20 CFR §234.30

When an applicant or an annuitant dies before being paid any annuities that may be due, the total of those annuities become payable to certain survivors in a lump-sum. Refer to § 234.31 through § 234.34 for information about when and to whom each type of unpaid annuity is payable. An application for an unpaid annuity must be filed within two years after the death of the person originally entitled to the annuity.