Nuclear Fuel Cycle

Uranium metal fuel was the first nuclear fuel to be used in reactors. Its key properties are:

• Good strength and ductility
• High thermal conductivity — this allows fuel elements to be thick in cross-section whilst still maintaining good heat transfer to the coolant
• High reactivity with non-metallic elements — uranium metal forms an oxide layer after only 3—4 days of exposure to air, which can reduce the thermal conductivity of the fuel

Crystal Phases of Uranium Metal

Uranium metal exists in three distinct crystal phases, each stable over a different temperature range:

The dimensional changes caused by the alpha and beta phases are known as the “growth problem”. When uranium metal is compacted into shape, the alpha and beta phase grains become misaligned. Under thermal cycling or irradiation these grains expand anisotropically, causing the overall shape of the fuel to change. This was a significant engineering challenge for early reactor designs.

Magnox Fuel

Magnox fuel is the most well-known metallic fuel. It was developed in Britain during the 1940s when the UK had no uranium enrichment capability, so a fuel using natural isotopic composition uranium was a necessity.

Key features of Magnox fuel:

• Rods of uranium metal alloyed with traces of iron and aluminium
• Clad in a magnesium oxide alloy (hence “Magnox” — MAGnesium Non-OXidising)
• Fuel element diameters of approximately 28 mm (thicker than oxide fuels due to the high thermal conductivity of the metal)
• Rod length of approximately 1 m
• Heat exchange fins are manufactured into the cladding to increase the heat transfer area
• Cladding temperature must be kept below 450 $^\circ$C (to avoid excessive oxidation) and below 660 $^\circ$C (to avoid phase change)

Magnox Fuel Manufacturing Process:

The manufacture of Magnox fuel elements involved several specialised metallurgical steps:

1. Casting: Uranium metal ingots were cast and machined to produce fuel bars of the required dimensions
2. Beta quenching: The fuel bars were heated to approximately 720 °C (into the beta phase) and then rapidly quenched in oil or water. This produced a fine, randomly oriented grain structure that minimised the anisotropic growth problem inherent in the alpha phase. Without beta quenching, the preferential orientation of alpha-phase grains during manufacturing caused severe dimensional distortion under irradiation.
3. Canning: The quenched uranium bar was inserted into the Magnox cladding can. The can incorporated machined cooling fins (herringbone pattern) to maximise heat transfer in the CO₂ gas coolant stream.
4. Anti-ratcheting features: A graphite disc was placed between the uranium bar and the end cap to prevent the fuel from “ratcheting” (progressively moving axially within the can due to repeated thermal cycling during reactor operation). Thermal cycling caused the uranium to expand against the can; without the graphite disc, friction prevented it from returning to its original position.
5. Sealing: The can was sealed by argon arc welding under a controlled atmosphere to prevent oxidation.

The fuel was manufactured at the Springfields site near Preston, Lancashire.

The first commercial Magnox reactor was Calder Hall 1, a 50 MWe reactor that came online in 1956 and operated until 2003. Following the completion of the Magnox programme, metallic fuels largely went out of favour in the UK.

Note on advanced metallic fuels: More recently, companies such as Lightbridge have been developing advanced metallic fuels — for example a 50:50 (by mass) Zr-U alloy with uranium enriched to almost 20%. This alloy has a melting point of about 1600 $^\circ$C, an average operating temperature of up to 370 $^\circ$C (compared to about 1250 $^\circ$C in normal oxide fuel), and a thermal conductivity five times better than oxide fuel.