Corrosion Characteristics of Studsvik R2 Al-Alloy by Hydrogen Evolution Under Simulated Repository Conditions

Other conference contribution, 2025

Large quantities of low and intermediate level radioactive waste are produced by the dismantling of nuclear reactors. It must be kept confined to ensure safety for extended time scales. To ensure this, the waste can be encapsulated in concrete which makes the interim storage and repository conditions alkaline (pH > 12). This is advantageous for some heavy-element radionuclides because it fosters their precipitation. For reactive metals and their corrosion characteristics, however, it can be problematic. The Swedish Studsvik R2 nuclear research reactor build contained an AlMg3.5 alloy similar to Al 5154 for its core internals and the reactor tank. The corrosion of aluminum at alkaline conditions is known to start at exorbitant high levels and then decrease very rapidly due to the formation of a protective oxide layer on the surface of the sample. The formation of corrosion products leads to a potential volume expansion which is accompanied by the evolution of hydrogen gas and the following pressure build-up. Both can lead to cracks in the concrete which creates possible pathways for radionuclides. Unirradiated rod-shaped samples were cut from a reference sample originating from the manufacture of the reactor vessel. Samples were embedded in regular Portland cement concrete with an elevated water/cement ratio of 0.7 to ensure
the availability of water throughout the experiment due to increased pore water content. The embedded samples were put into overpressure bottles alongside pressure, humidity & temperature sensors.
Corrosion rates were calculated from the evolving hydrogen pressure inside the bottle. Samples of the same geometry, from pure aluminum (99.95 %) were also used as references. The R2 alloy had an initial corrosion rate of 564 (± 180) µm during the first two weeks and then decreased to 434 (± 178) µm/y after one month. The reference material had a corrosion rate of 1622 (± 283) µm/y during the first two weeks and decreased to 606 (± 222) µm/y after one month which suggests a trend of the R2 alloy corroding slower than pure aluminum.