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° on a candy thermometer. Stir in walnuts; cook until 300° (hard crack stage). Remove from the heat and stir in baking soda., Spread immediately into a greased 15x10x1-in. pan. When cool, break into pieces.