The European Space Agency together with the National Nuclear Laboratory are looking for a potential use of americium-241 in developing RTGs. So they aren't completely nuke-averse.
Also, wasn't NASA's Advanced Stirling Radioisotope Generator canned in 2013 due to budget cuts? And recently there were problems with procuring plutonium-238. RTG isn't a silver bullet.
http://www.nnl.co.uk/news-media-centre/news-archive/space-ba... http://www.quora.com/How-does-ESA-plan-to-power-its-future-s...
Also, wasn't NASA's Advanced Stirling Radioisotope Generator canned in 2013 due to budget cuts? And recently there were problems with procuring plutonium-238. RTG isn't a silver bullet.
http://www.economist.com/news/science-and-technology/2164758...