Fracture control in the nuclear power industry

Fracture control of power reactor pressure vessels combines elastic fracture mechanics (LEFM) with materials testing. CANDU-PHW reactors employ small-diameter containment for the pressurized coolant. Fracture control philosophy is based on the demonstrated tolerance of pressure tubes to artifical flaws. A leaking, non-serious flaw signals the presence long before the pressure tube can rupture. The basis of this leak-before-break approach is found in LEFM theory. In most commercial nuclear power generators the parts of the steam generating systems outside the reactor core consist of conventional carbon steel and austentic stainless steel materials in piping, valves and pumps. Because of the need to ensure the integrity of the whole pressurized heat transport system, these materials are included in the fracture control plan and are subject to rigorous testing and non-destructive examination. Once the reactor is operating. its continued safe operation must be assured. In the pressure vessel types of reactors, small fracture specimens may be irradiated and withdrawn at intervals for testing. Changes in material properties due to neutron irradiation must be taken into account to ensure that the fracture control plant remains viable over the reactor's lifetime. The modular construction of the pressure tube reactor is more forgiving, the pressure tubes can be changed if necessary. For both systems, flaws found during in-service inspection may be assessed using LEFM principles to determine suitability for continued service