  A novel computational treatment of dense, stiff, coupled reaction rate
equations is introduced to study the nucleation, growth, and possible
coalescence of cavities during neutron irradiation of metals. Radiation damage
is modeled by the creation of Frenkel pair defects and helium impurity atoms. A
multi-dimensional cluster size distribution function allows independent
evolution of the vacancy and helium content of cavities, distinguishing voids
and bubbles. A model with sessile cavities and no cluster-cluster coalescence
can result in a bimodal final cavity size distribution with coexistence of
small, high-pressure bubbles and large, low-pressure voids. A model that
includes unhindered cavity diffusion and coalescence ultimately removes the
small helium bubbles from the system, leaving only large voids. The terminal
void density is also reduced and the incubation period and terminal swelling
rate can be greatly altered by cavity coalescence. Temperature-dependent
trapping of voids/bubbles by precipitates and alterations in void surface
diffusion from adsorbed impurities and internal gas pressure may give rise to
intermediate swelling behavior through their effects on cavity mobility and
coalescence.
