A Fuel Rod
Fuel Rods are the ammunition of several different types of Covenant weaponry. Different types of fuel rods exist depending the nature of their mission: for example most Fuel Rod Cannons fire Class-2 ammunition, sufficient against infantry and armor at short range, while Class-3 fuel rods have a longer range and are capable of tracking heat signatures, making it ideal for anti-aircraft missions.[1]
Essentially radioactive[2] material encased in a magnetic field, fuel rods are used in Fuel Rod Guns, Covenant Carbines, and the weapons mounted on the Anti-Air Wraith and the Banshee. These projectiles are guided to the target by their magnetic fields, with larger weapons capable of guiding their munitions to the target, while the Carbine's projectiles are faster and more accurate, but unguided.
A Covenant Anti-Air Wraith, with triple-barreled fuel rod cannons mounted.
Once they strike the target, the magnetic field dissipates, and the radioactive materials detonate.
These ammunition types are most often used in an anti-armor or anti-air role. They are also used as anti-personal weapons. They are essentially the Covenant equivalent to the UNSC's M41 SSR MAV/AW rocket launcher. Though the fuel rods are slower than the UNSC rockets, as well as dealing slightly less damage upon impact, they are better able to track their targets, and can fire rapidly, not to mention being able to store more of them.
A Halo 3 Covenant Fuel Rod Gun.
Rumors surround Fuel Rods, with many UNSC Marines under the apparent belief that even carrying a weapon that uses them for ammunition can cause Boren's Syndrome.[3]