NO Soviet republic has been more highly regarded - or treasured - by the Kremlin than the rich and powerful Ukraine, the breadbasket of the former Soviet empire. That's why what is left of the Soviet Union is so shaken by the swift departure of this large chunk of Eastern Europe from Soviet political control.On Dec. 1 Ukrainians vote on an independence referendum that seems certain to pass. The Ukraine, along with Georgia, Armenia, and Moldavia, has balked at the Kremlin's proposed "economic union." Last week the Ukrainian Parliament pledged to form a 400,000-strong national army. The Ukrainian defense ministry sent a chill up the world's spine by saying that the Soviet nuclear arsenal now on Ukrainian soil would stay there. The Ukraine houses multiple-warhead rockets. Intercontinental ballistic missiles are built in Ukrainian factories. Reactions in Moscow have been conflicting, reflecting the struggle between the Soviet center and the Russian Republic. Soviet defense chief Yevgeny Shaposhnikov says the Soviet Army will recognize national guards in the republics but that nuclear weapons must be controlled by the Soviet center. Russian leader Boris Yeltsin supports the right of republics to form armies, but says that if others do so, Russia will too. Vitaly Churkin, a spokesman for Mikhail Gorbachev, doesn't believe the Soviet military can "hold centralized control" over nuclear weapons if Yeltsin supports armies in the republics. Nonetheless, such forces are probably inevitable, given the reigning sentiment for autonomy - though it's doubtful the Ukraine can afford a 400,000-member army. No republic, however, should be allowed to become an independent nuclear power. General wisdom has it that the Ukraine isn't interested in nuclear weapons and that its claim to them is a bargaining chip. The Ukraine's first grass-roots movement in the 1980s was antinuclear, after Chernobyl. Presumably, Kiev doesn't want to send nukes back to Russia, but wants them destroyed on Ukrainian soil. The Soviets should agree. Who pays is the question. The next month is crucial in determining the character of the Ukrainian departure from the Soviet fold. It must be done amicably. Negotiations should not be tainted by self-serving nationalism, hard as that will be to avoid. Both sides must be willing to make concessions. The Ukraine will need relations with a Soviet center for some time; it is dreaming to think otherwise. With ethnic Russians making up 20 percent of the Ukraine's population, Russia too has an interest there.