Long‐lived products of uranium fission such as strontium‐90 and cesium‐137, with half lives of about 30 years, cease to be a problem in reactor wastes after a few centuries. But the alpha‐active transuranic actinides produced by transmutation in reactor fuel rods live much longer. Neptunium‐237, plutonium‐239, americium‐241 and curium‐246 have half lives ranging from five hundred to two million years. Although a waste disposal site would be no more radioactive after 500 years than a natural pitchblende ore deposit, it is widely felt that one should take reasonable precautions to keep these waste actinides out of the biosphere for hundreds of thousands of years.