Radiation chemistry of high-temperature (300–410 °C) water. Part 1.—Reducing products from gamma radiolysis

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Abstract
Using gamma rays, and dilute solutions of radical scavengers, the G values of the primary reducing products from water, namely H2, H and e– aq, were found to be 2.0, 0.3 and 0.4 at 400 °C, density 0.45 kg dm–3, in marked contrast to the G values 0.4, 0.6 and 2.7, respectively, for liquid water at 25 °C. At 300 °C, density 0.75 kg dm–3, G H2 was similar to the value at 400 °C. The high G value for hydrogen at 400 °C, coupled with low values for GH and G e– aq suggests that, in contrast to reactions at room temperature, only a small fraction of the hydrogen is formed by the reaction together of the species H and e– aq in spur reactions. The molecular hydrogen is thought to be formed together with oxygen atoms mainly in unimolecular decomposition processes, e.g. H2O*= H2+ O. The low dielectric constant of 8 at 400 °C compared with 80 at 25 °C is considered to be an important factor in determining the low G value for scavengeable electrons. The low overall G value for water molecules ultimately decomposed at 400 °C and density 0.45 kg dm–3 suggests a greater efficiency for the deactivation of excited molecules at that condition compared with liquid state at 25 °C. A reaction mechanism is proposed which accounts for the observed product against dose curves when computer simulation of these is carried out.

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