Greens have their historical grassroots in the anti-nuclear movement. In fact - I wasn't aware of this until recently, because there were legislative elections in my country - Belgium which has been one of the countries producing most of its electricity by nuclear power (> 60%), decided in 2003 to "close all nuclear power plants in 2015, or at latest in 2025". This was done because there was a green party in the government coalition. They ideally want to replace them with "solar and wind power", and if necessary with coal power plants.
Then, in their policy statement, they say that they have two main concerns: global warming, and "the nuclear problem", and that they did well on the second one.
In other words, when the green movement is confronted with a choice between a potentially REAL ecological problem, and their historical battle against "all things nuclear", then they do not mind agravating the first in order to be able to push their old agenda.