A quick search shows that there are plenty of alternatives. First, all nuclear power plants keep a excess supply of fuel on site, about 2-3 years. If Russia says tomorrow: "No more nuclear fuel exports to any 'unfriendly' European nations.", there are plenty of alternatives. US-based Westinghouse already produces nuclear fuel that is compatible with many Soviet-designed reactors. Plus France (Framatome and Orano), Netherlands (Urenco), Canada and Kazakstan can easily backfill any European needs, be it raw material or final fuel.