Limerick’s digital leap: Sustainability through modernization
The Limerick Digital Modernization Project began in 2019, the same year the Nuclear Regulatory Commission issued updated digital I&C interim staff guidance (ISG-06).
ISG-06 Rev 2 was to allow the NRC staff to rely on audits and reviews during a license amendment request (LAR) instead of requiring all design information up front, and Constellation planned to test that premise and share the results with other utilities.
Constellation worked closely with the Department of Energy’s Light Water Reactor Sustainability (LWRS) program and Westinghouse on a LAR that was submitted in September 2022 and approved in January 2026. Earlier plans envisioned system implementation at Unit 1 as early as spring 2026, but Constellation opted to have Unit 2 go first and is gearing up for system implementation in spring 2027.
Project director Steve Hesse has worked for Constellation and its predecessors as a nuclear engineer for over 40 years. He says he’s done some big projects in the past, but Limerick’s digital upgrade is “certainly one of the more interesting that I’ve had in my career.”
To hear more about what Constellation has learned, Nuclear News Editor-in-Chief Susan Gallier talked with Hesse, who was joined by three other project leaders: Jerry Segner, principal project manager; Mark Samselski, design engineering manager; and Scott Schumacher, lead systems rep for the project, who has interfaced between operations and engineering on design iteration testing.
What makes this fundamentally different from previous digital I&C upgrades?
Hesse: The intent is to convert most of our analog safety systems to a digital platform to improve reliability and performance and navigate away from parts obsolescence challenges, including escalating costs for old analog parts. We’re integrating quite a few safety systems into one digital platform, which makes it unique, at least in our experience inside Constellation.
Segner: Until now, most digital I&C upgrades have been of the nonsafety variety, on nonsafety platforms, such as turbine control upgrades and feedwater control upgrades. Since the advent of ISG-06, which paved the way for licensing safety-related digital upgrades, this will be the industry’s first safety-related digital I&C control upgrade of this scale.
Samselski: I’ll add that we have fundamentally changed the user interface for operators. We’ve transitioned from physical hand switches, lamps, and analog indicators to a touch screen or mouse click–based interface. We wanted to design the main control room for the next generation of plant operators.
What made Limerick the right plant for Constellation’s first I&C upgrade at this scale?
Hesse: Because Limerick is a hybrid BWR4/BWR5, it was not originally designed to leverage digital technology very heavily, as opposed to some of the later designs. So Limerick had significant opportunity to upgrade from analog to digital technology. It’s certainly one of the premier plants in the Constellation fleet, with lots of remaining plant life. The first unit was placed in service in the mid-80s. There are many opportunities to leverage digital technology for future operations.
Who are your industry partners in this project, and what is their role?
Segner: In the inception of this project, we worked with the DOE through Idaho National Laboratory and the Light Water Reactor Sustainability [LWRS] program. We had a public-private relationship with them to do the R&D phase of the project and produce four industry reports—including a business case analysis model, vendor independent design requirements, a human factors engineering plan, and a LAR framework—and everything we produced for that project is publicly available through the DOE. Constellation subsequently entered into an assistance agreement with the DOE to execute the full-scale demonstration phase of the project at Limerick in conjunction with the DOE LWRS program.
When we turned our full attention to conceptual design and work specific to the Limerick project the Electric Power Research Institute came into focus, and we were one of the early adopters of their Digital Engineering Guide.
We have quite a tally of other folks that are in boots-on-the-ground support. Westinghouse is the OEM [original equipment manufacturer] supplier of the digital platform that will host all of the new system elements. We have teamed with several other companies to supplement the Constellation team and set ourselves up for success through the design and installation phases.
What systems are being replaced or upgraded? What does this look like in practice?
Schumacher: The systems we’re replacing support essentially all Tier 1 safety functions for a BWR, including the scram function, emergency core cooling function with its attached piece of decay heat removal, reactor isolation, cooling, and containment isolation. We are also migrating our existing redundant reactivity control system—which is the system we use to comply with the ATWS [anticipated transient without scram] rule—to a nonsafety platform and executing those functions there.
If you visualize a main control room, you’re dealing with an environment that controls approximately 100 systems with a vertical information flow—where alerting indications such as annunciation and alarms tend to be up high, instrument readouts are at about eye level, and controls are below that—and a horizontal function distribution, where an operator shifts from left to right or moves around the room as necessary when switching trains of equipment or functions.
