As world attention focuses on how the Soviet leadership will react to President Bush's dramatic proposals to reduce nuclear arms, Moscow's eyes are focused on several of the Soviet Union's breakaway republics, where nuclear arsenals are located.Soviet President Mikhail Gorbachev reacted positively to the Bush plan, but he said it would be too hasty to offer an immediate and detailed response. Mr. Gorbachev will first examine strategic implications of the US proposals, both military and political. The latter depends on the views of leaders of the Ukraine, Kazakhstan, Byelorussia, and several Central Asian republics where nuclear weapons are deployed. Forging a consensus may become problematic as the possession of nuclear capabilities develops into an issue of republic sovereignty. For example, Vyacheslav Chernovil, mayor of Lvov in the Ukraine, said recently that he opposes relinquishing control over the nuclear arsenal in his republic. "What I'm really afraid of is the republics' reaction," says Sergei Blagovolin, director of military studies at Moscow's Institute of World Economy and International Relations." Both the Ukraine and Kazakhstan declared clearly that they have legitimate rights over nuclear materials on their territory. Whether their leaders agree [to work with the Soviet center to reduce nuclear capabilities] is now a test of their common sense. "By and large the Bush proposals will be accepted in Moscow," says Mr. Blagovolin. "The breakthrough in the sphere of arms control and in the whole spirit of US-Soviet relations is deeply satisfying for every Soviet person." Over a year ago, Blagovolin says, he published nuclear reduction proposals very similar to what Bush is now calling for.