The size distribution of dust particles in nuclear fusion devices is close to the power function. In the paper it is shown that function of this kind can be the result of brittle destruction. From the similarity assumption it follows that the size distribution obeys the power law with the exponent between -4 and -1. The model of destruction has much in common with the fractal theory. The power exponent can be expressed in terms of the fractal dimension. An additional assumption about the structure of fragmentation offers that the exponent is close to -3. The exponent for the case of the biggest ball removing equals -3.4.