Regulatory approaches in mitigating chemical degradation of zirconium alloys in the design and operation of light water reactors

Abstract

Zirconium alloys are the materials of choice for nuclear fuel cladding due to low thermal neutron absorption, with excellent thermal and mechanical properties. As part of defence in depth concept, zirconium-based cladding shall be able to maintain its integrity and prevent the release of fission products to the reactor core. In particular, chemical degradation, due to corrosion, hydriding and pellet-cladding interaction, can compromise the fundamental safety functions of fuel cladding. Due to the risks of such chemical degradation of zirconium alloys, various countries undertake divergent regulatory approaches to ensure the integrity of fuel cladding. This research paper is the first attempt to review these regulatory approaches and to provide a technical basis for future regulation development for the safety of nuclear power plants. This research comprises of literature review of various factors that cause fuel failures in the operation of light water reactors as well as the prevalent international safety standards and regulations developed by several countries. Corrosion can cause oxide layer build-up that reduce heat transfer during accident condition. Excessive hydrogen uptake can cause embrittlement during loss-of-coolant accident. Pellet-cladding interaction causes fuel failure during normal operation of nuclear reactors. The research on chemical degradation of zirconium cladding remains intensive that continues to enhance the protection of fuel cladding. Based on the available knowledge in nuclear research community, the International Atomic Energy Agency (IAEA) has established international standards to ensure the highest reliability of fuel cladding, from operation to accident conditions. Regulatory bodies around the world aspire to adopt these internationally agreed standards. However, they also implement the national codes and standards in addition to these international standards that are relevant to their existing nuclear fleet. There is no unique regulatory approach in ensuring the integrity of fuel claddings from chemical degradation, but all reflect the strong commitment of the international community to the highest level of nuclear safety. Bapeten and other regulatory bodies need to review such varying regulatory approaches and adopt the most sensible and reliable regulatory regimes that are relevant to their domestic needs, circumstances, and capabilities.