Nuclear reactors coupled to HTES are capital-intensive technologies, due to both the nuclear plant and the electrolysis plant. The development of economical and durable HTES unit materials, which can be similar to those of the solid oxide fuel cell materials, can contribute to cost reduction. The development of improved HTES units with low electrode overvoltage at lower temperatures can enable their use with lower-temperature and thereby lower-cost nuclear plants. Improved HTES cell designs are currently being investigated at Lawrence Livermore National Laboratory (Pham, 2000) and Idaho National Engineering and Environmental Laboratory (Herring, 2002). In addition, attaining high power cycle efficiency at the nuclear plant with relatively low temperatures can contribute to cost reduction. Finally, development of economic high-temperature radiation-resistant graphite or ceramic-coated graphite materials for the nuclear plant is needed.
A recent screening of several hundred possible reactions (Besenbruch et al., 2000) has identified two candidate thermochemical cycles that have the highest commercialization potential, with high efficiency and practical applicability to nuclear heat sources. These are the sulfur-iodine (SI) and calcium-bromine-iron (Ca-Br) cycles. The S-I cycle is being investigated by General Atomics and JAERI. The Ca-Br cycle, which is sometimes called UT-3 to honor its origin at the University of Tokyo, is being investigated by JAERI. Argonne National Laboratory (ANL) is currently working on achieving thermochemical water-splitting processes at lower temperatures than the SI and Ca-Br cycles. ANL has identified the copper-chlorine (Cu-Cl) thermochemical cycle for this purpose (Doctor et al., 2002).
Sulfur-Iodine Cycle and Other Sulfur Cycles The SI cycle has been proposed in several forms. (The SI cycle and other