  Understanding how properties of materials change due to nuclear
transmutations is a major challenge for the design of structural components for
a fusion power plant. In this study, by combining a first-principles matrix
Hamiltonian approach with thermodynamic integration we investigate quasi-steady
state configurations of multi-component alloys, containing defects, over a
broad range of temperature and composition. The model enables simulating
transmutation-induced segregation effects in materials, including tungsten
where the phenomenon is strongly pronounced. Finite-temperature analysis shows
that voids are decorated by Re and Os, but there is no decoration by tantalum
(Ta). The difference between the elements is correlated with the sign of the
short range order (SRO) parameter between impurity and vacancy species, in
agreement with Atom Probe Tomography (APT) observations of irradiated W-Re,
W-Os, W-Ta alloys in the solid solution limit. Statistical analyses of Re and
Os impurities in vacancy-rich tungsten show that the SRO effects involving the
two solutes are highly sensitive to the background concentration the species.
In quaternary W-Re-Os-Vac alloys containing 1.5% Re and 0.1% Os, the SRO Re-Os
parameter is negative at 1200K, driving the formation of concentrated Re and Os
precipitates. Comparison with experimental Transmission Electron Microscopy
(TEM) and APT data on W samples irradiated at the High Flux Reactor (HFR) shows
that the model explains the origin of anomalous segregation of transmutation
products (Re,Os) to vacancy clusters and voids in the high temperature limit
pertinent to the operating conditions of a fusion power plant.
