Conditioning of radioactive aluminium generated by the VVR-S Nuclear Reactor Decommissioning Laboratory Inactive Tests

Aluminium is a reactive amphoteric metal, readily forming a protective oxide layer on contact with air or water. However, as the oxides are amphoteric, aluminium is not resistant to corrosion in acidic and alkaline conditions, because the protective films dissolve. As a consequence radioactive waste containing bulk aluminium alloys can not be embedded in Ordinary Portland Cement (OPC). A potential encapsulating material for the radioactive aluminium is potassium magnesium phosphate (MKP). This paper presents the characterization results obtained from analyzing the potential magnesium phosphate formulations and assesses its potential to reduce the corrosion of aluminium. A series of experiments have been performed. The main conclusions of the paper are as follows. First, the pH values of magnesium phosphate formulation investigated increased gradually over the test duration, with pH measurement ranging from 8.1 - 9.1, indicating lower values compared with the reference composite OPC (pH ∼ 13). The reduction of pH is an important controlling factor for the corrosion of aluminium. Secondly, according to XRD, the hardened magnesium phosphate matrix is polycrystalline and the main reaction product of magnesium phosphate cement formulations was confirmed as MgKPO₄-6H₂O, which was found to dominate the crystalline phase composition. Thirdly, the compressive strengths obtained for magnesium phosphate matrices investigated are included in the accepted limits for the embedding matrix with cement (above 5 N/mm²). And fourthly, the corrosion of metallic aluminium in magnesium phosphate matrix is markedly reduced in comparison with the composite OPC