Void formation in irradiated Nickel 270

Nickel 270 (99.98 per cent nominal purity) was irradiated in EBR-II to fluences ranging from 1 × 10¹⁸ to 1.5 × 10²² neutrons/cm² at temperatures between 375 and 525°C. Voids were observed in all specimens in concentrations of 1 to 3 × 10¹⁴/cm³, independent of temperature and fluence. At low fluences the voids were non-homogeneously distributed. These observations are interpreted in terms of void nucleation on sites existing in the material prior to irradiation. The results are compared with other observations on nickel of comparable and higher purity. Large differences exist not only in the magnitude of void concentrations but also in temperature and fluence dependencies. These differences indicate that a single, void nucleation mechanism is not operative and that impurities play an important role in determining the nature and amount of damage produced by neutron irradiation at elevated temperatures.