**1. Introduction**

In recent years, research on hydrogen use, production methods, and economics has increased as countries have begun attempting to reduce their carbon footprints. As a power source, hydrogen offers flexible electricity generation, with the potential to serve as the load following or peaking power units. Hydrogen could also be used to shift electricity demand to off-peak hours, acting as a large-scale demand response or energy storage medium. Producing hydrogen via nuclear power and using it as a flexible load resource is being investigated by numerous organizations [1–3]. Several of these nuclear hydrogen configurations are also currently in development.

These nuclear integrated energy systems (IES) could provide economic benefits to nuclear power plants (NPPs). Competing with cheap fossil resources and declining renewable energy costs has left NPPs at an economic disadvantage [4]. Hydrogen production allows NPPs to diversify their revenue streams and has potential to increase NPP profitability [5].

In Japan, fossil fuel import requirements have led to high electricity prices and investigations into methods of producing electricity cheaply and locally [6]. Nuclear power could be advantageous in decreasing our dependence on fossil fuels, since uranium is much more energy dense, requires less frequent imports, and can be stored onsite for future use. Furthermore, following their initial installation, sources of renewable energy do not require any additional imports. Combining these technologies in a way that also reduces carbon emissions while maintaining low electricity prices is important for the future of Japan's

**Citation:** Richards, J.; Rabiti, C.; Sato, H.; Yan, X.L.; Anderson, N. Economic Dispatch Model of Nuclear High-Temperature Reactor with Hydrogen Cogeneration in Electricity Market. *Energies* **2021**, *14*, 8289. https://doi.org/10.3390/en14248289

Academic Editor: Muhammad Aziz

Received: 1 September 2021 Accepted: 31 October 2021 Published: 9 December 2021

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electricity system. An IES that enables NPPs to sell a secondary commodity instead of losing money on electricity sales could help boost overall system profitability.

Besides addressing cost and security concerns, hydrogen produced via nuclear energy could help in meeting the greenhouse gas reduction goals set by Japan's Ministry of Economy, Trade, and Industry (METI). METI has also set cost-reduction goals for hydrogen produced via low- or zero-emission sources [7]. With sufficient infrastructure, this clean hydrogen could be used to aid in decarbonizing Japan's industry or transport sections. Currently in Japan, hydrogen is sold at a wholesale price of ~100 JPY/Nm3. METI's goal is to reduce this price to 30 JPY/Nm3 by 2030, and to 20 JPY/Nm3 by approximately 2050 [7].

Government and research entities in Japan have also achieved expertise in nuclear high-temperature gas cooled reactors (HTGRs) and the applications thereof. The operating high-temperature engineering test reactor (HTTR) has aided in acquiring HTGR experimental and operational experience. The HTTR is a 30-MWt, helium-cooled reactor that uses graphite moderated prismatic fuel assemblies. The outlet temperature is 950 ◦C—high enough to integrate different process applications (e.g., hydrogen production) for testing purposes [8].

The iodine–sulfur (IS) cycle for hydrogen production appears to be a strong candidate for pairing with an HTGR [9]. The IS cycle utilizes a Bunsen reaction to convert water, I2, and SO2 into HI and H2SO4. The HI is then split up into its hydrogen and iodine components. A side reaction converts the H2SO4 into SO2, water, and oxygen, thus completing the cycle. The reactions are listed in Table 1.

**Table 1.** IS cycle reactions.


Several difficulties have inhibited the deployment of IS cycles, such as heat input and material requirements. This cycle requires high quality heat at upwards of 800 ◦C for the H2SO4 decomposition reaction, meaning that coupling with the current fleet of light-water reactors is difficult because they output steam at approximately 300 ◦C [10]. Additionally, material challenges associated with catalyst, reactant and container interactions or highly corrosive environments require special materials, such as Hastelloy C-276 [11], zirconium alloys [12], or special design features that isolate highly acidic environments from metals to avoid acidic oxidation.

Because of the unique positioning with an operating high temperature reactor, the Japan Atomic Energy Agency (JAEA) has emphasized the development of the IS cycle for hydrogen production [11], going so far as to design an HTTR and IS cycle cogeneration facility known as the HTTR-GT/H2.

The HTTR-GT/H2 is a design for coupling the HTTR with an IS cycle in order to demonstrate hydrogen–HTGR coupling capabilities. The process diagramed in [12] adds an intermediate heat exchange system to the HTTR in order to send heat to the nuclear-IS. A turbine for generating electricity is also planned. Thus, the demonstration could entail the choice of whether to dispatch and sell hydrogen or electricity, depending on regional electricity prices, hydrogen agreements, or other economic incentives.

While the technical development of the HTTR-GT/H2 has been detailed in previous studies, this report focuses on developing a techno-economic model to flexibly dispatch the HTTR-GT/H2 for electricity and/or hydrogen cogeneration. The goal is to investigate the potential impacts of different input assumptions or real-world conditions on the profitability of such a system. This work seeks to improve our understanding of the assumptions necessary for eventually making investment decisions pertaining to commercial hydrogen systems.

The HTTR-GT/H2 system was chosen for this economic model due to its simple design and the availability of process modeling data. Compared to commercial-scale systems, the HTTR-GT/H2 is relatively small, both in terms of nuclear plant size and hydrogen production. The small size means that electricity price feedback to the operation changes of the HTTR-GT/H2 would be minimal. The HTTR-GT/H2 has undergone detailed process modeling and has developed operation modes. Knowing the operating conditions for both the electricity sale and hydrogen sale modes makes the economic dispatch easier to model, and the smaller nature of this system helps further simplify the problem, since the system would participate in fewer electricity markets and have less of an impact on the electricity and hydrogen markets at large. This makes the impact of certain inputted data (e.g., electricity price data) more readily apparent. These effects and assumptions should be known prior to expanding this modeling methodology to larger, commercial systems as part of a broader study.

The HTTR-GT/H2 dispatch model acts as a price-taker model. Electricity is sold when regional electricity prices exceed the HTTR's operating costs and hydrogen is produced when the electricity price falls below HTTR-GT/H2 electricity production costs. The hydrogen is produced via the co-located IS cycle, as detailed in [13]. The price-taker assumption means that the model does not have any feedback between the changing load from the nuclear-IS cycle and grid electricity prices. This assumption is generally made for small generators and loads, such as the HTTR-GT/H2.

The goal of the dispatch model is to determine the price at which the system can sell hydrogen while breaking even economically. This price, also known as the levelized cost of hydrogen (LCOH), is the point at which sufficient money is made to justify building the hydrogen facility and dispatching energy to hydrogen production instead of selling only electricity.

The dispatch model was developed using the Risk Analysis Virtual Environment (RAVEN) model, developed at Idaho National Laboratory [14]. Two RAVEN plugins, the Holistic Energy Resource Optimization Network (HERON) and the Tool for Economic Analysis (TEAL)—also developed at Idaho National Laboratory—were used for creating the dispatch algorithm and tracking the economic parameters within the model [15].
