Corrosion of AlMg3.5 alloy from Studsvik R2 reactor under simulated cementitious repository conditions

Licentiate thesis, 2025

Decommissioning nuclear research reactors produces low- and intermediate-level radioactive
waste which has to be confined for extended time scales. The current repository concept is
predicated on encapsulation in concrete. The alkaline conditions of the cement pore water with
pH values up to 14 in early stages can be advantageous for certain radionuclides due to their
precipitation. However, for other materials, including reactive corrosive metals such as
aluminum, it can foster their corrosion. The decommissioned Studsvik R2 research reactor
contained an AlMg3.5 alloy for core internals and the reactor tank. Acute aluminum corrosion
is known to be fast at high pH and then slow down due to the formation of a protective oxidation
layer. The initial corrosion produces significant amounts of hydrogen which can lead to
pressure build up when encapsulated in concrete. This pressure increase can produce cracks
which can be migration pathways for radionuclides. The corrosion rates of the Studsvik R2
reactor alloy have been studied while exposing it to concrete with elevated water/cement ratios
of 0.7 (ca. 0.45 in commercial concrete). An artificial simplified weathered cement water was
used to examine its suitability to assess long-term corrosion rates in concrete. Corrosion rates
were determined by hydrogen-induced pressure measurements in overpressure bottles and
stainless-steel pressure vessels, as well as by mass loss evaluation. The initial corrosion rates
were high with more than 104 µm/y and then slowed down to values in order of 102 µm/y after
2000 hours of exposure in concrete. In pressure vessels, a delayed increase of the corrosion
rates in cement water was found which is assumed to be caused by the surface coverage of
hydrogen bubbles. Future and ongoing studies will contain different lower pH and climate
friendly concrete compositions, pre-treatment of the alloy and the examination of other reactive
metals that are relevant for Swedish repositories.