What's the policy on cancelling an r/t miles ticket after the first segment is flown? Right now we have an existing ticket that was priced 15.5K out and 11K back. Today a one-way outbound is 16.5K. We won't actually know until after travel begins if we'll want/need to cancel the return.

Does Delta not refund the miles on an unflown segment if travel has begun?
Or refund net of what a walk-up one way award would have been on the day of outbound travel? Or is it refunded based on the ticketed price?

Thanks for any insight you may have.