function ShowWindow(url, win) {
    var oWnd = window.radopen(url, win); //Opens the window  
    oWnd.add_close(OnClientClose); //set a function to be called when RadWindow is closed  
}
function OnClientClose(oWnd) {
    oWnd.setUrl("about:blank"); // Sets url to blank 
    oWnd.remove_close(OnClientClose); //remove the close handler - it will be set again on the next opening 
}