This paper describes the general view of Iodine-Sulfur cycles for thermochemical hydrogen production and the present state of IS process which has been studied at JAERI as a heat utilization system for HTGR’s. A bench scale apparatus is under operation in order to verify chemical applicability of the process. The three key conditions for operation, namely, (1) a maximum amount of SO₂ absorbable in Bunsen reaction, (2) relation between density and X(H₂O) under phase separation of Hi_x(aq) and H₂SO₄(aq), and (3) a removal of trace amount of H₂SO₄ from a Hi_x phase, are clarified.

This paper presents remarkable magnetic field effects on the chemical equilibrium for the system including ferromagnetic hydrides. The equilibrium hydrogen pressure in the β+γ region of the LaCo₅-H system was observed under the influence of magnetic fields up 26T. The pressure change for LaCo₅-H_4,2 is 60kPa in 26T at 293.2K. It is observed that the logarithmic pressure change, LPC, for this system has a non-linearlity with the magnetic fields. To investigate the origin of this behavior, the magnetization were measured in pulsed magnetic fields up to 28T. The magnetization change par mol hydrogen atom, ΔMS, increases with increasing magnetic fields. Moreover, the pressure-composition isotherms for the same system were measured in 12T as a -function of hydrogen contents.

In the GdCo₅H_x and the Y₂Co₇H_x systems, equilibrium hydrogen pressures were mostly decreased with increasing magnetic fields at the experimental temperatures and hydrogen compositions. The dependence of the LPC on the hydrogen composition or pressure is explained by the thermodynamic theory with the aid of the magnetic data. For the Y₂Co₇H_x system, the △Ms derived from the LPC predicts that a metamagnetic transition occurs in the critical field of 6T at 348.2K in the lower composition hydride of the β phase.