheeler, Editor
The concept of upgrading systems in nuclear power plants to digital is not new. In the early 1980s, some manufacturers of instrumentation and control (I&C) equipment were already converting to digital. The initial use of digital, even in safety-related applications, were in things such as displays, indicators, recorders, and other types of equipment where digital systems could be presented more usefully for operators to digest a lot of information in a convenient manner, said David Rahn, senior electronics engineer within the Division of Engineering of the Office of Nuclear Reactor Regulation at the Nuclear Regulatory Commission (NRC).
Following the Three Mile Island accident in 1979, different applications, such as radiation monitoring systems, were upgraded to digital. And shortly after, power generation functions and balance-of-plant systems began being upgraded to digital. In the U.S., digital I&C systems have been used in applications such as feedwater control systems, turbine control systems and demineralizer control systems.
“The analog controls were getting somewhat antiquated and hard to maintain and you could use digital technology to help improve reliability at the plant of that function,” said Rahn.
The NRC says digital devices used in safety-related systems make use of advanced technology that has the potential to improve plant safety and operation performance. Designing digital systems also gives operators fault-tolerant capabilities and online diagnostics. Richard Stattel, senior electronics engineer within the Division of Engineering of the Office of Nuclear Reactor Regulation at the NRC said analog systems, though, are safe and have operated very well over the past 30-plus years.
“But they are very problematic from a maintenance perspective because there is very little monitoring or diagnostic capabilities in those systems,” he said.
Several items have pushed the switch to digital, such as aging and obsolescence, said Joe Naser, program manager for the Electric Power Research Institute’s I&C program.
“Equipment is old and it is hard to find spare parts,” said Naser. “And vendor expertise at those vendors is disappearing in those certain types of equipment as well.”
Due to the systems being 30 to 40-years-old, failures are also occurring more and determining the corrective action to prevent failures in the future is becoming more difficult.
“Clearly one of the benefits, and enhanced capability, these systems provide is improved accuracy and response time,” said John Thorp, chief of the Instrumentation and Controls branch in the Division of Engineering, Office of Nuclear Reactor Regulation at the NRC.
With the NRC approval of the Eagle Series, the Common Qualified Platform (Common Q), Triconex, and the Teleperm XS systems for safety-related applications, operators may consider their use in retrofitting digital I&C systems at their nuclear facilities.
North Carolina-based Duke Energy has made the investment to update one of its plants to digital safety-related systems. According to Thorpe, Duke was the first operator to install an entire digital reactor protection system (RPS).
Duke completed the first RPS and engineered safeguards (ES) system installation at Unit 1 of the three-unit, 2,538 MW Oconee nuclear station in South Carolina during the spring outage of 2011. Unit 1 was commissioned in July 1973, followed by Unit 2 in September 1974 and Unit 3 in December 1974. Duke won approval from the NRC and delivery of testing began in 2009. After the spring 2012 outage, Duke has now completed digital RPS/ES upgrades at both Unit 1 and Unit 3. Duke has also won approval from the NRC to upgrade Unit 2, which is expected to take place in 2013.
|The three-unit Oconee nuclear station in Oconee County, S.C. generates 2,538 MW for Duke Energy. Unit 1 began commercial operation in 1973, followed by units 2 and 3 in 1974. All photos courtesy of Duke Energy.|
“The analog systems that were in place were becoming obsolete, in terms of being able to get replacement parts,” said Sandra Magee, Duke spokesperson for the Oconee station. “We knew we had to replace those.”
The digital control system upgrades being completed are part of $2 billion in upgrades Duke is making to keep the three units running safely for the next 30 years. The new digital control panels for the three reactors cost about $250 million.
“This is not a cheap prospect for (operators),” said Thorpe. “This is an expensive modification for them to make.”
The first installation at Unit 1 was completed in 68 days. Magee said by incorporating lessons learned and experience, Duke was able to complete the second installation in 54 days, three days ahead of schedule.
Duke selected and installed the Areva Teleperm XS platform, a system that has been installed or is on order at 74 units at 43 sites, across 14 different countries.
