U.S. nuclear power plants typically refuel every 18 to 24 months. Refueling outages normally occur in the spring and fall of the year when demand for electricity is at its lowest. The Energy Information Administration (EIA) tracks U.S. nuclear plant outages and provides the information on a webpage that is updated daily based on Nuclear Regulatory Commission (NRC) data. From 2018 through 2023, spring outages have peaked in mid-April with the average total nuclear power capacity out of service in the U.S. ranging from about 15 GW to slightly more than 23 GW. Fall outages, meanwhile, peak in late October with about the same capacity offline for maintenance and refueling as in the spring (Figure 1).
1. Daily U.S. nuclear capacity outage. Source: U.S. Energy Information Administration
Safety is always a priority whether a nuclear plant is in operation or conducting a refueling outage (Figure 2). During a refueling outage, plants typically optimize downtime by scheduling facility upgrades, repairs, and other maintenance work while the nuclear reactor is offline.
2. The High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) is shown here undergoing a routine refueling. While submersed, the spent fuel emits a luminescent blue glow due to Cherenkov radiation, in which shedding electrons move through the water faster than the speed of light. Source: Jason Richards/ORNL
In recent years, the average refueling outage duration has been about 32 days. That’s a marked improvement from earlier times. In 1997, for example, the average length of nuclear refueling outages in the U.S. was more than 80 days. A keen focus on continuous improvement helped bring the average duration down in subsequent years until in 2002 an average duration of about 33 days was achieved. Durations have fluctuated since then, but have remained constantly in the 30- to 46-day range over the past two decades, according to Statista, a market data provider.
EPRI Guidance on Refueling Outages
Sound planning is essential to a smooth refueling outage, which is why it’s no surprise that a great deal of time has been spent optimizing outage planning techniques. For example, in 1993, the Electric Power Research Institute (EPRI) issued a technical report titled “Effective Refueling Outage Preparation and Execution Guidance.” The guidance was specifically developed to provide the nuclear industry with a comprehensive resource for improving the planning, preparation, and execution of nuclear power plant refueling outages. The document aimed to help power plant operators optimize outage schedules, minimize downtime, enhance safety, and improve overall efficiency during the refueling process. Subsequent versions and updates of this guidance have been released by EPRI to reflect evolving industry best practices and technological advancements. The most-recent update was published in November 2022.
The latest abstract for the report notes, “Excellence in nuclear refueling outage nuclear safety, outage performance, optimization of outage duration, and management of costs is an ongoing industry effort.” It says the report provides guidance to assist personnel in the preparation and execution of a nuclear refueling outage, including:
■ Transitioning the plant from online and maintenance activities into an outage mode.
■ Returning the plant safely and effectively from outage mode to operating mode.
■ Information on how outage activities support station or fleet long-range and strategic planning.
■ Collection of human performance information related to the preparation and execution of a nuclear refueling outage, including techniques and good practices used by EPRI members.
EPRI says the information presented in the report is “a single reference for outage team members to consistently implement the processes associated with preparing for and executing nuclear refueling outages.”
ChemWorks Shutdown Calculator
Another useful outage planning tool developed by EPRI is its ChemWorks Tools. ChemWorks is a computer program that includes several modules for evaluating chemistry in a nuclear power plant. One of the modules in ChemWorks is a shutdown calculator (SDC) tool to model the shutdown chemistry evolution and predict cleanup times in pressurized water reactors (PWRs).
Recently, a team of researchers from Korea Hydro and Nuclear Power’s (KHNP’s) Central Research Institute studied the utilization of EPRI’s ChemWorks Tools for PWR shutdown chemistry evolution modeling. Results were published in June 2023 in Nuclear Engineering and Technology, an international journal of the Korean Nuclear Society.
The article notes that a shutdown chemistry evolution is performed in nuclear power plants at each refueling outage to establish safe conditions to open the system and minimize the inventory of corrosion products in the reactor coolant system (RCS). It explains that after hydrogen peroxide is added to the RCS during the shutdown chemistry evolution, corrosion products are released and are removed by filters and ion exchange resins in the chemical volume control system.
The researchers note that the shutdown chemistry evolution, including RCS cleanup time to remove the released corrosion products, impacts the critical path schedule during outages. The estimation of cleanup time prior to the outage can provide more reliable actions for RCS cleanup operations and transients to operators during shutdown chemistry. EPRI’s SDC tool estimates the cleanup time using operational data from the nuclear power plant, specifically, cobalt-58 peak activity.
The team evaluated the SDC tool using actual plant data from two refueling outages. In summary, the results showed that EPRI’s SDC provided reliable estimates of cleanup time and modeling can be used to predict shutdown chemistry plans more accurately. The researchers said this could contribute to improving the efficiency and safety of future shutdown chemistry evolutions in nuclear power plants.
—Aaron Larson is POWER’s executive editor.