Using exchange Monte Carlo (MC) simulations based on an ab initio-parameterized Cluster Expansion (CE) model, we explore the phase stability of low-Cr Fe-Cr alloys as a function of vacancy (Vac), carbon, and nitrogen interstitial impurity content. To parameterise the CE model, we perform density functional theory calculations for more than 1600 supercells containing Cr-Vac-C-N clusters of various size in pure bcc Fe, Cr, and Fe-Cr alloys. Our analysis shows that including three-body cluster interactions in a CE model is necessary for achieving agreement with experimental observations. MC simulations performed for 푇 = 650K show that Cr clustering in Fe-3.28%Cr alloys does not occur if there are no defects or if only vacancies are present. But the addition of a small amount of C or N impurities, at the level as low as 0.02 at.% in an alloy with no vacancies, routinely results in the formation of ordered interstitial compounds containing a high amount of Cr. We find that Cr segregates to interstitial impurities and that the local Cr content increases as a function of C and/or N concentration in a Fe-Cr alloy. In the presence of vacancies, C/N impurities aggregate to the core regions of vacancy clusters (voids), making Cr segregation effects less pronounced. The structure of Cr-rich clusters varies significantly, depending on the impurity content and on the N to C ratio. Predictions derived from MC simulations agree with experimental observations of Fe-Cr alloys exposed to ion irradiation. The concentration of Cr found in clusters with C and N interstitial impurities is in qualitative agreement, and the absolute Cr content found in the clusters simulated at 650 K is in quantitative agreement with experimental atom probe tomography (APT) observations of Fe-3.28%Cr alloys irradiated at 623 K, where the measured C and N content of 42±5 and 151±3 atomic ppm likely resulted from the contamination occurred during ion beam irradiation.