Tenn. Code § 17-518
A member may designate any person as beneficiary to receive benefits as provided in this chapter payable upon the death of the member. A change in designation may be made at any time by the member but such designation or change in designation shall be submitted in writing to the retirement board in such form and manner as the board may prescribe. No designation of beneficiary shall be effective unless and until it has been received by the retirement board prior to the date of death of the member. Upon any change in designation of beneficiary, the rights of all previously designated beneficiaries to receive any benefits provided by this chapter shall cease.
In the absence of a valid designation of beneficiary at the death of a member, the member's estate shall be beneficiary, except in those instances where the beneficiary is entitled to a benefit in accordance with Option B or Option C. In the absence of a valid designation of beneficiary and where the beneficiary is entitled to a benefit in accordance with Option B or Option C, the benefit shall be payable to the surviving spouse, if one exists, and in the event no surviving spouse exists no benefit will be payable unless otherwise determined by the retirement board.
T.C.A. § 17-518