Material Degradation in ACR-700 Fuel Channel Components

The ACR (Advanced CANDU Reactor)-700 shares many design features with operating CANDU (CANada Deuterium Uranium) reactors, such as horizontal pressure tubes, heavy water moderator, and on-line fueling. However, there are some departures from the operating reactors that could affect the performance of structural materials. These include use of light water coolant, higher coolant pressure and temperature, enriched fuel rather than natural uranium, and higher fuel burnup. More important, the ACR-700 has many design features that set it apart from conventional and evolutionary light water reactors. This paper reviews the available literature related to design and structural materials for ACR-700 fuel channel components (Zr-2.5Nb pressure tubes, calandria tubes, annular spacers, and end fittings). The objectives were to identify the potential degradation mechanisms, evaluate the main degradation mechanism causing field failures, and analyze the available data to assess the performance of various components over the design/service life of the ACR-700. The review concludes that delayed hydride cracking (DHC) has been a primary degradation mechanism causing failure of CANDU pressure tubes. The influence of light water coolant environment on DHC initiation in the ACR 700 needs evaluation. The main concern is the potential for generation of hydrogen due to corrosion of the pressure tubes and its ingress into them, thereby making the tubes susceptible to DHC.