In the IRR universe, Nuclear Power is still the King of Power Production. Smaller, cleaner, and greatly improved power production engines have made the use of Radioactive Isotope Engines possible.
Also, Nuclear Weapons are the weapon of choice when it comes to Weapons of Mass Destruction. Usually matters are rather extreme when Nations use Nuclear Devices.
The Monitor Corps only arms Warships with Nuclear Armaments, and they are to be used only in case of Interstellar War between the Union and Alien States. As it currently stands, no incident has called for the use of such devices.