Due to its high density (19.05 g/cm³) and its nuclear properties (fissionability of 235U), metallic uranium possesses different military uses, e.g. military tank armour, armour piercing ammunition, and civilian uses, e.g. fuel for nuclear reactors, radiation shielding,… At SCK•CEN, metallic uranium is used as fuel for two nuclear reactors. Natural unalloyed metallic uranium, coated with an aluminosilicate alloy bonding layer and an U(Al,Si)3 anti-diffusion layer, and encapsulated in an aluminium container, is used in the BR1. Uranium alloyed with aluminium is being used in the BR2. Aluminium alloys have been used as cladding materials for nuclear fuel and targets because of their low thermal neutron absorption cross-section.
One solution for the long-term management of these fuels, which will eventually become waste, is geological disposal. To do so, a direct embedding of the waste in a cement-based material could be used. In contact with the highly alkaline cement pore water, high corrosion rate is observed for aluminium and uranium to form e.g. aluminium oxides, uranium oxides and hydrides, and hydrogen gas. Different problems could appear during the geological disposal due to these products: (i) UO2 and UH3 possess a lower mass density than metallic uranium which could lead to stress in the encapsulated matrix and damage this matrix, (ii) the production of H2 could produce an extra pressure deepening the stress problem, (iii) UH3 is a pyrophoric product which can ignite in contact with oxygen to form uranium oxide and hydrogen. However, if a cement-based material possessing a low porosity is used, the diffusion of water through the material will be limited, and due to this mass transport limitation, the corrosion rate should drastically decrease.
Presently proposed disposal routes
At present, in Belgium, no disposal route is predetermined for the metallic uranium waste. Different disposal routes could be investigated. The simplest and cheapest route is the direct emplacement of the fuel capsules (uranium, bonding and anti-diffusion layers, and aluminium container) in a cementitious matrix. To evaluate this option implies (i) the study of the metallic aluminium corrosion rate and mechanism in contact with a cement-based material, (ii) an analysis of the electrochemical corrosion of the aluminium cladding (iii) an analysis of the couple aluminium – aluminosilicate alloy to study the galvanic corrosion of the cell after breaching the aluminium cladding, and (iv) an analysis of the couple aluminosilicate – uranium alloy to study the galvanic corrosion of the cell after breaching the bonding layer. To keep this project feasible, the U(Al,Si)3 anti-diffusion layer will not be taken into account. Moreover, the metallic uranium corrosion will be the subject of a future project.
If the first route would prove to be unacceptable, mainly due to a too high corrosion rate of the aluminium container and the production of a huge volume of hydrogen, the second route would be the mechanical removal of the aluminium container, the bonding layer and anti-diffusion layers, and mixing afterwards the metallic uranium with a cement-based material. Evaluating this route implies the study of the corrosion rate and mechanism of the metallic uranium in highly alkaline solutions as well as the electrochemical corrosion of uranium. The influence of the mass transfer by encapsulating the metallic uranium in a cement-based material should be also analysed. If this second route is still not acceptable, a third route would be the preoxidation of the metallic uranium. This implies an iteration of calcination steps to oxidise the metallic uranium followed by the mechanical removal of the oxidised layer until the whole rod is treated. Finally, the last route foreseen whould be the reprocessing of the spent fuel to separate the plutonium from the uranium for a possible further use.