The Safety Concept
safety is based on the multiple barriers principle. Radioactive substances are
contained in several barriers that support each other, yet are as independent
from each other as possible so that the failure of one barrier will not
endanger the overall functioning of the isolation.
Posiva's final disposal plans are based on the KBS-3 concept developed by SKB, the company responsible for nuclear waste management in Sweden. In this final disposal solution, spent fuel will be packed into strong water-tight canisters. The canisters will be placed into the bedrock at a depth of 400 to 700 metres, where they are isolated from the population and where they will require no maintenance to remain leak-tight for as long as their content could in any way harm living nature.
State of Matter
of the Fuel: The ceramic state of the fuel forms the first release
barrier in itself. The uranium within the gas-tight metal rods is solid and
dissolves in water only slowly, which diminishes the rate of release of
Final Disposal Canister: The fuel is packed in a gas-tight, corrosion-resistant canister made of copper and cast iron that protects the fuel assemblies from the mechanical wear occurring deep inside the bedrock.
Bentonite Barrier: The final disposal canister is surrounded with bentonite clay that protects the canister from any potential jolts in the bedrock and diminishes the movement of water in the proximity of the canister.
Bedrock: The bedrock provides the canister and bentonite with conditions where changes are slow and predictable. Deep in the bedrock, the canisters are protected from any changes occurring aboveground, such as future ice ages, and kept away from people’s normal living environment.
Radioactivity of Spent Fuel Diminishes Over Time
The radioactivity of spent nuclear fuel quickly
diminishes during the first decades after the fuel has been removed from the
reactor. During the first 40 years, activity is reduced to approximately one
tenth of what it is one year after the fuel is removed from the reactor.
Already at the time of final disposal, the walls of the canister and a couple
of metres of bedrock are enough to completely block any radiation that the fuel
The lowering trend then continues so that within the first thousand years, activity is reduced to approximately a thousandth part of the first year's level. At the same time, the radiation level on the surface of the canister is reduced to approximately a one-hundredth part of the level prevailing at the time of final disposal.
A small portion of the radioactive substances contained in spent fuel have a very long lifespan and require long-term isolation from the biotic nature. For this reason, the final disposal canisters are designed to remain leak-tight in their final disposal location for as long a period as possible.