Abstract: A method for converting a vertically downward flow of bypass coolant through coolant flow holes in a core barrel and former plates in a nuclear reactor to a vertically upward flow. Coolant flow holes are provided in the normally solid top former plate by suitable means such as drilling. These new coolant flow holes are located so as to be substantially in coaxial alignment with the existing coolant flow holes in the intermediate and lower former plates. Existing coolant flow holes in the core barrel adajcent the top former plate are plugged. Existing coolant flow holes in the lower former plate are plugged in an alternating pattern with each fifth hole being plugged having only forty-four percent of its flow area plugged.