“We incorporated lessons learned from the first installation of our digital I&C system in the U.S. and successfully delivered another on-schedule installation,” said Mike Rencheck, chief executive officer of Areva Inc.
|Duke Energy’s Oconee Nuclear Station used Areva’s TELEPERM XS technology package to upgrade its reactor protection system and engineered safeguards protection system from analog to digital technology.|
The Teleperm XS I&C system is a fully digital system used to monitor and control the processes and equipment of a nuclear plant.
“The new system will help operators deal with situations much quicker and easier,” said Magee. “And it is more resilient to problems with instruments.”
The NRC in July approved Areva’s Simulation Validation Testing tool, known as SIVAT, which was developed to perform validation testing on digital I&C systems. Areva said the simulation testing will enable nuclear utilities to more accurately and easily test systems.
In Virginia, testing is also taking place at the Center for Advanced Engineering and Research (CAER), an industry-specific research and development center. The CAER has four major programs taking place, two of which focus on the development of the next generation of nuclear plants. One of those programs is the Center for Safe and Secure Nuclear Energy, which is centered around the control room, digital I&C, human factors and simulator capabilities.
Nearly 70 percent of the 30,000-square-foot facility is dedicated to research. Of that space, 3,000 square feet is designed to be a reconfigurable control room for the next generation of nuclear plants. In that room, there is a strong focus on the shift from analog to digital controls. At CAER, researchers can monitor operators as they train on and learn how to use digital systems, such as flat screen monitors and touch screens, rather than antiquated analog components such as dials, lights, knobs and switches.
“For most plants, you are talking about changing technology and changing expertise,” said Ray Torok, senior project manager at EPRI. “This is not something you do in six months. This is a multi-year process that involves careful orchestration of multiple hardware and software upgrades as well as training and modernization of processes to address digital technology issues.”
In January 2012, Areva started conducting human factors testing at the Center. Using a highly-digital, full-scope main control room simulator, the human factors piece to the control room can capture human performance data. From a separate observation room, operators are being watched by researchers as they control the simulator.
“Some things are not visible up-front, such as processes and training,” said EPRI’s Naser.
|Oconee reactor operator Chad Morgan participates in one of the verification steps involved in the upgrade.|
All eyes are now on Pacific Gas & Electric (PG&E), NRC’s Thorp said, as the utility works to upgrade the two-unit, 2,300 MW Diablo Canyon plant in California. During a scheduled outage in June, PG&E installed a new digital process control system (PCS) at Unit 1. The PCS both controls and monitors various systems and provides indications for the control room, PG&E said.
In 2009, though, PG&E submitted plans to the NRC to upgrade the reactor trip system and engineered safety features actuation system to digital. The existing Eagle 21 system at Diablo Canyon is being replaced with a combination Tricon Triconex and CS Innovations Advanced Logic Systems system.
Invensys Operations Management in July said version 10 of the Triconex Tricon controller was approved by the NRC for use in safety-related, nuclear power plant I&C applications, such as RPS. The Tricon controllers, of which more than 10,000 are in use today in safety control applications, are being installed at Diablo Canyon as PG&E continues to make the shift from analog to digital.
|Oconee teammates to work to complete equipment installation at Oconee during the Unit 1 outage.|
“Partnering with Pacific Gas & Electric and the Diablo Canyon team to renew nuclear safety certification for the Tricon controller gives other nuclear plants wanting to modernize their safety systems and meet the latest regulatory guidelines a strong point of reference,” said Harry Forbes, senior analyst with ARC Advisory Group.
The Tricon controller’s fault tolerant, triple-modular-redundant architecture removes single points of failure by integrating three isolated, parallel control systems and extensive diagnostics into one system.
Reviews and meetings are taking place between PG&E and the NRC as Diablo Canyon moves forward with the upgrade program. The NRC said the project is expected to be complete by 2014.
EPRI is in the midst of a project evaluating the benefits and risks associated with digital upgrades as compared to the benefits and risks associated with maintaining analog systems. This type of systematic assessment will help ensure regulators and nuclear plant owners make informed decisions that encourage safety and reliability improvements.
“There are going to be some analog systems that keep humming nicely as can be. And that’s great,” said Naser. “But you are going to see increasing amounts of digital.”