What you are saying is true, in that it is potentially possible for Superman to absorb nearly a 100% of the radiation (all of it by the way) that comes into contact with his body. What you obviously don't understand it tha it still doesn't come close to providing nearly enough energy to perform his most impressive feats.
This site demonstrates this ....
http://www.bluetights.net/theplanet/showthread.php?t=22265
Note this is only for the benefit for the guy i have quoted in this post, and others who are actually interested. I can't be arsed to read a million more prentious posts about how 'ridiculous' realism is in comics !