Modelling of spatial radiolysis in primary water

The radiolysis of water in LWRs (light water-cooled reactors) by gamma, neutron and alpha radiation leads to the production of molecular (H2O2, H2) and radical (H, OH, e-(aq), H+) species. These species subsequently react and achieve steady state concentrations. The concentrations of the oxidizing species (O2, H2O2) and dissolved hydrogen significantly influence the corrosion potential of an alloy exposed to the primary coolant and, hence, affect the resistance of the alloy to localized corrosion and stress corrosion cracking particularly. A common approach to estimate the corrosion potential of in-core components is numerical calculation. The steady state concentrations of dissolved hydrogen, oxygen and hydrogen peroxide are estimated by conventional radiolysis codes. These concentrations are used afterwards to calculate the corrosion potential. However, the analytical expressions employed in the calculations for approximation of the mass transport do not take into account radiolysis in the diffusion layer. This can cause significant errors in the estimation of the corrosion potential even for a simple electrode geometry; hence also for occluded regions like baffle bolt crevices. Therefore, to improve the numerical calculation of radiolysis one should take into account simultaneously mass transport effects and radiolysis. This approach has been called spatial radiolysis calculations. A typical PWR radiolysis model contains some 50 chemical reactions involving some 20 species. The implementation of all these reactions into a finite element code in order to calculate mass transport and radiolysis simultaneously has not been successful