In This Issue
Summer Bridge on Engineering the Energy Transition
June 26, 2023 Volume 53 Issue 2
This issue explores the energy transition needed to address the mounting threats of climate change. The articles are an excellent resource to help inform meaningful decisions and steps for energy-related contributions to reduce carbon emissions.

Shifting the Paradigm: Nuclear-based Integrated Energy Systems to Achieve Net Zero Solutions

Wednesday, June 7, 2023

Author: Shannon M. Bragg-Sitton, Richard D. Boardman, and Aaron S. Epiney

Achieving a net zero carbon society demands a new approach that will use all clean energy generation options available.

Governments and private industry around the world have established aggressive goals to achieve net zero carbon emissions for the power, industrial, and transportation sectors by 2050. These goals require a sharp paradigm shift in how energy demands are met. Research laboratories and private companies are developing holistic, integrated solutions that seek to efficiently utilize an array of clean energy generation sources to meet the growing demand for heat, steam, and electricity from nonemitting sources.

Current State

Electricity demands in the United States[1] and across many developed countries (Ritchie et al. 2022) account for approximately one-third of overall energy use and about one-third of CO2 emissions. Most energy demands have traditionally been met via generators that support a single energy sector. The chemicals and fuels industries draw some electricity from the grid, but most large industrial facilities independently cogenerate heat and power for their internal utility duties, contributing to ~20 percent of CO2 emissions. Only recently has the transportation sector been more directly linked to the grid with the increasing adoption of electric vehicles, but it is still responsible for more than one-third of CO2 emissions.

A coordinated, integrated approach can link energy demands across sectors to optimize energy generation and use while maintaining grid reliability and resilience.

The prevailing low-carbon generation sources for electricity, industry, and transportation ultimately will come down to economics, but it is increasingly clear that options analysis should include not only the energy intensity, reliability, resilience, and security of generation sources but also the associated energy storage needs and cross-sector energy supply possibilities.

The buildout of variable wind and solar electricity production has already negatively impacted traditional baseload power plants, requiring them to operate more flexibly to accommodate supply/demand mismatch. In the future, more baseload plants may be forced to turn down their output, potentially reducing their economic viability; divert energy to a secondary, off-grid electricity or thermal energy user; or permanently shut down, putting grid resilience at risk.

Abating emissions from industrial processes presents a greater challenge than decarbonizing the grid because of limited options for nonemitting sources of heat. Electrification may help (assuming electricity is sourced from nonemitting generators, including nuclear), but some processes require a direct heat source to maintain efficiency and economic competitiveness.

Similarly, today’s transportation sector primarily relies on internal combustion engines that use liquid fuels. Although most light-duty electric vehicles (i.e., passenger vehicles) can switch to plug-in batteries with buildout of charging infrastructure, and electric buses and trains can support commuter systems, heavy-duty transport vehicles, maritime vessels, and aircraft cannot be easily electrified. For these vehicles, hydrogen fuel cells or synthetic liquid fuels produced with renewable carbon sources can significantly reduce greenhouse gases (GHGs) and other pollutants. These options will further increase the demand for nonemitting thermal and electrical energy sources.

Proposed Solution: An Integrated Energy System

Amid efforts to achieve net zero emissions across all energy generation and use sectors, grid reliability and resilience, as well as customer affordability, must be maintained.

An integrated energy system (IES) can offer solutions that leverage desirable attributes of each energy resource. This could include a shift from the single--output systems commonly used today (e.g., generators operating independently to support electric grid demand) to other configurations, such as multi-input multi-output (MIMO) systems integrating multiple resources to provide both heat and electricity to multiple energy users (Arent et al. 2021). While some CO2-emitting multi-output (cogeneration) systems are in operation today, IES development efforts focus on providing heat and electricity to multiple energy users without CO2 or other GHG emissions.

MIMO systems could be deployed in an energy park configuration connected to a regional grid balancing area; alternatively, they may operate as an independent microgrid. Through coordinated dispatch from the various installed generators, energy can be redirected to storage or coupled energy users (e.g., hydrogen production) as needed to ensure efficient use rather than reducing dispatchable output when the installed variable generation is sufficient to meet demand.

