Development of a method for decontaminating the primary circuit of a helium-cooled high-temperature reactor

In a helium-cooled high-temperature (HHT) reactor with gas turbine the turbine must be accessible for maintenance. If the activity of the primary circuit becomes excessive due to fission and activation product contamination, decontamination procedures must be undertaken to make the system accessible. For two years several groups have been working on the development of a decontamination process for an HHT reactor and promising results have already been obtained. A method has been developed for artificially contaminating specimens out of pile with 134Cs, 137Cs and 110Agsup(m). The adhesion of caesium to specimens contaminated out of pile is comparable with that observed in specimens contaminated in pile. In-pile experiments have shown that the contamination depends qualitatively and quantitatively on the material used and its surface condition. Treatment of specimens with solutions containing oxidizing agents and substances of complex formation has led to the determination of more than 100 decontamination factors associated with an acceptable material corrosion rate. In parallel with these experiments the corrosion of materials has been investigated using electrochemical methods, in particular contact corrosion and crevice corrosion. The initial results of studies of the interactions between helium contaminants and metallic materials are also available. The first part of the programme was to determine the mechanical characteristics before and after decontamination, especially under alternating stresses. All these investigations will be pursued with a view to establishing a decontamination process which will enable the gas turbine and other parts of an HHT reactor to be decontaminated without damage to the materials. (author)