The Secretary's review shall be based upon the hearing record. The findings of fact in the decision under review shall be conclusive if supported by substantial evidence in the record as a whole. The Secretary's review of conclusions of law shall be de novo. Upon review of the decision, the Secretary may affirm, reverse, modify, or vacate the decision, and may remand the case to the Office of Administrative Law Judges for further proceedings. The Secretary's final decision shall be served upon all parties and the Chief Administrative Law Judge, in person or by mail to the last known address.