[en] The density changes measured in three Fe--Cr--Mn--Ni solute-bearing austenitic alloys irradiated in FFTF-MOTA indicate that nickel additions do not decrease swelling as had been anticipated, but actually increase swelling initially. These data were confirmed by microscopy examination but are contrary to the behavior observed during electron irradiation of these same alloys. Nickel and commercially relevant solute additions also appear to suppress the formation of ferrite and sigma phases, which were observed in solute-free Fe--Cr--Mn ternary alloys irradiated in the same experiment. 8 refs., 11 figs