Subject: Reprocessing Air In Submarines

A friend of mine and myself were discussing something relating to 
submarines and we stumbled on this problem.

These days nuclear submarines can basically stay submerged indefinately if 
it wasnt for crew needed some daylight, or for food.

How do these submarines reprocess the air to create that required by the 
crew? And how can they get it to last so long?

We know that it is some type of chemical reaction used, but we are wanting 
to know in a little bit of detail.

Re: Reprocessing Air In Submarines