From Halopedia, the Halo wiki
Čerenkov radiation (also spelled Cerenkov or Cherenkov) is electromagnetic radiation emitted when a charged particle (such as a proton) passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Čerenkov radiation. It is named after Russian scientist Pavel Alekseyevich Čherenkov, the 1958 Nobel Prize winner who was the first to characterize it rigorously. Čerenkov radiation only occurs in a medium such as air or water because the speed of light in the medium is slowed. Contrary to popular belief, this phenomena could occur in space, due to there being an extremely low density of particles, along with the container of the fissile material itself.
When a starship transitions from slipspace to normal space, or vice versa, the glow of Čerenkov radiation emitted by fissile materials, such as plutonium, can give its presence away to enemy forces even if the ship possesses stealth systems. As a result, if complete stealth is required, prowlers are forced to jettison any on-board nuclear weapons before transitioning back to normal space to avoid detection. In addition, nukes launched from slipspace can easily be detected by enemy sensors, despite the use of stealth coatings or lead linings.
Components of the MJOLNIR armor, likely its microfusion plant, produce Čerenkov radiation. The design of the Recon and Scout variants specifically reduces the amount of radiation emitted for stealth purposes.