Fig. 1-6 Reaction scheme of IS process The IS process is composed of three chemical reactions which in combination spilit H2O. An acid production reaction produces H2SO4 and HI from H2O, sulfur dioxide (SO2) and iodine (I2). Heat decomposes the HI to form H2 and I2. Heat also decomposes the H2SO4 to produce SO2, O2, and H2O.

Fig. 1-7 Experimental facility The experimental facility produces hydrogen by the IS process. This facility, constructed from fluorinated resin and glass, operates at atmospheric pressure. The heat required for its operation is supplied by electricity.

Fig. 1-8 H2 and O2 production vs. time H2 production was nearly stable at a rate of 32 normal liter (NL)/h, and the production ratio of O2 to H2 was 0.5:1. These results indicate that the water-splitting reaction was stable.

We have been conducting R&D on hydrogen (H2) production technologies using nuclear energy to obtain a large-scale, economical H2 production process that is environmentally friendly. Among the H2 production methods that split water using heat from a HTGR, the iodine and sulfur (IS) process has attracted worldwide interest. The demonstration of a stable, long-term, continuous closed-cycle operation is indispensable in industrializing this process. To achieve this operation, maintaining a balance of reacting constituents of the three chemical reactions is essential. In this operation, all chemicals except water (H2O), H2, and oxygen (O2) must circulate through the process (Fig. 1-6) without being discharged. A new control method was developed to maintain the acid production solution composition in a stable state, which is vital to obtain this balance. A H2 production test using the experimental facility (Fig. 1-7) was performed to verify the control method. The acids produced during acid production separate into two phases, the hydriodic acid (HI) phase, and the sulfuric acid (H2SO4) phase. These acids were periodically sampled during the test and analyzed. Their compositions were found to remain at nearly regular mole fractions. This confirms the suitability of the control method and H2 production for 20 h were successfully accomplished (Fig. 1-8). This R&D was conducted under a contract between JAERI and the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Reference S. Kubo et al., A Demonstration Study on a Closed-Cycle Hydrogen Production by the Thermochemical Water-Splitting Iodine-Sulfur Process, Nucl. Eng. Des., 233(1-3), 347 (2004). (http://dx.doi.org/10.1016/j.nucengdes.2004.08.025)

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