From Halopedia, the Halo wiki
Fuel rods are caseless, transparent projectiles, containing a green, radioactive substance composed of energetic meta-materials and strained-bond lattices—an imperfect form of Forerunner infusion gel. Once they strike the target, the radioactive materials contained in the projectile detonate. The contents and effects of the fuel rods are similar to the incendiary gel used in Mgalekgolo's assault cannons. Fuel rods are most often used in an anti-armor or anti-air role, but are also devastatingly effective against infantry. They are essentially the Covenant equivalent to the UNSC's M19 surface-to-surface missiles.
At least two UNSC classifications of the Covenant's fuel rods are known.
The fuel rod cannons mounted on the Anti-Air Wraith and the Banshee use the heavier Class-3 fuel rods, which have a greater range and a capability to track heat signatures. The Seraph is additionally equipped with four fuel rod cannons, which are capable of locking onto air targets.
Though widely used among UNSC personnel, "fuel rod" is actually an informal term supposedly originating from common popular culture depictions of nuclear fuel rods. 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.
List of appearances