In a saucepan, cook sugar, corn syrup and water until sugar dissolves and mixture comes to a boil. Add butter; cook until mixture reaches 280°F on a candy thermometer. Stir in walnuts; cook until 300°F (hard crack stage). Remove from the heat and stir in baking soda. Spread immediately into a greased 15x10x1-inch baking pan. When cool, break into pieces. Yield; 1 1/4 pounds.