"Radioactive" refers to something that emits harmful energy in the form of radiation. This energy can be harmful or even deadly to living organisms, and can also cause damage to the environment. Full definition
The iodine is taken up by the active (abnormal) thyroid tissue, but not by any other tissues, resulting in a selective local accumulation of radioactive material in the abnormal tissues. (icatcare.org)
It is now a form of radioactive waste that has to be kept in a safe status for thousands of years afterward. (thegreenlivingexpert.com)
The reactor is designed to never be opened on site and thus there is little opportunity for diversion of radioactive material from operating plants. (watertechbyrie.com)