Neutron and electron irradiation experimental studies conducted on body-centered cubic Fe and Fe−Cr alloys have established two prismatic dislocation loop populations, which have Burgers vectors of either a/2⟨111⟩ or a⟨100⟩. The loop formation depends on factors such as dose (D), dose rate (D rt ), temperature (T), chromium content (Cr%), and other alloying elements. Hence, it is important to understand how irradiation-induced dislocation loops evolve conditional upon the loop characteristics, such as loop density (DD), average loop size d̅ , and irradiation parameters (D, D rt , T, and irradiation type), which is still an active area of research. To understand these complex structure−property relationships, machine learning (ML) is employed in a three-step approach. This includes imputing missing data with a k-nearest neighbor, generating functionalized features, and assessing feature importance with random forest classification and regression. Physics-based features are incorporated in a hypothesis-driven active learning scheme to overcome data unavailability challenges. Insights obtained from ML models (i) to categorize dislocation loop types, show the highest correlation with d̅ ; (ii) Log(DD), obtained through mathematical formulations involving D, Cr%, d̅ , and T (e.g., Log(DD) ∼ D + exp(−Cr%) + 1/d̅ and log(DD) ∼ D + exp(−Cr%) + 1/T). Hypothesis-driven active learning is able to predict Log(DD) in which the experimental date is not known. Causal models verify cause−effect relationships for dislocation loop classification and irradiation factors in FeCr alloys.