A time-dependent model is formulated for the chemical reaction between sodium colloids and gas bubbles, which are brought into contact with each other by the growing voids. It is shown that in this exothermic reaction, heat is released much faster than it is dissipated by conduction to the surrounding salt. This results in a large and sudden temperature and pressure increase in the void, which is analysed in detail. The effect on the mechanical stability of NaCl under irradiation is discussed