Many observations contradict the FP3DM. These include the void lattice formation11-14 and higher swelling rates near GBs than in the grain interior in the following cases: high-purity copper and alumi­num irradiated with fission neutrons or 600 MeV protons (see original references in reviews117,118); aluminum irradiated with 225 MeV electrons119 and neutron-irradiated nickel120 and stainless steel.121 Furthermore, the swelling rate at very low dislocation density in copper is higher,122-124 and the depen­dence of the swelling rate on the densities of voids and dislocations is different,125 than predicted by the FP3DM. It gradually became clear that some­thing important was missing in the theory. There was evidence that this missing part could not be the effect of solute and impurity atoms or the crystal structure. Indeed, austenitic steels of significantly different compositions and swelling incubation peri­ods exhibit similar steady-state swelling rates of ~ 1% per NRT dpa.3 , And, although generally the bcc materials show remarkable resistance to swelling, , the alloy V-5% Fe showed the highest swelling rate of ^2% per dpa: 90% at 30dpa.34

As outlined in Section 1.13.3.1, the primary dam­age production under neutron and ion irradiations is more complicated; in addition to PDs, both vacancy

and SIA clusters are produced in the displacement cascades. This is the reason the FP3DM predictions fail to explain microstructure evolution in solids under cascade damage conditions. In fact, it has been shown that it is the clustering of SIAs rather than vacancies that dominates the damage accumu­lation behavior under such conditions. The PBM proposed in the early 1990s and developed during the next 10years (see Section 1.13.1) essentially resolved many of the issues; the phenomena men­tioned have been properly understood and described. This model is described in the next section.