The biomass, Ganoderma Lucidum, is a naturally growing macro-fungus that grows in rubber plantations, and has been found to remove uranium from low level radioactive wastewaters. Subsequent to the sorption of uranium, the loaded biomass needs to be safely and scientifically disposed. This is done by first immobilizing the waste biomass material in a solid matrix such as concrete or resin, and then disposing it in deep sea or geological repositories after due enclosures for safety. Despite immobilization there remains a risk of leaching of radioactivity from the loaded biomass material. This requires to be verified by testing leachability in lab under varying pH conditions that mimic sea water and geological repository environments. Short terms desorption and leachability experiments were conducted at the materials lab at MANIT, Bhopal to determine the leachability of radioactivity from the concrete matrix enclosing the loaded biomass. It was found that leaching was rapid in acidic conditions (0.2 N HCl), with 27.2 % uranium leaching out within 8 days. This indicated the suitability of alkaline sea-waters for safer disposal of uranium laden immobilized biomass. Mathematical models were subsequently developed based on the laboratory data to predict the long-term leachability for assessing the safety of disposal mechanism, and indicated the prominence of diffusion as the leachability mechanism. This paper details the experiments conducted, the materials used, the lab observation results, and the mathematical models developed to determine safety aspects related to possible leaching from immobilized concrete matrix in which radioactive biomass has been loaded.