Sauté celery, green pepper, and onion in butter or margarine in a large saucepan over medium heat until tender but not brown. Add Shrimp, cornstarch, seasonings, and radishes. Stir quickly to coat Shrimp with seasonings. Add water and bring to a boil. Reduce heat and simmer 15 to 20 minutes. Prepare grits according to package directions. Simmer stew to desired thickness, check seasonings, and serve over grits. Cornbread sticks are excellent with this.