UNSC Nukes

The use of proximity-fused nuclear warheads in space engagements was widespread early in the Covenant War, but thermal shock and direct radiation proved ineffective against energy-shielded ships. Later developments used the warhead to power x-ray lasers and focused plasma spears. Conventional Nuclear weapons are still deployed in terrestrial combat as a tool of last resort.

Warfleet Pg 16

Spear: General term for nuclear shaped-charge warheads fitted to UNSC anti-ship missiles late into the Covenant war. Spears use a fusion bomb to create and accelerate a plasma jet to relativistic velocities. Also known as a Casaba Howitzer.

Warfleet Pg 92