In many countries rock salt formations are candidates to host nuclear waste repositories. One of the aspects that needs careful consideration before such a repository can be put into operation is the formation of radiation damage in the salt. A model has been developed that provides a fundamental understanding of the buildup of radiation damage in NaCl. This model is based on kinetic rate reactions and takes into account the effect of impurities and the colloid nucleation stage on the growth of metallic sodium colloids. With this model, we have calculated the amounts of NaCl that can be converted into metallic sodium and molecular Cl2 for various options for repository design and intermediate storage times. It is shown that the concentrations of these defect aggregates, even very close to the high-level radioactive waste containers with steel walls 5 mm-thick, will be limited to a few mole percent.