Radiation of spent fuel
Nuclear fuel becomes strongly radioactive during its use. Various materials provide protection from radiation. In addition, radiation decreases with time.
Radioactivity refers to the instability of the atom nucleus, or its tendency to change into another kind of nucleus. This change event is often referred to as radioactive decay. The final result is a stable, non-radioactive substance. Some substances only take a few fractions of a second to decay, others need billions of years.
In connection with the decay, the atom nucleus may emit matter in the form of alpha or beta particles or energy as gamma radiation. These particles and energy waves emitted from the radioactive nucleus are referred to by the common name of radiation.
The radiation produced by a radioactive substance decreases with time. The radiation decreases rapidly at the beginning, but slows down over time.. Spent nuclear fuel is very radioactive immediately after use, but within a year the activity is already reduced to one-hundredth of the original level. At the time of final disposal, no more than one-thousandth of the original radioactivity remains. In the final disposal facilities, the radioactivity reduces to the level of a rich uranium ore deposit in 250,000 years.
Various materials isolate radiation. A few years after removal from the reactor, spent fuel is transferred to interim storage. In there, it is stored in water pools. It is possible to go around at the pool edges, as the eight metre mass of water on top of the assemblies stops the radiation of the assemblies. At the time of final disposal, even a few metres of rock fully stops the radiation from the assemblies.