The decommissioning and dismantling of nuclear research reactors can lead to a large amount of low- and intermediate-level radioactive waste. For repositories, the materials must be kept confined and safety must be ensured for extended time spans. Waste is encapsulated in concrete, which leads to alkaline conditions with pH values of 12 and higher. This can be advantageous for some radionuclides due to their precipitation at high pH. For other materials, such as reactive metals, however, it can be disadvantageous because it might foster their corrosion. The Studsvik R2 research reactor contained an AlMg3.5 alloy with a composition close to that of commercial Al5154 for its core internals and the reactor tank. Aluminum corrosion is known to start rapidly due to the formation of an oxidation layer, which later functions as natural protection for the surface. The corrosion can lead to pressure build-up through the accompanied production of hydrogen gas. This can lead to cracks in the concrete, which can be pathways for radioactive nuclides to migrate and must therefore be prevented. In this study, unirradiated rod-shaped samples were cut from the same material as the original reactor tank manufacture. They were embedded in concrete with elevated water–cement ratios of 0.7 compared to regular commercial concrete (ca. 0.45) to ensure water availability throughout all of the experiments. The sample containers were stored in pressure vessels with attached high-definition pressure gauges to read the hydrogen-induced pressure build-up. A second set of samples were exposed in simplified artificial cement–water to study similarities in corrosion characteristics between concrete and cement–water. Additionally, the samples were exposed to concrete and cement–water in free-standing sample containers for deconstructive examinations. In concrete, the corrosion rates started extremely high, with values of more than 10,000 µm/y, and slowed down to less than 500 µm/y after 2000 h, which resulted in visible channels inside the concrete. In the cement–water, the samples showed similar behavior after early fluctuations, most likely caused by the surface coverage of hydrogen bubbles. These trends were further supported by mass loss evaluations.

Decommissioning of research reactors leads to significant amounts of low- and intermediate-level radioactive waste that is planned to be stored under cementitious conditions in Swedish repositories. These materials contain important amounts of aluminum and its alloys which are reactive under alkaline conditions found in concrete pore water. The corrosion produces hydrogen that can lead to cracks in the concrete which can be pathways for radioactive ions. As an attempt to optimize repository conditions for aluminum-based reactor waste, parts of the ordinary Portland cement I in the concrete were replaced by fly ash. This addition may reduce pH and alkali–carbonate reactions increase the durability of the concrete. Testing of fly ash concretes is also of interest from a future availability perspective given the potential phase out of ordinary Portland cement in favor of formulations such as fly ash variants with lower CO₂ emissions. Decreased corrosion rates, more uniform corrosion accompanied by ATR-FTIR (Attenuated Total Reflectance—Fourier Transform Infrared Spectroscopy) spectra and µCT (Micro computed tomography) as well as SEM-EDX (Scanning Electron Microscopy with Energy Dispersive X-ray analysis) images suggest less aggressive corrosion of the Al alloy from the decommissioned Studsvik R2 reactor in concrete with varying amount of fly ash replacement.