Now, we’re taking a broad swath of controls and functions and bringing them all together, transitioning the concept of operations to a central workstation with distributed informational presentation and awareness.
Limerick employees in the control room simulator. (Photo: Constellation)
What’s being done to prepare operators for the transition?
Schumacher: Training needs were a primary consideration. We built and are currently running training on a fully digital glass-top simulator.
The operators have gone through six full cycles of training. It started off with a design that wasn’t fully mature. It was, “Okay, we’re not using hand switches anymore. Here are very simple, very basic equipment lineup changes using a touch screen.”
As the design matured and the operators got familiar with it, training became more sophisticated, getting into minor plant manipulations, minor accident scenarios, and then all the way through to the design basis accident. By approaching training in this way, we could really start to develop operator buy-in and trust. We were able to receive feedback, both solicited and unsolicited, on our design and on the enormous number of changes to operators’ concept of operations.
How will operators’ work be changing?
Schumacher: Far and away, the biggest change is a series of automations that enable operators to perform routine operations, surveillance tests, and, in transient conditions, some plant parametric control with a greatly simplified user interface. The advantage of putting this automation on the nonsafety platform is the operator can be in much more of a supervisory and oversight role than in an active role, walking across the control room to take compensatory actions for whatever evolution they find themselves in.
We’re incentivized to keep the safety platform simple and capable of performing its license functions and not much else. Where they’re really going to be interfacing and touching the plant in the day-to-day winds up being on the nonsafety platform, which provides situational awareness. You can think of it like a car, the safety system is your steering wheel and your instruments, and the nonsafety platform is your windshield that shows you where you are right now and what’s coming up.
What are the most meaningful improvements you expect to gain?
Samselski: It piggybacks on what Scott just described: improved operator interfaces. We have significantly reduced operator risk by automating surveillances, because digital technology surveils itself. Software doesn’t physically degrade over time. We have continuous self-diagnostics and diagnostics on all trains of equipment that are impacted by the modernization project.
We also have improved the spectrum of information and quality of information reaching the main control room. Rather than trying to maintain situational awareness with old analog meters a couple of feet away or an annunciator that says something generic, we have higher fidelity information that we bring to the operator so that they can make a more informed decision during any emergency situation.
Because it’s software-based, because we have self-surveillances, we’re not putting the plant in a lineup that can challenge nuclear safety. We will have less hands-on time spent ensuring the equipment can perform its safety function when called on.
Going down this path of modernization is really about transformation of your operating model, your business model. Moving toward a technology-centric model, using technology as a force multiplier to enable long-term lifespans for the existing LWR fleet.
How long did the project take, from planning to initial LAR submission to NRC approval? Did that timeline meet your expectations?
Samselski: We entered the relationship with the DOE and some of the other industry partners at INL and the LWRS program in mid- to third-quarter 2019. In the fourth quarter of 2020 we really started gaining traction and established the project relationship with Westinghouse. In January 2021 we started the process of preparing for the LAR submittal, and from that point it was about an 18-month process. The team worked hard through that time. It took maybe a month or two longer than we had hoped, but it was in September of 2022 when we submitted the license amendment request.
We used the ISG-06 process of meetings to streamline the review timeline, and once the LAR was submitted, the NRC then came up with a timeline for their review. Ultimately, earlier this year, we received the formal safety evaluation from the NRC. During the NRC review timeline, we had open item assessment reviews, which allowed us to provide clarity to the NRC for some questions and point to docketed information to help them resolve any outstanding questions. We did work through the hurdles to resolve any questions that the NRC had, and we ultimately had a safety evaluation that was favorable.
What did you learn as you went through that process with the NRC?
Samselski: When we started this project, the NRC had just released ISG-06, the alternate review process. As part of the research phase, we collected information to follow the new ISG-06 process, and we completed a LAR framework.
Our work predates the reform now going on at the NRC, including the ADVANCE Act, and we’ve been working with the Nuclear Energy Institute over the past few years to incorporate any lessons learned from the reports to get that feedback to the NRC. The culmination of that is really to get feedback to other key stakeholders in industry so that we could show that there is a streamlined process to modernization across the U.S. fleet to keep operating LWRs running for up to 100 years.