Nuclear energy is the primary source of non-emitting thermal energy and can flexibly provide clean heat and electricity. Deploying nuclear and renewables in a more tightly coordinated, integrated approach can link energy demands across the electric, industrial, and transportation sectors to optimize energy generation and use, maintain the grid reliability and resilience offered by thermal generators, and support decarbonization of industry and transportation.

Bragg-Sitton Figure 1.gif

A variety of options can be pursued for an integrated energy park (figure 1). Prioritization of options may depend on opportunities to reduce emissions from a selected process (based on both process scale and overall market size; McMillan et al. 2016), reduce operational costs relative to competing technologies, enhance domestic industrial opportunities, or other factors, such as ensuring access to clean water in regions experiencing water scarcity or enhancing social, environmental, and energy justice. This paper focuses on use of nuclear energy alongside renewables in these conceptual energy parks.

Commercial implementation of technologies is strongly dependent on cost, deployability within a desired timeframe, and technology availability at the desired scale. The science-based development of IES involves three key pathways:

  • Energy system and process modeling, simulation, and analysis: These are required to characterize and optimize the intersection of multiple energy use sectors from both a technical and economic perspective.
  • Component development, testing, and demonstration: Experimental facilities are required to validate the modeled behaviors. Facilities designed to reflect real system responses support validation of simulation results to build confidence and assurance in the proposed system design.
  • Process and system monitoring, control, and maintenance: Process monitoring and control must be demon-strated at each development scale (bench, pilot, or engineering) in preparation for prototype deployment to ensure safe, reliable, and secure system operation before commercialization.

Status of Development

Multiple technologies that can support nuclear--renewable IES are under development around the world. Many of these technologies are operating commercially as independent units today, but they are not integrated to create a multi-application clean energy park.

Nuclear Energy Systems

Nuclear energy has been powering the US grid since the Experimental Breeder Reactor-I first did so in 1951. Current US fleet plants are all light water reactors (LWRs), most of which produce about 1 gigawatt of electricity (GWe) and provide steam outlet temperatures of ~300°C. The field of reactor design options is, however, poised to change with the development of microreactors (~10s MWe), small modular reactors (~50–300 MWe), and non-water-cooled advanced reactor (AR) technologies that provide higher temperatures and offer higher power generation efficiency. Each of these systems offers different IES opportunities.

Light Water Reactors

The current US LWR fleet faces economic challenges in regions of the country where subsidized renewable energy buildup and low-cost natural gas have reduced the wholesale price of electricity to levels that are difficult for nuclear power plants to clear the market throughout the year. For several large-scale nuclear plants, particularly those in deregulated markets, these challenges are leading to early plant closures (before plant license expiration) (Szilard et al. 2017).

Advanced nuclear reactors can extract heat at higher temperature to drive industrial processes.

Recent studies have demonstrated the value of flexible operation of grid-connected LWRs (Boardman et al. 2019; Epiney et al. 2019; Frick et al. 2019). IES configurations offer enhanced flexibility with continued operation at nominal power levels by dynamically apportioning energy to meet grid demand while sending excess energy to yield a secondary product, such as hydrogen, at a market-competitive price. These analyses helped promote LWR hydrogen demonstration projects at multiple US nuclear plant sites in partnership with the US Department of Energy: Constellation’s Nine Mile Point Nuclear Station (New York), Energy Harbor’s Davis-Besse Nuclear Power Station (Ohio), and Xcel Energy’s Prairie Island Nuclear Generating Plant (Minnesota).[2] Nine Mile Point reached a major milestone in March 2023 with operation of a 1.25 MWe low-temperature electrolysis system to produce 560 kg of hydrogen per day.

Advanced Nuclear Reactors

Numerous AR concepts are under development by private industry, in many cases with support from federal research laboratories. These concepts focus on inherent safety, waste minimization, generation of cost--competitive electrical power, and nonproliferation, but the characteristic most relevant to IES is the potential to extract heat at higher temperature to drive industrial processes.

