40 ILCS 5/15-120

"Beneficiary": The person or persons designated by the participant or annuitant in the last written designation on file with the board; or if no person so designated survives, or if no designation is on file, the estate of the participant or annuitant. Acceptance by the participant of a refund of accumulated contributions or an accelerated pension benefit payment under Section 15-185.5 shall result in cancellation of all beneficiary designations previously filed. A spouse whose marriage was dissolved shall be disqualified as beneficiary unless the spouse was designated as beneficiary after the effective date of the dissolution of marriage. After a joint and survivor annuity commences under the portable benefit package, the survivor annuitant of a joint and survivor annuity is not disqualified, and may not be removed, as the survivor annuitant by a dissolution of the survivor's marriage with the participant or annuitant.