Long-lived fission products
The inventory of the radiotoxicity of spent fuel shows that the contribution of fission products decreases fairly quickly - everything is relative - until about 600 years. At that time sclae, the fission products like cesium-137 and strontium-90 whose periods are 30 years give way to a handful of radionuclides having very long periods. The activities varying in inverse pto eriods, these fission products contribute little to the radioactivity and much less toxicity. Their radioactive toxicity is less than 0.5% of that of the initial uranium ore and less than one hundred millionth of the initial radiotoxicity of the spent fuel.
Source : Clefs CEA