Radiation damage in materials

Changes in a material caused by irradiation, including displacement defects, chemical changes and their subsequent microstructural evolution. A nominal dose does not uniquely specify the surviving defect population or final material response.

Retention of helium produced by alpha decay or nuclear reactions. helium can be trapped at defects and form bubbles; its long-term effects are distinct from the immediate ballistic collision cascade.

A helium-containing cavity in an irradiated material. helium helps stabilise small cavities and can accumulate at grain boundaries, contributing to embrittlement.

Volume increase caused by irradiation-induced defects and microstructure changes. Accumulating helium and defect clusters can contribute; radiation-induced swelling is not identical to radiation-induced amorphization.

Transport accelerated by irradiation-maintained populations of mobile point defects. It can change precipitation, segregation and recovery. Its consequences depend on defect production, recombination and defect sinks, rather than simply on the nominal displacement dose.

A reduction in resistance to plastic deformation associated with irradiation. Removal of pre-existing strengthening obstacles can exceed new irradiation hardening, for example when an initially hardened microstructure undergoes recovery or precipitate dissolution. Compare an appropriate thermal control before attributing a measured change to radiation.

Loss of mechanical integrity associated with retained helium, including the growth and coalescence of gas-containing cavities at grain boundaries. Gas location, trapping, temperature, stress and the initial microstructure influence the result. It can occur in austenitic steel without the ferritic ductile-to-brittle transition.^[ref]

Loss of long-range crystalline order through irradiation damage. radiation-induced amorphization does not itself demonstrate loss of chemical containment or a specified radionuclide release rate.

The fraction of a material lacking the specified long-range crystalline order. It is a structural measure, not directly a dissolved mass fraction or radionuclide release fraction.

A random-overlap model in which each independent radioactive decay damages a fixed volume. At constant event rate density $R$ and damaged volume $v$, the surviving crystalline fraction is $e^{-Rvt}$ and the amorphous fraction is $1-e^{-Rvt}$.

The effective volume converted to an amorphous state by one independent radioactive decay event in a direct-impact model. The subscript alpha can index the event without identifying the emitted alpha particle as the dominant source of ballistic radiation damage.

The number of alpha decay events per material volume accumulated over time. For constant alpha-decay event rate density $R$, $D=Rt$; otherwise $D(t)={\int }_0^{t}R(s)ds$. Multiplying by a damaged volume per event gives a dimensionless overlap parameter.

Macroscopic volume increase caused by a population of growing cavities in a material. Stable nucleation, defect supersaturation and competition among sinks determine its onset and evolution.

The derivative of volume void swelling with respect to displacement dose, $dS/dD$. It differs from a time rate; conversion requires a dose rate and a specified irradiation history.

The transient dose interval before substantial measurable cavity void swelling. Nucleation and evolution of competing defect sinks make it sensitive to starting microstructure and irradiation conditions.

A vacancy-rich cavity generated under irradiation. Its growth is governed by the balance of vacancy and interstitial fluxes and differs from a strongly gas-pressure-stabilised bubble.

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]

If a fixed matrix volume $V_0$ acquires cavity volume $V_c$, then $S=V_c/V_0$ and the cavity fraction of the current total volume is $f=V_c/(V_0+V_c)$. Eliminating $V_c$ gives $S=f/(1-f)$; for small cavity fractions, $S\simeq f$.

An increase in resistance to plastic flow caused by irradiation-produced obstacles to dislocation glide, including loops, clusters and precipitates. Increased strength can coexist with reduced ductility.

An irradiation-induced dimensional change that can occur without external stress, often through anisotropic defect accumulation in a textured crystal. Opposite strains along different directions can leave a small net volume change.

An ideal orientation average $\overline{\mathbf{\epsilon }}={\sum }_g{v}_g{R}_g{\mathbf{\epsilon }}^{(g)}{R}_g^T$. Random orientation averages a traceless single-grain growth tensor to zero, while crystallographic texture can retain a directional bulk strain. crystal grain constraints can modify this simple independent-grain estimate.

In an ideal axisymmetric volume-conserving crystal grain model, directional growth is proportional to $1-3⟨{\cos }^2\theta ⟩$, where $\theta$ is the angle between that direction and the crystal c-axis. Random crystallographic texture has $⟨{\cos }^2\theta ⟩=1/3$.

A population-balance description of defects and clusters during irradiation. Production, recombination, diffusion to sinks, emission and changes in cluster size are represented by coupled rate equations. Parameter and population choices determine which regimes the model can describe.

A feature that absorbs mobile point defects, such as a free surface, grain boundary, dislocation or cavity. Capture efficiency and relative preference for defect types are as important as visible size or number.

Preferential absorption of self-interstitials relative to vacancies by a sink. Dislocation bias contributes to the unequal defect fluxes that can sustain void swelling.

In a mean-field diffusion model, an effective inverse-area measure for removal of mobile defects. The loss rate $D{k}^{2}c$ has number-density-per-time units when $D$ is diffusivity and $c$ is defect number density.

Damage initiated when energetic particles transfer enough momentum to displace lattice atoms. A recoil can generate a displacement cascade, while many displaced atoms recombine during the ensuing relaxation.

Removal of an atom from its original lattice site. Whether it leaves a surviving defect depends on subsequent collisions, recombination and relaxation.

Annihilation of a vacancy and an interstitial, or related relaxation of irradiation defects. The surviving damage after a cascade can be much smaller than its initial number of atomic displacements.

atomic displacements produced by collision-driven momentum transfer. It differs from the much larger electronic energy deposition that a fast alpha particle can produce along its track.

The minimum recoil energy required, for a specified atom and direction or a stated averaging convention, to produce a surviving displacement defect. It depends on material, direction and the defect-survival criterion.

A dimensionless estimate of displacement events per target atom under a specified damage model. It does not count the vacancies still present after recombination and microstructural evolution.

The rate of accumulation of displacements per atom. Equal final displacement dose reached at different rates can produce different defect populations because diffusion and recovery continue in elapsed time.

A sequence of atomic recoils and local relaxation following an energetic impact in a solid. It can leave isolated Frenkel pairs, clusters and regions of altered local structure, rather than one permanent defect per initial collision.

A lattice atom given a recoil by an incident particle. Its subsequent collisions can generate a displacement cascade; its energy distribution depends on the incident radiation and material.