(89c) Life Cycle Assessment of the Sulfur-Iodine Cycle

There are several thermochemical processes which can be used for large-scale production of hydrogen from nuclear reactors, including sulfur-iodine cucle, UT-3 cycle (University of Tokyo), and ISPRA Mark 9. The U.S. Department of Energy proposes to use the sulfur-iodine cycle combined with a new-generation nuclear reactor as the source of energy for the process. The decision to use this process was based on a study performed by General Atomics, University of Kentucky, and Sandia National Laboratories. That study ranked numerous thermochemical processesusing qualitative parameters such as number of reactions involved, number of chemical separations, and number of reports and studies published.

Life cycle assessments (LCA)have been published on different processes for hydrogen production; however, each tends to focus on parameters that increase the attractiveness of the process being advanced and decrease the attractiveness of others. Life cycle assessment should be performed which examine each process objectively, consistemtly, and equitably.

The goal of this life cycle assessment is to evaluate the environmental impacts of producing hydrogen using the sulfur-iodine thermochemical cycle and a nuclear reactor heat source. The life cycle assessment will identify significant environmental aspects and assess their impacts. The assessment can be used "stand-alone" or may be compared with similar life cycle assessments.

The sulfur-iodine cycle requires a heat source capable of producing temperatures of 1000 degrees Centigrade. Since current light-water reactors operate at nominal 600 degrees, new generation high-temperature gas-cooled reactors must be used. Materials used for construction must be able to withstand high temperatures. The LCA will examine the environmental impacts of using high temperature alloys and ceramic materials.

Another significant difference among thermochemical processes involves chemicals used in the process. For example, the UT-3 process uses bromine, calcium, and iron. ISPRA Mark 9 uses iron and chlorine, whereas the sulfur-iodine process uses sulfuric acid and iodine. The LCA addresses the method of production for each chemical, energy used, and relative abundance. Iron, chlorine, and calcium are relatively abundant and easily produced in the quantities necessary. Bromine and iodine, on the other hand are less abundant and require energy-intensive separation techniques. Some chemicals, such as sulfuric acid and iron, may be recycled from other industrial processes and may actually be a positive influence on the LCA.

Ultimately, the results of an LCA must be examined in the context intended. If the purpose of hydrogen generation is to replace hydrocarbon-based fuels in automobiles and enhance energy independence, it may be acceptable to emit higher greenhouse gas emissions from production of raw materials. On the other hand, if reducing greenhouse gas emissions is the goal of the process, various LCAs can be compared and the appropriate production technology selected.