Validation of ¹³¹I yields from fission and irradiation of ¹³⁰Te

¹³¹I, a radioactive (T1/2 ~8 days) isotope of element iodine, is an important isotope due to its wide applications especially in the field of healthcare. It is used in medical therapies for diagnosis as well as treatment, especially of ailments related to thyroid. It is also used as an industrial tracer. ¹³¹I is produced in nuclear reactors by neutron irradiation of ¹³⁰Te. ¹³¹I also appears as a product in fission of heavy nuclides. In order to get the maximum specific activity of ¹³¹I, it is required to have a suitably optimized irradiation scheme for the specific reactor system and an efficient extraction methodology to deduce maximum ¹³¹I from irradiated samples. The optimization of activation and decay procedures requires an accurate estimation of the yield of ¹³¹I as a function of time during the generation process and subsequent radioactive cooling. Unfortunately, the literature contains inconsistent and erroneous results of the ¹³¹I yield as a function of neutron fluence, target mass, irradiation time and post-irradiation decay. We have developed an analytical model to calculate the yield of ¹³¹I produced either in fission or by irradiation of ¹³⁰Te as a function of irradiation time, cooling time, sample mass and neutron flux. Our model is based on general and mathematically exact solution of governing rate equations and does not build upon the approximations on which conventional analytical methods are based. We have tested our model by comparing its results with those from other studies and found it satisfactory. Estimation of activity calls for a careful and judicious selection of cross-section data as they are found to vary widely across different studies. To estimate the activity for Dhruva, we calculated the flux depression factor and microscopic cross-section using the neutronic code DRAGON by simulating the actual target sample geometry and properties in Dhruva fuel matrix. Flux depression factor was also estimated by a diffusion theory-based model and was found close to that obtained from DRAGON. We have used our model with thus obtained cross-section and flux depression factor to estimate ¹³¹I activity produced in Dhruva at different tray rod locations and found them in agreement with the measured values. (author)