(210f) Radiolytic Water Splitting: Demonstration at the Pm3-a Reactor

The current interest in nontraditional methods for the generation of hydrogen has prompted a revisitation of radiolytic splitting of water, where the interaction of ionizing radiations (α, β, and γ) with water produces molecular hydrogen. This reevaluation was further prompted by the current availability of large amounts of radiation sources contained in the fuel discharged from nuclear reactors. This spent fuel is usually stored in water pools, awaiting permanent disposal or reprocessing. The yield of hydrogen resulting from the irradiation of water with β and γ radiation is low (G-values = <1 molecule per 100 electron-volt of absorbed energy) but this is largely due to the rapid reassociation of the species arising during the initial radiolysis. If impurities are present or if physical conditions are created that prevent the establishment of a chemical equilibrium, the net production of hydrogen can be greatly enhanced. In the operation of nuclear reactors, where intensive radiation fluxes are created by fission and radioactive decay processes, the generation of hydrogen could be hazardous. Also, the radiolytic production of reactive oxygen-containing species may lead to excessive corrosion of materials of construction. Thus, considerable effort has been expended to suppress the radiolysis of water. This author was involved in 1962-63 with the PM3-A, a small nuclear power reactor at the McMurdo Station in Antarctica. During the first year of operation, considerable quantities of hydrogen was continuously generated in the closed containment tank in which the reactor pressure vessel, surrounded by shielding water, was located. This unexpected development was caused by the improper design of the shield water recirculation loop. Water was withdrawn at a point located in the high radiation zone near the pressure vessel were radiolytic hydrogen was generated. After passing through a cooler and ion-exchange purifier, the water was returned through a sprinkler ring located above the shielding water level. This provided for an efficient degassing and subsequent buildup of hydrogen in the closed containment tank. Temporary remedial methods, such as continuous purging of the containment atmosphere with air or the use of a catalytic recombiner, were adequate to permit continuing operation of the reactor. Eventually, modifications of the recirculation loop completely suppressed the generation of hydrogen. An analysis of this occurrence and a review of the remedial actions, which curtailed the release of hydrogen into the containment tank atmosphere, are very useful in the evolution of conceptual designs of radiolytic hydrogen generating systems. Calculations were performed to estimate the theoretical limits for the yield of hydrogen obtainable by radiolysis. The effects of other factors (catalysts, increased contact surfaces, temperature, phase conditions and separations, ion scavengers, and materials of constructions) are examined. Radiolysis could also be useful when used in conjunction with other techniques for hydrogen production.