[en] The motion of point defects in a solution is of a stochastic nature. Therefore their number in any finite volume fluctuates. The void growth rate undergoes stochastic fluctuations. The amplitude of these fluctuations depends on the sum of vacancy and interstitial fluxes to the void. A spatially random void distribution results in small spatial variations of the swelling rate from void to void. The condition for the stochastic fluctuations of point defect concentrations to increase crucially the amplitude of the small variations concerned is obtained on the basis of the stochastic description of the void evolution under irradiation. The instability of the spatially homogeneous void distribution arises due to the essential difference of the parameters characterizing the systems of voids and point defects. It is also shown that voids of sizes typical for void lattices in Mo can become spatially ordered by means of a Brownian-like motion after a dose of about 10 dpa. (orig.)