Highly accessible radionuclides decay in a few hundred years, inaccessible radionuclides decay very slowly. After about 180,000 years, the spent fuel's radioactivity will be comparable to radia-tion from the natural uranium from which it was made. The waste system designers' goal is to devise a series of barriers that will isolate the fuel from contact with the habitable environment until the fuel no longer presents a hazard. The conservative approach adopted by SKB requires that the copper canis-ter itself remain intact for more than 100,000 years. Why Sweden chose copper Corrosion resistance is the canisters' most important technical requirement.
Copper is naturally resistant to corrosion.
Moreover, because copper is a pure element and not an alloy, it is possible to calculate conditions under which the metal will remain stable indefinitely. Pure metals like copper also hold fewer surprises from a corro-sion standpoint. Over a period of more than 15 years, Swedish scientists and engineers have built up an extensive database on the country's groundwater. It is, in fact, a rather high-quality groundwater. Water that soaks into the bentonite buffer will, in addition to the minerals listed above, contain some sulfate, carbonate and bicarbonate.