The U.S. Department of Energy's Office of Scientific and Technical Information

The increased penetration of intermittent renewable energy technologies such as wind and solar power can strain electric grids, forcing carbon-based and nuclear sources of energy to operate in a load-follow mode. For nuclear reactors, load-follow operation can be undesirable due to the associated thermal and mechanical stresses placed on the fuel and other reactor components. Various methods of thermal energy storage (TES) can be coupled to nuclear (or renewable) power sources to help absorb grid variability caused by daily load demand changes and renewable intermittency. Two TES techniques are investigated as candidate thermal reservoirs to be used in conjunction with a small modular reactor (SMR): a two-tank sensible heat storage system and a stratified chilled-water storage system. The goal when coupling the two systems to the SMR is to match turbine output and demand and bypass steam to the TES systems to maintain reactor power at approximately 100%. Simulations of integral pressurized water reactor dynamics are run in a high-fidelity FORTRAN model developed at North Carolina State University. Both TES systems are developed as callable FORTRAN subroutines to model the time-varying behavior associated with different configurations of these systems when connected to the SMR simulator. Simulation results reveal the sensible heat storage system is capable of meeting turbine demand and maintaining reactor power constant while providing enough steam to power four absorption chillers for chilled-water production and storage. The stored chilled water is used to supplement cooling loads of an adjacent facility.

Approximately 19% of the electricity produced in the United States comes from nuclear power plants. Traditionally, nuclear power plants, as well as larger coal-fired plants, operate in a baseload manner at or near steady state for prolonged periods of time. Smaller, more maneuverable plants, such as gas-fired plants, are dispatched to match electricity supply and demand above the capacity of the baseload plants. However, air quality concerns and CO₂ emission standards have made the burning of fossil fuels less desirable, despite the current low cost of natural gas. Wind and solar photovoltaic power generation are attractive options due to their lack of carbon footprint and falling capital costs. Yet, these renewable energy sources suffer from inherent intermittency. This inherent intermittency can strain electric grids, forcing carbon-based and nuclear sources of energy to operate in a load-follow mode. For nuclear reactors, load-follow operation can be undesirable due to the associated thermal and mechanical stresses placed on the fuel and other reactor components. Various methods of thermal energy storage (TES) can be coupled to nuclear (or renewable) power sources to help absorb grid variability caused by daily load demand changes and renewable intermittency. Our previous research has shown that coupling a sensible heat TES system to a small modular reactor allows the reactor to run at effectively nominal full power during periods of variable electric demand by bypassing steam to the TES system during periods of excess capacity. In this paper we demonstrate that this stored thermal energy can be recovered, allowing the TES system to act as a peaking unit during periods of high electric demand or used to produce steam for ancillary applications such as desalination. For both applications the reactor is capable of operating continuously at approximately 100% power.

This paper provides a comprehensive overview of the development of a NuScale power module in the Modelica process model ecosystem at Idaho National Laboratory (INL) as part of the U.S. Department of Energy’s Office of Nuclear Energy Integrated Energy Systems (IES) program. Model development has led to the creation of a dynamic NuScale module in the Modelica language that operates under natural circulation and is consistent with design parameters set forth in the design certification documentation for the Nuclear Regulatory Commission (NRC). Controllers have been developed that are consistent with controllers mentioned by NuScale design certification documents. While characteristic time constants may differ due to controller gains, the actual control actions of the system are consistent with NuScale prescribed actions. In addition to natural circulation and controllers, the NuScale power module includes hot channel calculations, reactivity control, and a pressurizer. Hot channel calculations include bulk fluid temperature, outer clad temperature, fuel centerline temperature, and departure from nuclear boiling ratios. Reactivity control incorporates moderator and fuel temperature feedback mechanisms, boron concentration, and control bank positions. A pressurizer with sprays and heaters was developed to maintain primary system pressure within bounds. Further, the model has been developed to allow the user to input beginning-, middle-, or end-of-cycle reactivity conditions. Three simulations were run that demonstrate the capability of the model controllers to meet target parameters while preserving system values within reasonable bounds. The first simulation is an initialization study at 160 MW(thermal)/50 MW(electric) that compares the model steady-state values with the design certification values. This simulation demonstrates the capability of the model to operate in natural circulation mode and closely matches operating values set forth in the NRC design certification documents. The second simulation is a power uprate from 160 MW(thermal)/50 MW(electric) to 200 MW(thermal)/60 MW(electric), and the final simulation is of a typical summer day where the turbine demand oscillates throughout the day. These illustrate the capabilities of the system to operate under natural circulation while maintaining system parameters within controller bounds and to match turbine output with turbine demand. This work adds a systems-level model of the NuScale power module with associated control systems, reactivity coefficients, control banks, coolant through fuel temperature profiles, and associated mass flow rates throughout the system that have been added to the Modelica ecosystem. This model will be the basis of work that is utilized in the Joint Use Modular Plant program at INL, which proposes to utilize a single power module to support research, development, and demonstration activities, with a specific aim to demonstrate various IES configurations. As part of the ecosystem, the NuScale module can be quickly integrated with existing Modelica modules (e.g., thermal energy storage, reverse osmosis, high-temperature steam electrolysis, gas turbines) to model energy grids in different geographical locations.

This report discusses the different options for coupling thermal energy storage (TES) systems to advanced nuclear power plants (A-NPPs) in order to enable flexible and hybrid plant operation. An advanced light-water reactor (A LWR), a high-temperature gas-cooled reactor (HTGR) and a liquid-metal fast reactor (LMFR) were selected as the initial use cases for demonstrating a thermally balanced energy storage coupling design for thermal power extraction. The models presented herein showcase several design considerations, focusing on optimal deployment methodologies for achieving steady-state and transient-state operation with minimum disruption to the nuclear power cycle. This first part of the study presents steady-state models developed using Aspen HYSYS®, with the thermal energy bypass for NPP-TES coupling being varied at up to 50%. The various components were sized using the Aspen Process Economic Analyzer (APEA) and Aspen Exchanger Design and Rating (EDR), when applicable. Cost functions from these models were developed using the latest publicly available data obtained from APEA V11. The TES-coupled A-NPP steady-state models and cost functions then provided a baseline for dynamic operation and process optimization by using Idaho National Laboratory (INL)’s Framework for Optimization of Resources and Economics (FORCE) tools. A stochastic optimization of the various energy storage systems coupled to the A-NPPs was then performed using the Risk Analysis Virtual Environment (RAVEN) and its dispatch optimization plugin, the Holistic Energy Resource Optimization Network (HERON). The signal processing and synthetic history capabilities of RAVEN were used to account for the unpredictable behavior of electricity markets. An autoregressive moving average (ARMA) model was used to analyze price signals from the Pennsylvania-New Jersey-Maryland (PJM) market and were applied to the HERON analysis in order to optimize a system with the best economics. Transient modeling evaluation was then performed using Modelica models within the HYBRID repository, which was developed at INL for the Department of Energy Integrated Energy Systems program for the characterization of dynamic integrated system behavior and feedback. This includes evaluation of the TES-coupled A-LWR systems’ impact on physical and thermal system response during imposed system demands. Additional TES-coupled reactor types, coupling approaches, markets, and TES technologies will be evaluated in future work.

The U.S. Department of Energy's Office of Scientific and Technical Information