Cecile Lecomte, 27, rappelled down a motorway bridge near the western city of Münster late on Monday night to hang suspended over a rail line, forcing the 25-car train carrying the enriched nuclear fuel to stop.
Police climbing specialists then had to dangle off the bridge to remove her. It was the third time that Lecomte had succeeded in blocking trains from the Gronau processing plant in western Germany.
Police said that the train was headed for France but a protest group, AKU Gronau, said it was going to Russia.
Polls show that a majority of Germans remain opposed to nuclear power and believe the technology remains highly dangerous because of potential accidents and terrorist attacks.
Shipments of waste to the Gorleben storage site for nuclear waste regularly spark angry protests.
Former Chancellor Gerhard Schröder's government decided to mothball the last of Germany's 17 nuclear reactors - which produce a quarter of Germany's power, compared to 80 percent in France - by about 2020.
But current Chancellor Angela Merkel's conservatives say that going back to nuclear, as fellow EU member Italy is doing, would reduce Germany's dependency on oil and gas imports from Russia and the Middle East.
The conservatives, who argue that abandoning nuclear power undermines the goal of cutting greenhouse gases, want to look at extending the life of some nuclear power stations.