  The economical lifetime of the divertor is a key concern for realizing
nuclear fusion reactors that may solve the world's energy problem. A main risk
is thermo-mechanical failure of the plasma-facing tungsten monoblocks, as a
consequence of irradiation hardening induced by neutron displacement cascades.
Lifetime extensions that could be carried out without prolonged maintenance
periods are desired. In this work, the effects of potential treatments for
extending the lifetime of an operational reactor are explored. The proposed
treatments make use of cyclic recrystallization processes that can occur in
neutron-irradiated tungsten. Evolution of the microstructure under
non-isothermal conditions is investigated, employing a multi-scale model that
includes a physically-based mean-field recrystallization model and a cluster
dynamics model for neutron irradiation effects. The model takes into account
microstructural properties such as grain size and displacement-induced defect
concentrations. The evolution of a hardness indicator under neutron irradiation
was studied. The results reveal that, for the given microstructure and under
the assumed model behaviour, periodical extra heating can have a significant
positive influence on controlling the irradiation hardening. For example, at
800 C, if extra annealing at 1200 C was applied after every 100 hrs for the
duration of 1 hr, then the hardness indicator reduces from maximum 140 to below
70.
