Hi,
Either you have to use javascript or vbscript to open a popup window.
or you can set
autopostback
of that radio button to true and write some line to open that popup window. you can write
ClientScript.RegisterStartScriptBlock(this.gettype(),"window.open('http://www.codeproject.com');","startscript",true);
This will register this script and executed after the page is reloaded on client's browser.