The Canadian Program on Thorium Fuel Cycles in CANDU Reactors

Abstract

Over the past three decades, a number of fission power reactor systems have been developed to the stage where they are economically competitive, safe and reliable sources of electrical energy. Almost all installed and projected nuclear generating capacity, regardless of reactor type, use mined uranium in a once-through cycle. These cycles release less than one per cent of the energy available in uranium and do not tap any of the energy available in the more abundant thorium resources. Projections of accessible and economic uranium resources are such that there must be a change to fuel cycles with significantly improved uranium utilization characteristics of fission power is to play a growing, major role in meeting world energy needs through the next century. While other nations may be obliged to develop Fast Breeder Reactors to unlock the full potential of nuclear energy, the unique fuel conserving characteristics of the CANDU reactor enable it to approach full resource utilization with only minor modifications to the reactor itself. A major deriving force underlying the CANDU reactor development program was the concept of neutron economy with allows the use of natural uranium as fuel in a once-through cycle. This was achieved by using heavy water as both coolant and moderator; low-absorption zirconium alloys as both core structural and fuel bundle materials, and by adopting on-load bidirectional refueling to minimize fuel management reactivity holdup. This basic development allows the identification of a broad range of potential advanced fuel cycle options which combine improved uranium utilization with protection against higher uranium prices. These include improved once-through cycles utilizing either low-enriched uranium or uranium recovered from spent LWR fuel and plutonium-uranium recycle. However, the use of thorium cycles in CANDU offers the most potential for efficient resource utilization including the possibility of near-breeder cycles in which there is no net consumption of fissile material under equilibrium fueling conditions. To investigate this potential, Atomic Energy of Canada Limited is conducting an applied research and development program on all aspects of thorium cycles including thorium fuel fabrication, inreactor fuel behaviour and performance, reprocessing and safeguards. In parallel, the Nuclear Fuel Waste Management program has been developing safe and economical methods of immobilization and disposal of reprocessing wastes. The objective of this paper is to review the rationale for the program and its current status, and indicate the future focus of the research and development program.