The three primary technologies being pursued in the United States are liquid metal (sodium-cooled) fast spectrum reactors, high-temperature gas-cooled reactors, and molten salt reactors. The potential to achieve much higher temperatures (500–750°C) with ARs opens possibilities of meeting the thermal and electricity needs of multiple industrial users while reducing industrial emissions.

Renewable Energy Options

Multiple renewable generators may be integrated with nuclear energy to establish clean energy parks. Options may include wind, solar (photovoltaic [PV] or concentrated solar), geothermal, biomass, or hydropower. Some of these generators (wind, solar PV) produce electricity directly, while others could be integrated via a thermal energy manifold that distributes heat as needed to electricity or secondary product production.

Energy Storage

Energy storage—electrical, thermal, and chemical—may play a key role in coupling diverse technologies to ensure that energy is delivered when needed and of the desired quality. Many IES require both thermal and electrical integration; however, coupled energy users may require heat augmentation to achieve the needed quality.

In addition, many industrial applications operate best at steady state. If an IES dispatches energy alternately between the grid and the industrial user, then energy storage components may be required to manage flexibility of operation without deprioritizing any coupled users (Knighton et al. 2021). Stored energy may support peak power production in deregulated markets for systemwide profit maximization (i.e., storage when electricity production exceeds demand, causing the electricity selling price to be low or negative, and sale when the price is high). Stored energy can also smooth the transition of energy use between process applications that operate on different characteristic time scales.

One of the most versatile energy carriers is hydrogen, which enables chemical energy storage. It can be compressed, stored, transported, and later combusted to produce electricity, or it can become feedstock for many of the processes shown in figure 1.

Interface Technologies

IES may require thermal integration, electrical integration (behind the grid), or both, with each option posing different technical, operational, and regulatory challenges. Researchers are working to advance at-scale demonstration of suitable integration technologies to accelerate IES deployment.

Thermal interconnection can be accomplished using heat exchangers and heat transfer loops. Design considerations include materials compatibility, working fluid characteristics, operational temperature, and temperature limitations. Interconnections must provide efficient heat transfer without creating unnecessary interdependence among subsystems.

Intermediate heat transfer loops can effectively isolate the nuclear island from heat users, eliminating the potential for radioactive contamination of products for both normal and off-normal operations. Isolation can also reduce the number of components and subsystems required to adhere to nuclear quality levels in coupled facilities, thus reducing overall system cost.

In some applications it may be necessary to boost the temperature of the heat transfer media. This can be accomplished using electric heating, a fired heater (including hydrogen-fired), compression, or heat pumping. Chemical heat pumps can achieve very high temperature amplification if the cost-benefit is justified (Gupta et al. 2022).

Some IES configurations benefit from power transactions behind the grid—before the electrical switchyard. Such interfaces can increase the efficiency of energy delivery to coupled users and may also reduce operational costs.

Industrial Energy Applications

The manufacturing industry uses about 25 exajoules of energy per year, of which approximately 20 percent is from electricity (with about one-third produced onsite for captive use), 40 percent from steam (all generated onsite), and 40 percent from fossil-fired combustion as a source of either direct heating (as in a cement kiln) or indirect heating (as in fired heaters) (Ruth et al. 2014). Over 90 percent of the primary energy required is derived from combustion of fossil fuels. Hydro-electric dams and biomass combustion in concentrated heat and power plants are still the main sources of nonfossil energy used by the industrial sector.

Over 90 percent of the primary energy required for industrial energy applications is derived from combustion of fossil fuels.

A key tenet of IES is apportioning energy between power production and heat provision to industrial applications to both enhance energy use efficiency and reduce industrial CO2 emissions by using nonemitting generation sources. The US manufacturing industry can be broken down by electrical duties, heat or steam duties and temperature requirements, and heat transfer media and methods for direct heating (Pellegrino et al. 2004; summarized in Bragg-Sitton et al. 2020). Key markets include feedstock drying, petroleum distillation, biomass and coal pyrolysis, hydrotreating, steam cracking, oxidative coupling, and calcination.

