Possibilities of radiation dose-rate reduction in pressurized-water reactors based on an adsorption model

Cobalt is 'adsorbed' more or less reversibly on the surface of the oxide layers on stainless steel and on Zircaloy. The dose rate that is built up outside the core but inside the primary system of PWRs can be explained by this adsorption model. Based on this model, the most promising possibilities for reducing the dose-rate are: (1) reduction of the growth rate of zirconium oxide on Zircaloy, (2) the application of hard-facing materials without cobalt and of stainless steels with a low cobalt content, and (3) preoxidation of stainless steel. It can also be concluded from the model that fast recontamination will take place after decontamination of parts of the primary circuit. The dose rate of the decontaminated parts will eventually be even higher than that of the non-contaminated parts. Lengthening of the fuel cycle by using more highly enriched uranium will result in thicker zirconium oxide layers and hence in an increase in dose rates in the system. (orig.)