= Solution
\b[<helium> changes cavity stability and <ductility>; <void swelling> changes bulk volume and component dimensions.] These effects interact, but a <helium> bubble and a vacancy-rich void are not interchangeable descriptions.
<neutron> reactions produce <helium> from alloy constituents and impurities. For example, capture by boron-10 can produce <helium> through $^{10}\mathrm B+n\longrightarrow{}^7\mathrm{Li}+{}^4\mathrm{He}$; the production per displacement dose depends on composition and <neutron> spectrum. Retained <helium> has low solubility and is trapped at defects or in bubbles. Bubble nucleation and growth at <grain boundaries> can assist intergranular cavitation, lowering tensile <ductility> and creep-rupture resistance, particularly when <temperature> permits the relevant transport. This is important even before a large bulk <void swelling> is measurable. A primary study linked high-temperature <ductility> loss to observed bubbles and grain-boundary weakening in irradiated <stainless steel>.<High-temperature Embrittlement of Stainless Steel irradiated in Fast Fluxes by A. S. Fraser and others>[Nature 211, 291–292, abstract: mechanical observations, thin-foil bubbles and boundary-bubble interpretation]{ref}
<displacement damage> continuously creates <vacancies> and <self-interstitials>. Their different capture efficiencies at <dislocations> and other sinks can leave a <vacancy supersaturation> available to cavities. A stable cavity grows when its net <vacancy> uptake exceeds <vacancy> emission and interstitial absorption. <helium> can help stabilise small nuclei, but the later volume increase is not simply the volume of gas inserted into an unchanged matrix. Changing <helium> content can also change cavity number, size and competition for <point defects>; a universal proportionality between <helium> concentration and <void swelling> is not justified.
For <nuclear fuel cladding>, <void swelling> can enlarge the diameter, alter the fuel–<nuclear fuel cladding> gap, reduce clearances and distort <reactor coolant> channels or nearby structures. Nonuniform <void swelling> generates additional <stresses>. Bubble-assisted embrittlement and stress-dependent <creep (deformation)> then affect the ability of the wall to contain the fuel and internal pressure. The relevant measurements combine <density> or volume change, dimensional <strain>, cavity microscopy and mechanical tests. Pressurised-tube experiments explicitly separated <void swelling>, <creep (deformation)>, plastic <strain> and <precipitation (chemistry)> <strain>; <void swelling> itself was approximately isotropic in that study.<The influence of cold-work level on the irradiation creep and swelling of AISI 316 stainless steel by E. R. Gilbert and F. A. Garner>[Journal of Nuclear Materials 367–370, 954–959; PNNL abstract, separation of strain components]{ref}
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