Nuclear energy has the potential to provide heat (primarily via steam and indirect heating) and electricity to meet many industrial process needs. Use of high--temperature ARs would reduce the need to augment steam heating, but these designs require additional development time. In the near term, heat augmentation technologies represent a key opportunity that could enable utilization of LWRs for high-temperature process applications.

Hydrogen Production

Hydrogen (H2) can be used to incorporate clean energy in existing or new industries, and nuclear-supported H2 production is versatile as both an energy carrier and a feedstock for numerous industrial applications. Today, hydrogen is mainly used in petroleum refineries and for ammonia production. In the future, it may also be used as a combustion fuel, to refine iron ore, and to power gas turbines.

Hydrogen may be used
in the future as a combustion fuel, to refine iron ore,
and to power gas turbines.

Two types of H2 generation technologies are used currently: (1) hydrocarbon cracking, reforming, and shifting with steam and (2) water splitting (Boardman 2021). Reforming technologies convert fossil fuels and biomass into hydrogen, emitting the carbon in the feed material as CO2. Water splitting technologies, which do not result in carbon emissions when the source of heat and electricity is carbon-free, fall into two categories: chemical looping, which involves formation of an aqueous mineral acid followed by high-temperature acid decomposition, or electrolysis—low-temperature water electrolysis (LTE) and high-temperature steam electrolysis (HTE). With HTE, the additional heat reduces the amount of electrical work needed and thus increases the H2 production efficiency—HTE can be 20–50 percent more efficient than LTE.

The choice of nuclear reactor design to support hydrogen production ultimately depends on the cost of producing electricity and heat relative to the capital and operation and maintenance costs of the H2 plant. In addition, high reactor outlet temperatures yield high thermal-to-electricity efficiencies, further enhancing H2 production efficiency (McKellar et al. 2018).

Coordination of Multiple Regulatory Entities

In the United States, civilian nuclear reactors are licensed and regulated by the US Nuclear Regulatory Commission. Its role is to protect public health and safety related to nuclear energy generation as well as other radiological sources. Industrial facilities are bound by the code of federal regulations under the National Energy Policy Act, Environmental Protection Agency, Occupational Safety and Health Administration, and codes and standards for building and operating potentially hazardous process operations. Industrial siting and operations generally fall under individual state agencies, and the Federal Transportation Agency or Department of Commerce oversees the transport of chemicals and fuels. All industrial practices are subject to the International Organization for Standardization.

Nuclear-based IES must demonstrate that nuclear-industry integration will not increase the risks of operating the nuclear facility. Preliminary probabilistic risk assessments—completed for integration of a generic pressurized or boiling water reactor with a large-scale HTE plant—support the colocation of hydrogen production at a nuclear plant without increasing safety risk (Vedros et al. 2020). A Hydrogen Regulatory Research Review Group, comprising laboratory researchers, nuclear plant operators, architectural engineering firms, and licensing experts, is also evaluating potential technical and safety risks for nuclear-hydrogen integration (Remer et al. 2022); its work supports nuclear--integrated H2 production demonstration facilities in clearing regulatory hurdles at current fleet nuclear plants.

Modeling, Simulation, and Optimization Tools

The laboratory-developed Framework for Optimization of ResourCes and Economics (FORCE) ecosystem of tools supports design, analysis, and optimization of novel IES solutions.[3] Key tools include:

  • Reactor Analysis and Virtual Control ENvironment (RAVEN), to process raw market data to create synthetic data that represent the market and its stochastic nature
  • Holistic Energy Resource Optimization Network (HERON), to stochastically optimize both the individual IES component sizes and real-time dispatch of IES resources to the grid or coproduct markets
  • Tool for Economic AnaLysis (TEAL), used in conjunction with HERON for multiyear financial optimization of the IES in agreement with the financial figures of merit appropriate to the type of market
  • HYBRID, a repository of transient process models, with detailed models of various nuclear reactors, energy storage, and ancillary processes (e.g., water desalination, H2 production) that can be used to understand dynamic behavior, integration, and control of IES across time scales
  • Feasible Actuator Range Modifier (FARM), a RAVEN plug-in used to ensure that control actions requested by the dispatch optimizer do not violate operational constraints for various hardware components
  • Optimization of Real-time Capacity Allocation (ORCA), for real-time IES control and optimization
  • Dynamic Reliability Analysis Framework and Toolbox (DRAFT), which uses HYBRID operational data to construct component reliability and failure probability data, which are used in HERON to differentially motivate dispatch decisions.

