Modern steels are strengthened by finely dispersed nano-particulates, particularly in high-temperature structural materials. Radiation damage and temperature can cause these to change their shape, size, and distribution, leading to embrittlement. Oxide dispersion strengthened (ODS) steels are designed for high-temperature operation; they contain oxide nano-clusters (e. g. yttrium-titanium oxide) in a ferritic-steel matrix.

The size and morphology of these particles may change with exposure to stress, temperature, or radiation; all conditions which occur in nuclear-power systems, typi­cally increasing the size and decreasing the number of strengthening particles. Changes in the volume fraction, surface interface and size distribution of these particles can cause significant changes to the strength, ductility, and toughness of these materials. Advanced reactor-materials use nano-particulates to act as sinks for radiation-induced phenomena such as vacancies, self-interstitials, and as trapping sites for He bubbles from nuclear reactions in the material. Small-angle neutron scattering (SANS) can non­destructively analyse these nanoscale features over a large sample volume.

The use of a strong magnetic field can differentiate between magnetic and non­magnetic scattering at right angles to each other, and the strengthening nano-par­ticulate oxides are typically non-magnetic. With ODS steel samples the SANS curves for no magnetic field (nuclear scattering only) and with field (nuclear + magnetic scattering) are compared. If they have no difference then the scattering is entirely non-magnetic and it can be assumed that the scattering is from the non-magnetic oxide nano-clusters. If there is a difference then the contribution from iron-based magnetic scatterers can be removed. This experimental technique should be used in conjunction with an oxide-free reference sample of the same manufacturing route (if possible), for correct de-convolution of the SANS from oxide nano-clusters.

The size of strengthening nano-particles is in the range 1-20 nm, and the few techniques available for studying these are transmission-electron microscopy (TEM), atom-probe tomography (APT), and SANS, however TEM and APT only sample a small volume. As a result, SANS has become a critically important technique for the development, understanding and characterisation of these irradi­ation-resistant materials [16, 17].