HAVOK tactical nuclear weapon
From Halopedia, the Halo wiki
Summary
The HAVOK nuclear warhead is mainly used in missiles fired by UNSC Bombers or Cruisers. However, an alternate use is to simply place the warhead on a ground target and detonate it with a remote. Its yield is thirty megatons.
Around July of 2552 during the battle of Sigma Octanus IV, Blue Team detonated a HAVOK nuclear warhead at the city of Cote D'Azur.