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The key imbalance is preferential loss of self-interstitials to biased defect sinks, leaving a vacancy supersaturation that can feed void swelling. The initial displacement cascade creates both vacancies and self-interstitials, often in equal net numbers as Frenkel pairs; it need not create more vacancies than interstitials at birth.

Many nearby Frenkel pairs recombine. Others separate, become trapped or form dislocation loops. Surviving self-interstitials are usually much more mobile than vacancies over the relevant range. Dislocations also have an interstitial bias: their elastic interaction with defects makes them more efficient sinks for self-interstitials. Preferential interstitial removal to the dislocation structure can leave the mobile vacancy population above its thermal equilibrium value.

A minimal radiation damage rate theory makes the distinction explicit:Here $c_i,c_v$ are defect number densities, $G$ is the surviving mobile-pair production rate per volume, $K$ the recombination coefficient, $D_i,D_v$ the diffusion coefficients, and $k_i^2,k_v^2$ the respective sink strengths. The vacancy loss term includes thermal emission in an effective form. Equal production terms do not require equal steady populations or equal partitioning among sinks. This simplified model omits an explicit cluster-size distribution.

A stable cavity receiving a greater vacancy influx than interstitial influx and vacancy emission grows. Vacancies therefore condense into voids, producing void swelling. Helium can stabilize small cavity nuclei, but a growing vacancy void need not be a high-pressure gas bubble. Dislocation loops, precipitates and changes in the dislocation density evolve alongside the voids and modify the competing sink strengths.

There is no violation of atom conservation: atoms represented by removed self-interstitials have joined extended defects or surfaces, while the corresponding unfilled lattice volume can accumulate in cavities. The classic electron-microscope observations of cavities in irradiated type316 steel provide direct evidence for this microscopic basis of swelling.^[ref]

The resulting dislocation density reflects competing production, growth and recovery rates. The irradiation spectrum and displacement dose rate set the supply of recoil cascades and surviving point defects. Cascade structure controls the fraction born as isolated defects or clusters; equal nominal displacements per atom can therefore give different defect populations.

Temperature sets vacancy and interstitial mobilities and the rates of thermal emission and recovery. A simple radiation damage rate theory model illustrates the competition using number densities $c_i,c_v$:Here $G$ is the surviving mobile-pair production rate per volume, $K_{\mathrm{rec}}$ is the recombination coefficient, $D_i,D_v$ are diffusion coefficients, and $k_i^2,k_v^2$ are sink strengths. The model omits explicit cluster-size populations and thermal-equilibrium emission; those must be added for a quantitative description of a particular exposure.

The initial dislocation density, grain size, free surfaces, precipitates and existing cavities all change sink strengths. Dislocations can have an interstitial bias, so their capture rates for the two defect types differ. Solutes and transmutation gases alter defect binding and mobility. Crystal structure, elastic anisotropy and applied stress affect which loop orientations survive and grow.

For a number density $N_{\ell }$ of circular loops of radius $R$, their contribution to line length per volume isThus loop nucleation and loop growth both increase dislocation density, while shrinkage, annihilation and absorption at other sinks can decrease it. A change in loop count alone does not determine the change in total line density.

Schematically, ${\dot{\rho }}_d$ is a production-and-growth contribution minus recovery and annihilation. An initial rise followed by approximate saturation is possible when these balance, but the saturation value and approach rate depend on the evolving microstructure. There is no universal dislocation density fixed by dose alone.