NUCLEATION AND GROWTH OF HELIUM-VACANCY CLUSTERS IN VANADIUM AND VANADIUM ALLOYS - V-5TI, V-3TI-1SI, V-5TI-5CR

Thermal helium desorption spectrometry (THDS) is applied to the study of helium trapping and clustering in vanadium and a number of selected vanadium alloys. A comparative analysis of the desorption spectra rev eals the presence of at least two different trap sites responsible for the helium desorption observed in the investigated materials. These a re (1) helium-vacancy clusters nucleated at interstitial impurities (C , N, O) and developed during the irradiation, and (2) pre-existing tra ps, e.g. precipitates and undersized alloying elements. The contributi on of these two types of traps in pure vanadium and the alloys appeare d to vary in magnitude depending on the material. In addition to the e xperiments, a Monte Carlo computer program has been developed for the simulation of both the irradiation and the annealing of the materials. The results obtained for nucleation, growth and dissociation of heliu m-vacancy clusters are compared to the experimental results. The model takes into account interstitially dissolved impurities, i.e. C, N and O interacting with helium-vacancy clusters.