Greenpeace said yesterday that it had collected enough signatures to force Russian President Vladimir Putin to call a referendum on what it says are his government’s plans to change Russia’s environmental laws and allow the country to import, store, and dispose of nuclear waste from foreign nations. Greenpeace said the change would make the country the “world’s nuclear waste dump.” Russia’s Atomic Energy Ministry, however, said it did not intend to import nuclear waste but only nuclear fuel that could be used again. And it says the fuel could be stored safely while earning Russia billions of dollars for importing it.