= Solution
\b[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:
$$
\begin{aligned}
\dot c_i&=G-Kc_ic_v-D_i k_i^2c_i,\\
\dot c_v&=G-Kc_ic_v-D_v k_v^2(c_v-c_v^{\rm eq}).
\end{aligned}
$$
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.<Voids in irradiated stainless steel by C. Cawthorne and E. J. Fulton>[Opening description of Dounreay-irradiated specimens examined by thin-foil electron microscopy]{ref}
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