You'll find the NPR directory here:

http://www.mediafly.com/Areas/NPR/Feeds

To make things simple, it's very worthwhile to [link] download Apple's free 'iTunes' . In my opinion, this is exceptionally good software. It's a first rate way to find and download audio and video podcasts. You can use it to browse the 'iTunes Store' where you can subscribe to any of hundreds of podcast feeds (including NPR):

Here are a few of the dozens of science podcast sources:

SETI Institute "Are We Alone?" - http://radio.seti.org/listening-options.php
Astronomy Cast - http://recap.ltd.uk/podcasting/sciences/astronomycast.php
Astronomy Magazine - http://www.astronomy.com/asy/default.aspx
CERN - http://www.cernpodcast.com/
IRrelevant Astronomy - http://www.jodcast.net/amp/irrelevant.html?num=irrelevant_5
NOVA E-mc2 - http://nova_e-mc2.vitalpodcasts.com/
NOVA Science Now - http://www.pbs.org/wgbh/nova/sciencenow/rss/