In a large nonreactive saucepan, combine the celery, carrot, onion, peppercorns, bay leaf, thyme and 8 cups of water. Bring to a boil over high heat. Reduce the heat to moderately low, cover and simmer for 10 minutes. Add the chicken breasts to the pan skin side down. Place a plate directly on top of the chicken to keep it submerged. Reduce the heat to low and poach very gently for 30 minutes. Remove from the heat and let rest for 15 minutes.