4.9. For the most highly concentrated radioactive waste, such as spent nuclear fuel (if designated as radioactive waste) and vitrified waste from fuel reprocessing, it is necessary that the engineered barriers provide practically complete containment over a period of several hundreds of years to several thousand years. This will ensure that the majority of shorter lived radionuclides decay in situ and uncertainty associated with the degradation of the waste form and migration of radionuclides when pronounced thermal gradients are present is reduced (i.e. any release of radionuclides would occur only after the heat generated by radioactive decay has substantially decreased).