document: Ferritic and martensitic steels are candidate materials for use in nuclear reactors [1,2]. The transmutation-created inert gas, especially He, plays an important role in the microstructural evolution of these steels under neutron irradiation. In a previous paper [3] the mechanisms by which He in a perfect body-centred-cubic (bcc) Fe lattice, can agglomerate into bubbles was discussed. It was shown that small He interstitial clusters are highly mobile but become effectively pinned with the emission of Fe interstitials when the clusters contain 5 or more He atoms. Small bubbles up to around 1.5 nm in diameter can easily form at room temperature from such seed points but larger bubbles are more difficult to form by diffusion alone due to the induced strain in the bcc lattice which increases the energy barriers for diffusion towards the bubbles whilst reducing them in a direction away from the bubbles. Subsequent bubble enlargement can then only occur either through increased temperature or by radiation induced mechanisms which increase the number of vacancies in the bubble and reduce the lattice strain. Emission of interracial loops from such a bubble was not observed in molecular dynamics simulations.
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