  The evolution of point defect concentrations under irradiation is controlled
by their diffusion properties, and by their formation and elimination
mechanisms. The latter include the mutual recombination of vacancies and
interstitials, and the elimination of point defects at sinks. We show here that
the modelling of this evolution by means of atomistic kinetic Monte Carlo
(AKMC) simulations, necessarily using small system sizes, introduces strong
space and time correlations between the vacancies and interstitials, which may
strongly affect the recombination rate and the point defect concentrations. In
such situations, standard rate theory models fail to predict the actual point
defect concentrations. The effect is especially strong when the elimination of
point defects occurs only by recombination, but can still be significant in the
presence of sinks. We propose a new Correlated Pair Theory that fully takes
into account the correlations between vacancy and interstitial pairs and
predicts point defect concentrations in good agreement with AKMC simulations,
even in very small systems. The Correlated Pair Theory can be used to modify
the elimination rates in AKMC simulations to yield point defect concentrations
as predicted by the standard rate theory, i.e. representative of large systems,
even when using small simulation boxes.
