The behavior of radioiodines in the primary coolant water of the heavy water-moderated and -cooled, natural uranium fueled 100-MW research reactor has been studied. The release-to-birth ratios of 131I, 133I, and 135I were compared during normal reactor operations and during incidents of fuel failures by estimating the isotopic concentrations using gamma spectrometry. From these results, conclusions on the severity and the size of the failure are drawn, which will help in the fuel performance evaluation and cladding development studies.