I think you forgot some important characters in your url ; see below:
var url = "RR_Txn_Detail.aspx?RRID=" + RRID + "&BucketID=" + SLA_Bucket + "&AccNum=" & AccountNum;
But it would be even better to construct your URL this way:
var url = string.Format("RR_Txn_Detail.aspx?RRID={0}&BucketID={1}&AccNum={2}", RRID, SLA_Bucket, AccountNum);
Much more clear to read and eventually debug.
Hope this helps.