Wait, PBS Nova and Nature went away!? How? I just got done playing with them recently. Here's the links I used, and they still work.

http://www.pbs.org/wgbh/nova/rss/nova-vodcast-pb.xml
http://feeds.feedburner.com/NaturePodcasts

I did take a look at your xml file, and you've got " PBS|" in front of the URLs, thus breaking them. Correct that small typo in your original file (or just add these back in) and you should be just fine.