Nuclear-assisted hydrogen production presents a promising pathway for large-scale, low-carbon hydrogen generation. Among the critical considerations for this method is water consumption, which varies depending on the production technology, reactor type, and system configuration. High-temperature electrolysis (HTE) and thermochemical cycles are the two primary nuclear-assisted hydrogen production methods, each with distinct water requirements and cooling demands. Additionally, the integration of desalination offers potential solutions to mitigate freshwater consumption, particularly in water-scarce regions.

Water is a fundamental input for hydrogen production, as it serves as the feedstock for splitting into hydrogen and oxygen. In nuclear-assisted systems, water consumption occurs in two main ways: as a direct feedstock and as a coolant for the nuclear reactor. The total water requirement depends on the efficiency of the hydrogen production process, the reactor’s thermal output, and the cooling system employed.

High-temperature electrolysis utilizes both electricity and heat from a nuclear reactor to split water into hydrogen and oxygen. The process operates at elevated temperatures, typically between 700°C and 900°C, which improves electrolysis efficiency compared to low-temperature electrolysis. However, HTE still requires a substantial amount of water for feedstock. On a molar basis, one molecule of water yields one molecule of hydrogen, translating to approximately 9 kilograms of water per kilogram of hydrogen produced. In practice, additional water is needed for system cooling and auxiliary processes, increasing total consumption.

Thermochemical cycles, such as the sulfur-iodine (S-I) cycle or the copper-chlorine (Cu-Cl) cycle, use a series of chemical reactions to split water, driven by nuclear heat. These cycles often achieve higher theoretical efficiencies than HTE but may require more complex water management. For example, the S-I cycle involves multiple steps where water is consumed and regenerated, with net water usage comparable to HTE. However, side reactions and inefficiencies can lead to slightly higher water demand. The Cu-Cl cycle, being a hybrid thermochemical-electrolytic process, has intermediate water requirements.

Cooling demands in nuclear-assisted hydrogen production are significant, as nuclear reactors generate substantial waste heat that must be dissipated to maintain safe operating temperatures. The choice of cooling system directly impacts water consumption. Three primary cooling methods are employed: once-through cooling, wet recirculating cooling, and dry cooling.

Once-through cooling systems withdraw large volumes of water from a nearby source, such as a river or ocean, pass it through the reactor’s heat exchangers, and return it at a higher temperature. This method is highly water-intensive, with typical withdrawal rates ranging from 90 to 180 cubic meters per megawatt-hour of thermal energy. While the consumed fraction (lost to evaporation or blowdown) is relatively small, the environmental impact of thermal discharge can be a concern.

Wet recirculating cooling systems, such as cooling towers, reduce water withdrawal by recycling cooling water but increase consumption due to evaporation losses. These systems typically consume 1.5 to 3 cubic meters of water per megawatt-hour of thermal energy. For a nuclear reactor coupled with hydrogen production, this translates to significant additional water use beyond feedstock requirements.

Dry cooling systems, which use air instead of water for heat rejection, eliminate water consumption for cooling but are less efficient and more expensive. They are rarely used in large-scale nuclear applications due to performance penalties but may be considered in arid regions where water scarcity outweighs efficiency losses.

The type of nuclear reactor also influences water usage. High-temperature gas-cooled reactors (HTGRs) and molten salt reactors (MSRs) are often considered for hydrogen production due to their ability to deliver heat at the required temperatures. HTGRs typically use helium as a coolant, which reduces water dependency compared to light-water reactors (LWRs). However, LWRs, which dominate the current nuclear fleet, require substantial cooling water, making them less ideal for hydrogen production in water-constrained areas.

Integrating desalination with nuclear-assisted hydrogen production offers a potential solution to freshwater scarcity. Nuclear reactors can provide both the heat and electricity required for desalination processes, such as multi-stage flash distillation or reverse osmosis. By co-locating desalination plants with hydrogen production facilities, brackish water or seawater can be treated to meet both cooling and feedstock needs. This approach not only reduces reliance on freshwater sources but also enhances the overall sustainability of the system.

For example, a nuclear reactor producing hydrogen via HTE could divert low-grade waste heat to a desalination unit, generating freshwater for electrolysis and cooling. Studies indicate that coupling a 300 MW thermal reactor with a desalination plant could produce up to 50,000 cubic meters of freshwater per day, sufficient to support large-scale hydrogen production while minimizing net freshwater withdrawal.

The potential for water recycling within nuclear-assisted hydrogen systems further reduces consumption. Closed-loop water management, where process water is treated and reused, can significantly decrease the demand for fresh inputs. Advanced purification technologies, such as membrane filtration or electrochemical treatment, enable the recovery of high-purity water from waste streams, optimizing resource utilization.

In summary, nuclear-assisted hydrogen production involves substantial water use, primarily as feedstock and coolant. High-temperature electrolysis and thermochemical cycles have comparable water requirements for hydrogen generation, but cooling system choices dramatically influence overall consumption. Reactor type plays a critical role, with high-temperature gas-cooled reactors offering advantages in water efficiency over traditional light-water reactors. The integration of desalination and water recycling presents viable strategies to mitigate freshwater demand, particularly in regions facing water scarcity. By carefully optimizing these factors, nuclear-assisted hydrogen production can achieve sustainable water use while contributing to decarbonization goals.