Immerse and soak plank in water for 1 hour; drain.
Prepare grill, heating one side to medium and one side to high heat.
Combine mayonnaise, rind, juice, and chives; stir well with a whisk. Chill.
Lightly coat top of tuna with cooking spray. Sprinkle the tuna with salt; firmly press peppercorns into tuna.
Place plank on grill rack over high heat; grill 5 minutes or until lightly charred. Carefully turn plank over; move to medium heat. Place tuna on charred side of plank. Cover and grill 10 minutes or until desired degree of doneness. Serve tuna with orange mayonnaise.
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