Nuclear fission power currently provides over 11% of the world\’s electricity demands and allows reliable electricity production, with negligible CO2 emissions . As nuclear fusion becomes increasingly recognized as a potential alternative to fission, great efforts are being made worldwide towards developing nuclear fusion reactors. These are aimed to offer a secure source of Cy5.5 NHS ester with no production of greenhouse gases, no long-lived radioactive waste and almost unlimited fuel supplies. In particular, ITER, a major international effort to build a commercial reactor-scale fusion device, is currently undergoing construction . Both fission and fusion reactors operate in a highly aggressive environment of high temperatures and high doses of energetic neutrons [3–7]. Bombardment by neutrons, characteristic of these environments, initializes atomic scale changes in the microstructures of the materials inside the reactor. Primary mechanisms of such initial changes are either displacement of atoms from their lattice positions by neutron collisions , or the generation of hydrogen and helium through chemical transmutation reactions that cluster into bubbles leading to undesirable swelling of the matrix . These initial atomic scale changes further develop into a variety of chemical and structural features [9,10] which can ultimately undermine critical mechanical properties through phenomena such as radiation induced embrittlement , radiation hardening  and radiation induced clustering . It is therefore crucial to fully understand mechanisms that are in operation from even the earliest stages of atomic scale damage. Such knowledge will allow the accurate evaluation of operational lifetime of the different components, and the development of new, sustainable materials towards internal components of nuclear reactors.
Atom probe tomography (APT) has been established over the past decades as a key technique for the characterization of radiation-induced nanoscale damage in materials for fission and fusion applications . Among these, case studies include radiation-induced clustering in reactor pressure vessels (RPV) steels [14–16], particle stability and distribution of oxide particles in oxide dispersion strengthened steels (ODS) [17–19], helium bubble trapping at oxide particles in ODS steels  and surface oxidation processes of stainless steels  and zirconium alloys  in corrosive environment. With the rise to prominence of APT, the potential of its forerunner, field ion microscopy (FIM), has largely been overlooked. While APT has proven capabilities for the study of small scale radiation induced chemical changes, it lacks the necessary spatial resolution and detection efficiency to image individual sites on the crystal lattice and therefore struggles in the direct imaging of atomic scale crystal damage. FIM, on the other hand, is comparatively more limited in terms of analytical capabilities. However, it enables direct imaging of complete crystallographic arrangements of atoms on the surface of the sample and can therefore constitute as a highly beneficial complementary technique to APT.
FIM and APT are based upon the concept of field ionization and field evaporation respectively . Both techniques require a very sharp needle-shaped specimen, held in an ultra-high vacuum chamber, and exploit the fact that an intense electric field can be generated at its apex by application of a DC voltage. In APT, high voltage/laser light pulses are superimposed on the standing DC voltage to trigger the field evaporation of atoms on the surface of the specimen, which are in turn projected onto a position-sensitive detector. Each hit on the detector can be directly related to the pulse responsible for the single corresponding field evaporation event, facilitating highly accurate time-of-flight measurements and hence chemical identification. In the final step an inverse projection algorithm, combined with the sequence of evaporation and an assumed model for specimen shape enable a 3D atom-by-atom reconstruction of the analyzed volume .