How do you play this river?

[CardSharpCook]

Pokerroom $1000+0. Blinds 75/150. UTG open raises to 450. UTG has been a little loose PF, so I don't given the position raise as much respect as I might with other players. Action folded to me in MP2 with KK. I decide to take the line of call, probably fold on an ace flop, but we'll see how it plays out. All others fold. I have a 15K stack, UTG has a 13.5K stack. My table image is as a big bluffer.

(T1125)Flop: AA3
Seeing two aces makes me reevaluate the odds of him having a third aces, so when he bets(T800), I call - planning to fold to a turn bet or check/call down.

(T2725)Turn: 8
He checks. I check.

(T2725)River: A
He checks. OK, so I am sitting on the nut boat, but the case ace beats me. What do you do here?

CSC