Diffusion-kinetic modelling of the effect of temperature on the radiation chemistry of heavy water

Abstract

The extended spur diffusion model has been adapted to examine the influence of temperature on D₂O radiolysis with radiation of linear energy transfer (LET) up to 90 eV nm⁻¹. The temperature range 20–315°C has been investigated to predict the effects of γ-rays and fast neutrons on the coolant in pressurised water-cooled nuclear power reactors. The model has been tested for three sets of parameters describing initial concentrations of the primary species and their spatial distribution. The best accord with the experimental data has been obtained with an initial yield of G°(e_aq⁻) of 0.425 μmol J⁻¹. Calculations show that the decay of e_aq⁻ is controlled by the reactions: (i) e_aq⁻ + OD → OD⁻, (ii) e_aq⁻ + D⁺ → D and (iii) e_aq⁻ + e_aq⁻ → D₂ + 2OD⁻ whilst the chemistry of the OD radical is mainly determined by (iv) OD + OD → D₂O₂, and (i). The observed slower decay and higher yields of e_aq⁻ in comparison with those in H₂O are explained as resulting from a broader initial spatial distribution of the electron after thermalisation. A larger initial spur size is also found for the other radiolytic products in D₂O. Analysis of the sensitivity of the calculated radiation chemical yields to the precision of the values of the diffusion coefficients and the reaction rate constants shows that their uncertainty increases with LET and can be as much as 0.02 μmol J⁻¹. Thus better quality experimental data are required for more accurate extrapolation to high temperatures so that the parameters used for modelling radiation effects in reactor cooling water can be tested more rigorously.