The influence of boron on the clustering of radiation damage in graphite

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Abstract
A model for describing the clustering of interstitial atoms in graphite is set forth, based on chemical reaction rate theory. Vacancies and interstitials are assumed to be homogeneously generated by the neutron flux. The interstitials are allowed to move freely between the layer planes whilst the vacancies are considered to be stationary. Two types of nucleation are considered: (1) homogeneous nucleation when one unbound interstitial meets another unbound interstitial, (2) heterogeneous nucleation when one unbound interstitial encounters another interstitial atom bound already to a boron atom. It is shown that heterogeneous nucleation may be taken into account in the reaction rate theory by reducing the velocity of the interstitials by an amount which corresponds to the time they spend bound to an impurity. Differential equations are formulated to allow for the variation of loop density and average loop radius with time. Numerical and asymptotic solutions are obtained and fitted to data obtained from electron microscope observations (Kelly and Mayer 1969); the theory predicts that the loop density is proportional to the square root of the impurity concentration and that the loop radius is inversely proportional to the sixth root of the impurity concentration, provided the interstitial velocity is ‘impurity controlled’.