Analysis tools are available for public use via GitHub.

Potential benefits of deploying integrated assets can only be assessed based on their technical and economic performance relative both to the current state of the art and to independently operated systems that produce the same products for the market. As such, nuclear-based IES performance might be evaluated relative to comparably scaled renewable generators coupled with energy storage systems, or to natural gas combined cycle systems with coupled carbon capture and storage, ensuring that both the benchmark systems and the integrated systems can achieve equivalent emissions reduction and system reliability.

Case Studies

Numerous case studies conducted for application of nuclear energy in IES support further development of the dynamic models and tools. A few of these case studies are summarized below.

Hydrogen Production

Steady-state and dynamic analyses of nuclear-supported H2 production have been completed both for current fleet and advanced nuclear systems. One of the earliest demonstrations of the FORCE toolset was for a collaborative study among Constellation (previously Exelon), Fuel Cell Energy, and national laboratories to evaluate the potential of using existing nuclear plants in the US Midwest to produce hydrogen via HTE while continuing to support grid electricity demands (Frick et al. 2019). The analysis indicated that during times of low grid pricing (ample supply), H2 production is more profitable to the plant. When grid demand and grid electricity pricing are high, selling energy to the grid is more profitable.

Hydrogen storage provides additional flexibility in plant operations, ensuring that both grid and H2 demand can always be met. The 2019 FORCE analysis indicates a potential revenue increase to the evaluated nuclear plant of $1.2 billion over a 17-year span with optimally sized HTE and storage systems. This work resulted in Constellation’s decision to demonstrate H2 production at Nine Mile Point, which started operation in March 2023.

Sustainable Fuels

Sustainable fuels (synfuels) can be produced from carbon and H2 chemical building blocks, including methane (a substitute for natural gas), olefins (as substitutes for -diesel and jet fuel), and oxygenates (as substitutes for motor gasoline). The most common pathways to produce synfuels are methanation, the Fischer-Tropsch process, and the methanol-to-gasoline process.

Synthetic fuels produced from clean hydrogen
and a renewable carbon resource or CO2 sourced directly
from the atmosphere are sustainable.

Synthetic fuels produced from clean hydrogen and a renewable carbon resource or CO2 sourced directly from the atmosphere are sustainable. Biomass can also be converted directly into sustainable fuels using emission-free hydrogen to hydrotreat pyrolysis oils. Development of these nonemitting pathways to liquid hydrocarbon fuels supports decreased atmospheric CO2 and reduced emissions from hard-to-abate transportation systems (Wendt et al. 2022).

Bragg-Sitton FIgure 2.gif

Carbon Conversion Product Pathways

Use of nuclear energy to support carbon conversion pathways (e.g., converting coal feedstock into higher-value products versus combustion for electricity) provides alternative revenue opportunities to communities that rely on exports of fossil fuel resources to support their economy. Initial analysis of a carbon conversion refinery for the Appalachian region focuses on optimizing the chemical process for coal conversion to products based on market needs (Worsham et al. 2022).

Energy Storage

The influx of solar and wind power has increased net demand variability on the grid. Energy storage in an IES can be used to shift power production to periods when demand is high or when variable power generation is low, or to transfer energy to industrial users (Saeed et al. 2022).

A recent study compared utility-scale battery storage with hydrogen and thermal energy storage systems (Knighton et al. 2021). Results indicate that the best option is situational and depends on a variety of factors, most importantly the time scale. Batteries are consistently better for short storage durations, typically less than 4–6 hours. At longer time scales, hydrogen and thermal energy storage can be more economical. Because the cost of energy storage is significant, it can be justified in the electricity sector only when there is a substantial differential between the selling price of electricity during normal and peak demand periods.

