you cannot convert radioactive material into nonradioactive material, you can only shield it, dilute it or place it at a safe distance.
cell mutations
It gets frozen. None of its other properties are affected. It is still radioactive material.
a. placard
It varies, depending on the particular radionuclide involved. The half-life of Carbon-14, for instance, is 5730 years, while Iodine-131 is 8 days, Cesium-137 is 30 years, and Uranium-235 is 700...