5 C.F.R. § 1651.17

(a) Right to disclaim. The beneficiary of a TSP account may disclaim his or her right to receive all or part of a TSP death benefit. If the disclaimant is a minor, the parent or guardian of the minor must sign the disclaimer.

(b) Valid disclaimer. The disclaimer must expressly state that the beneficiary is disclaiming his or her right to receive either all or a stated percentage of the death benefit payable from the TSP account of the named participant and must be:

(1) Submitted in writing;

(2) Signed or acknowledged, in the presence of a notary, by the person (or legal representative) disclaiming the benefit; and

(3) Received before the TSP pays the death benefit.

(c) Invalid disclaimer. A disclaimer is invalid if it:

(1) Is revocable;

(2) Directs to whom the disclaimed benefit should be paid; or

(3) Specifies which balance (traditional, Roth, or tax-exempt) is to be disclaimed.

(d) Disclaimer effect. The disclaimed share will be paid as though the beneficiary predeceased the participant, according to the rules set forth in § 1651.10. Any part of the death benefit which is not disclaimed will be paid to the disclaimant pursuant to § 1651.14.