I think you are displaying report in a WebPage and you need to open it in a new tab.
if so pls call this from which page you you need to open the report page.
String s= "<SCRIPT language='javascript'>" + " window.open('" + sPgName + "')</SCRIPT>";
if(!this.Page.ClientScript.IsStartupScriptRegistered("OpenWindo"))
this.RegisterStartupScript("OpenWindo", s);
sPgName is your repot page name