The various options for nuclear hydrogen production are given in Table G-4. The basic chemistry, projected efficiency, established experience, and other related issues for each technology option are now briefly addressed.

The electrical energy demand in the electrolysis process decreases with increasing water (or steam) temperature. While the demand for heat energy is increased, the decrease in the electrical energy demand improves the overall thermal-to-hydrogen heat conversion efficiency. Higher temperatures also help lower the cathodic and anodic overvoltages. Therefore, it is possible to increase the current density at higher temperatures, which yields a significant increase in the process efficiency. Thus, the high-temperature electrolysis of stream (HTES) is advantageous from both thermodynamic and kinetic standpoints. The electrodes of the HTES unit can be made of ceramic materials, which avoids corrosion problems, though hydrogen embrittlement might still be a problem for electrode durability. High-pressure operation would also be preferable, in order to reduce the size of the chemical units and transmission lines.

The HTES process is potentially advantageous when coupled to high-efficiency power cycles and can consequently yield high overall thermal-to-hydrogen energy efficiency. The efficiency of hydrogen production via coupling of HTES to either of two high-temperature nuclear reactors is given in Figure G-5 (Yildiz and Kazimi, 2003). One reactor is the gas turbine modular high-temperature reactor (GT-MHR) (LaBar, 2002). The second is an advanced gas-cooled reactor (AGR) coupled to a direct supercritical CO₂ power cycle. The cycle was originally proposed for fast reactors (Dostal et al., 2002). The supercritical AGR (S-AGR), also referred to as the S-CO₂, necessitates upgrading the AGR design pressure from the current 4 megapascals (MPa) to about 20 MPa, which has not been attempted before in a concrete containment. A reference HTES design called HOTELLY (high-operating-temperature electrolysis) (Doenitz et al., 1988) is chosen as the basis for this example.

Implementation of the GT-MHR-HTES at the temperature of 850°C for the near term appears possible, while achieving temperatures of 950°C and higher might be expected for the years 2025 and beyond. Similarly, for the S-AGR-HTES, the near-term and far-term goals may be 650°C