Hesse: From Constellation’s perspective, the presubmittal meetings that were structured around the alternate review process were very effective. It allowed us to vet the design and answer many questions prior to submittal so that we didn’t have to answer those questions in a more cumbersome RAI [requests for additional information] setting.
What were some challenges of working through the NRC process?
Samselski: The initial challenge was overcoming the cultural change of COVID during our development and presubmittal process. All of our presubmittal meetings were remote. That was probably one of the biggest hurdles for us, but that would not apply to the next licensee that follows this process.
Hesse: For the licensing action, you have to demonstrate human factors adequacy, and that is a challenge to do with a design that is not fully mature. So, we had to work through timing issues with the regulator on that. Similarly, for environmental qualification activities you have to submit your EQ reports when the design is not necessarily mature to the point where you know every part that needs to be qualified. Those were learnings for the NRC and for us, and that’s the kind of material that’ll go out there to help the industry evolve.
Now you have the go-ahead from the NRC. When will work begin during outages, and does that represent any schedule delay?
Hesse: The platform has been manufactured for both Limerick-1 and Limerick-2. In late November and early December, we’ll start to move the equipment to Limerick. Now we’re doing prework, infrastructure work, while the platform itself is still going through the final phases of testing.
In terms of schedule delays, I wouldn’t characterize us as being in a delay mode. I will tell you, however, that we’ve worked through the design iteration churn, and now we’re working through that in testing to make sure that all of the logic and interfaces in the control room are as they should be. We’re very methodically working through that with Westinghouse. We’re being deliberate and making sure that the product that gets delivered here is everything that it needs to be.
When will installation begin?
Segner: We’ll tackle installation in phases. Even as we’re continuing factory acceptance testing at the Westinghouse facility, we’ve started installing the infrastructure—so the cabling, the raceways, the nuts and bolts and conduit that will connect the systems together from an infrastructure perspective.
We’ve also taken advantage of working ahead in what we call an N-1 outage. In the outage prior to the primary installation outage, we began making room in the main control room by moving components that will be displaced by the modernized human-machine interfaces in the main control room, as well as removing abandoned cabinets in other areas of the plant for logistical reasons, to ensure materials aren’t encumbered by things that are no longer needed by the plant. We have undergone two N-1 outages successfully, making cosmetic changes and moving tactile controls in the main control room over the past two years.
Currently, we’re in the infrastructure installation phase. We’re wrapping up Unit 1 and we’ll begin Unit 2, which will take us up to next year. We’ll have a pre-outage phase where we have an exemption to the ATWS rule that allows us up to 30 days prior to the refueling outage to remove that ATWS mitigation system from service, both divisions, and begin decommissioning that and replacing those portions that will be part of the new ATWS system. From there, we transition to the actual refueling outage, when the main system with all the cabinets and interfaces will be installed. That will happen over two outages. The lead unit, Unit 2, is currently slated for spring of 2027 and the lag unit for the spring of 2028.
Will the main system installation affect outage critical path?
Hesse: Yes, the installation will likely be the critical path of the outage.
What are the next steps at Limerick, and what digital modernization project might Constellation tackle next?
Hesse: We’re right now heavily into final planning activities for the outage installation. We’re in tag-out preparation activities, and we’re continuing to revise several thousand procedures to get them in place and ready to go. We’re also into preparations for site acceptance testing. As soon as the new systems get here, we will build them out and test them locally on-site to ensure that nothing was damaged in transit and that we know how to best integrate the system with our other existing plant systems, our plant process computer, and our cyber systems as well.
Within the Constellation fleet, there are other smaller but equally challenging safety-related digital upgrades that are in flight or under consideration, and certainly we’re going to apply the learnings from this project to those activities.
Does Constellation see this project as a model for the rest of the U.S. nuclear fleet?
Hesse: That is certainly the intent, that’s why the DOE embarked on this partnership. The expectation is that other utilities avail themselves of the materials being placed out there through INL and the DOE. We are sharing lessons learned because certain other utilities are going down very similar paths here, just slightly behind us on the safety side.
My advice is that you write thorough and robust requirements specifications in advance so that you have a clear road map for moving forward, engage the regulator early, and avail yourselves of the presubmittal meeting process because it definitely yielded benefits for us.
Limerick sits on the Schuylkill River in eastern Pennsylvania. (Photo: Constellation)