Experimental Systems

Laboratory testing and demonstration of individual or coupled technologies are needed to evaluate IES performance characteristics, integration approaches, and system control options. Scaled testing also provides data for validation of computational models used in broader system design and optimization before demonstration on a nuclear system. Advancing technologies from lab to commercial scale can entail unique challenges that can impede deployment if not addressed.

Nonnuclear, electrically heated test facilities can be useful for characterizing system integration approaches and controllability of system operation under normal and off-normal operating conditions. The Dynamic Energy Transport and Integration Laboratory (DETAIL) at Idaho National Laboratory (figure 2) has multiple experimental systems integrated both thermal-hydraulically and electrically. A Microreactor Agile Nonnuclear Experimental Testbed (MAGNET) supports testing of various microreactor concepts, and the Thermal Energy Distribution System (TEDS)—a network of valves, pipes, and heat exchangers—moves thermal energy between connected subsystems, serving as the backbone of DETAIL. TEDS can also deliver thermal energy to HTE systems for H2 production.

Real-time digital simulators enable electrical integration of DETAIL with systems at other laboratories (PowerGrid International 2023) and/or customer sites to extend IES demonstration capabilities. A battery storage and charging laboratory and a microgrid test facility demonstrate how each system can respond to changes in demand or supply on the grid. And integration of DETAIL and the Human Systems Simulation Laboratory allows demonstration of control approaches for industrial use of nuclear-generated thermal energy and/or steam, in addition to electricity production, and will provide valuable information on the human factors aspects of operating integrated systems.

Advancing Technology through Collaboration

Both developed and emerging economies around the world are seeking diverse energy generation and delivery options as they pursue a net zero future. Global partnerships among energy planners and technology developers are necessary to accelerate adoption of clean energy pathways that will increase electricity access, energy security, and environmental sustainability in both developed and developing regions. These collaboration platforms provide opportunities to share experiences in clean energy technology options, operational or deployment challenges, project financing, and -community engagement.

In addition, IES research brings together nuclear and renewable technology developers and energy users to evaluate opportunities and establish a new paradigm for clean energy deployment. Stakeholder engagement is essential to ensure that research and development (R&D) is well positioned to support rapid technology advancement. As such, industry expert groups have been established to engage in laboratory research activities, providing feedback on key gaps to commercial IES deployment, for both current fleet LWRs and future ARs.

Summary and Path Forward

Achieving a net zero carbon society demands a new approach that will best utilize all clean energy generation options. Integrated energy systems are imperative for meeting clean energy needs across all energy sectors. They couple diverse energy generation sources, including variable renewables, with high-capacity clean energy generated by nuclear plants and fossil plants that capture and manage carbon emissions. These systems ensure more efficient energy use and increased revenues for plant owners by supporting multiple product streams while ensuring power grid reliability and resilience.

The path forward requires a concerted private-public effort in which innovative technologies and system integration are reduced to practice through modeling and simulation; technology testing, proving, and scale-up; and financial structures that help overcome the risks taken on by first movers and commercial adoption. Technology performance and reliability testing will reduce the technical, safety, and financial risks inherent in disruptive technologies and physical/temporal systems integration.

Successful deployment of the integrated clean energy solutions proposed here will require true partnership among research organizations, technology developers, energy users, investors, policymakers, and communities.

The clean energy transition away from fossil fuel dependence cannot leave communities behind. -Nuclear and renewable-based integrated energy systems can usher in job creation and economic development through deployment of new infrastructure and industry.


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[1]  Lawrence Livermore National Laboratory, Energy Flow Charts,

[2]  Other demonstrations have been proposed, but contracting has not been completed.

[3]  For additional information about the US DOE Integrated -Energy Systems program and the FORCE tool suite, see

About the Author:Shannon Bragg-Sitton is director, Integrated Energy & Storage Systems Division; Richard Boardman is directorate fellow, Energy & Environment Science & Technology; and Aaron Epiney is lead for modeling and simulation, DOE-NE Integrated Energy Systems program, all at Idaho National Laboratory.