Bomb explosion kills head of Russia's nuclear defense forces in Moscow / The head of Russia's nuclear defense forces has been killed in an explosion in Moscow / Fact-check: What do we know about Russia's nuclear arsenal? / Putin outlines new rules for Russian use of vast nuclear arsenal / Russia's nuclear weapons can turn US into radioactive ashes -- Kurchatov Institute chief / Vladimir Putin's criteria for nuclear war amid WW3 fears as Russia announces tactical nuke drills / Russian troops to practice employing non-strategic nuclear weapons in missile drills / Russia is fueling more tension with the U.S. over nuclear weapons in space / Putin says Russian nuclear weapons 'more advanced' than in US