Helium accumulation in metals during irradiation – where do we stand?

In the present contribution, the basic ideas pertinent to modelling helium accumulation in metals are reviewed. Topics of earlier work are: Diffusion of atomic He and bubble nucleation under irradiation, low temperature (diffusion controlled) vs. high temperature (dissociation controlled) nucleation, cascade induced He resolution and continuous bubble nucleation at high doses, homogeneous in the bulk vs. heterogeneous nucleation at extended defects; bubble coarsening upon annealing, migration and coalescence vs. Ostwald ripening; bubble state, upper limit vs. ‘equilibrium’ pressure, high pressure equation of state of He, bubble-to-void transformation under a stress or irradiation induced effective vacancy supersaturation. More recent topics are: formation and growth limitation of He platelets (He-filled nano-cracks) in some metals and ceramics, coupled two-component Ostwald ripening of bubble–loop complexes. Possible effects of bubble formation on mechanical properties are briefly addressed: hardening and embrittlement, particularly at high temperature where intergranular fracture is induced by the transformation of bubbles to voids at grain boundaries. Finally, important but unsolved problems are identified and their relevance are briefly discussed.