During nuclear fission and fusion, matter that seems to disappear is actually converted into massenergyvolumenuclei.

energy

The answer is; mass-energy

Radioactive elements are heavier unstable isotopes of the element that sponteneously decay into other elements. In nuclear fission, the isotopes decay into light more stable elements. The change in mass is because some of the matter is converted to electromagnetic radiations. This is why nuclear fission produces harmful radiation such as gamma rays that can be harmful to genetic material of living organisms.