Spent fuel

At the time of final disposal, only one-thousandth of the fuel’s original radiation remains. The basic principle of nuclear safety is to ensure that radioactive materials cannot contaminate the environment.

Multi-level safety systems are put into place to prevent emissions. Safety is developed by analysing risks and preparing for them. In addition to the laws of physics, nuclear safety is based, among other things, on the organisation’s high safety culture and multiple physical protections.

The fission reaction that occurs in electricity production makes the nuclear fuel highly radioactive. Protections against radiation can be implemented using a variety of materials. A few years after being removed from the reactor, spent nuclear fuel is transported to interim storage, where it is stored in water pools. The areas by the pools are safe as the 8-metre mass of water on the bundles stops the radiation. At the time of final disposal, even a few metres of rock will stop the radiation emitted by the bundles entirely.

Spent nuclear fuel is highly radioactive immediately after use, but its activity will decrease to one-hundredth of the original level in a year. The radiation of nuclear fuel decreases rapidly during the first decades following removal from the reactor. When spent fuel is disposed of after 40 years, only one thousandth of the original radioactivity will remain. Upon final disposal, the canister walls and a few metres of rock are enough to stop the radiation released by spent fuel entirely.

The radioactivity of spent nuclear fuel decreases to the level of a rich uranium ore deposit in 250,000 years. After this, the activity will continue to drop, arriving at roughly one-thousandth of the first year’s level over a thousand years. At the same time, radiation level on the surface of the canister will decrease to roughly one-hundredth of the level upon final disposal.

A small portion of the radioactive materials contained by the fuel have an extremely long life, which necessitates their isolation from nature. For this reason, the final disposal canisters are designed to remain tight and impervious in their final deposition place long enough for the radioactivity of spent fuel to decrease to a level not harmful to the environment.

Read more about the journey of nuclear fuel from the reactor to the final disposal facility on the tvo.fi websiteOpen link in a new tab.

Radioactive decay

  • Refers to the instability of the atomic nucleus, or its tendency to transform into another kind of nucleus.
  • The end result is a permanent non-radioactive material. For some materials this takes fractions of a second, for others it takes billions of years.
  • In the context of decay, the atomic nucleus can release material in the form of alpha or beta particles and energy in the form of gamma radiation. These particles and energy waves released by the radioactive nucleus are jointly referred to as radiation.
  • The radiation emitted by a radioactive substance decreases over time. The reduction in spent fuel radioactivity is initially rapid but it slows down later. This is due to the different substances contained in the fuel elements and their different half-lives. In general, radiation from a radioactive substance attenuates at the rate of the inherent half-life of that substance.