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In 1967, C. Cawthorne and E. J. Fulton reported electron-microscope thin-foil examinations of type316 stainless-steel fuel cladding and mechanical-test specimens irradiated in the Dounreay Fast Reactor. The work helped identify microscopic cavities in the dimensional-instability problem.^[ref]

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]

At low accumulated displacements per atom, a stainless steel can undergo substantial microscopic damage while its macroscopic volume changes little. Irradiation-generated vacancies and self-interstitials recombine, reach defect sinks, or form small clusters and dislocation loops. These processes can harden the steel before a large population of growing cavities appears. Thus dimensional stability during this interval does not imply an undamaged microstructure.

Void swelling commonly has an incubation interval, a transition into appreciable swelling, and a later interval with an approximately steady swelling rate per unit dose. The transition reflects the evolution of both stable cavity nuclei and the competing sink population. Gas atoms can help stabilize small cavities, but a vacancy void need not be filled with high-pressure gas.

The driving imbalance is a vacancy supersaturation. Dislocations generally capture interstitial defects more efficiently than vacancies, giving an interstitial bias. Once stable voids exist, the resulting excess vacancy supply can enlarge them. Growing and evolving dislocation structures compete with the cavities for defects. A roughly steady balance can give an approximately linear volume increase with dose over part of the exposure range, rather than proving indefinite linear growth.

If cavities occupy a fraction $f$ of the current volume and the matrix volume is otherwise unchanged, the void swelling volume fraction relation isSince cavity volume scales as radius cubed, numerous tiny clusters and a smaller population of larger voids can have very different dimensional consequences.

The original Cawthorne-Fulton observation of voids in irradiated steel used thin-foil electron microscopy on type316 stainless steel exposed in the Dounreay Fast Reactor. It connected microscopic cavities with the dimensional-instability problem that had emerged during fast-reactor fuel development.^[ref]