Magnesium alloys are mixtures of magnesium and other alloying metal, usually aluminium, zinc, silicon, manganese, copper, and zirconium. Since the most outstanding characteristic of magnesium is its density, 1.7 g/cm3, its alloys are used where light weight is an important consideration (e.g., in aircraft components). Magnesium has the lowest melting point (923 K (1,202 °F)) of all the alkaline earth metals. Pure magnesium has an HCP crystal structure, is relatively soft, and has a low elastic modulus: of 45 GPa. Magnesium alloys also have a hexagonal lattice structure, which affects the fundamental properties of these alloys. At room temperature, magnesium and its alloys are difficult to work cold because plastic deformation of the hexagonal lattice is more complicated than in cubic latticed metals like aluminium, copper, and steel. Therefore, magnesium alloys are typically used as cast alloys. Despite the reactive nature of the pure magnesium powder, magnesium metal and its alloys have good corrosion resistance.
Magnox Alloy – AL80
Magnox, an abbreviation for “magnesium non-oxidizing,” is a high magnesium alloy mainly of magnesium with small amounts of aluminium and other metals. This alloy, designated as alloy AL80, is used as a clad for nuclear fuel employed in the UK gas-cooled, graphite-moderated power station reactors of the same name. The claddings from magnesium alloys of type Magnox are well compatible with metallic uranium at temperatures up to 500°C and are highly resistant to oxidation.
This material has the advantage of a low neutron capture cross-section but has two major disadvantages:
- It limits the maximum temperature (to about 415°C), and hence the thermal efficiency, of the plant.
- The disadvantage of these alloys is a high tendency to grain growth, loss of strength properties, and oxidation in the presence of water steam.
The magnox alloy Al80 has a composition of 0.8% aluminium and 0.004% beryllium.
As was written, magnox alloy AL80 was used primarily as the fuel clad in Magnox-type reactors. On 30 December 2015, Wylfa Unit 1, the world’s last operating Magnox reactor, was closed. As the Magnox design was being rolled out, work was already underway on the Advanced Gas-cooled Reactor (AGR) with the explicit intention of making the system more economical. The AGR design retained Magnox’s graphite moderator and carbon dioxide coolant but increased the cooling gas operating temperature to improve steam conditions. This increased temperature (about 650 °C (1,202 °F)) would greatly improve the thermal efficiency of the power plant. But this was too hot for the magnox alloy, and the AGR originally intended to use a new beryllium-based cladding, but this proved too brittle. This was replaced by a stainless steel cladding. However, steel has a higher neutron cross-section, and this change required the use of slightly enriched uranium fuel to compensate for higher absorption cross-sections.