Lightly coat the grill with cooking spray and preheat for 5 minutes.
Grill the chicken for 5-7 minutes.
Cool and cut into small cubes.
In a large salad bowl, combine the chicken, grapes, celery, onion and walnuts.
Mix together the mayonnaise, mustard, salt, pepper and juice to make a dressing.
Pour over the chicken salad and toss lightly