22 C.F.R. § 19.11-4
If a participant or former participant has a spouse or former spouse whose whereabouts are unknown, such participant may elect to reduce or eliminate the share of a regular survivor annuity provided for that person under § 19.11-2 or § 19.11-3 by filing an affidavit with PER/ER/RET stating that his/her spouse or former spouse is missing and giving full name, last known address, date last heard from, circumstances of the disappearance and a description of the effort that has been made to locate the individual. Thereafter, the participant shall take such additional steps to locate the missing person as may be directed by PER/ER/RET. That Office shall also attempt to locate the missing person by sending a letter to the individual's last known address given in the Department's files, to the address given on the affidavit, and, if a Social Security number is known, to the Social Security Administration for forwarding. The election and affidavit may be filed at any time before commencement of annuity. It must remain on file with PER/ER/RET for at least one year before being given irrevocable effect by the Department. If the annuity to the former participant becomes effective prior to the expiration of this one year period, the annuity shall be computed and paid without reference to the election filed under this section. Following this one-year period, or at the commencement of annuity, if later, if the missing person has not been located, the affidavit may be reaffirmed by the participant, after which an election by the participant to reduce or eliminate the share of regular survivor annuity for the missing person shall be given irrevocable effect by the Department. If the annuity to the former participant has commenced, it shall be recomputed and paid retroactively to give effect to any election made under this section.
22 C.F.R. § 19.11-4