Utilization Potential of Thorium in Fusion–Fission (Hybrid) Reactors and Accelerator-Driven Systems

Abstract

World thorium reserves are approximately three times more abundant than natural uranium reserves. Furthermore, nuclear power plants are producing nuclear waste materials in substantial quantities in the form of minor actinides (MA). Large quantities of reactor-grade (RG) plutonium have been accumulated in the course of nuclear electricity generation over the past 50 years in civilian reactors as nuclear waste. Two emerging nuclear energy system for the utilization of thorium and reactor-waste actinides have been investigated. (D, T) fusion reactions produce highly energetic neutrons at 14.1 MeV, which can easily fission thorium. Calculations on a laser-fusion-driven fusion–fission (hybrid) reactor has led to fissile burnups exceeding 400,000 MWd/MT. In addition to fission energy production in situ, such a reactor would produce ~160 kg of 233U per year. Highly energetic protons near the GeV range can destroy heavy nuclei so that each proton can initiate creation of multiple highly energetic neutrons, called evaporation neutrons or spallation neutrons. These secondary neutrons will themselves lead to the production of new neutrons in a cascade so that the proton energy will be multiplied through fission processes and, additionally, new fissile fuel will be bred from relatively passive nuclei, such as 232Th and 238U. Calculations have shown that the spallation neutron spectrum in infinite medium by incident 1 GeV protons can range from thermal up to 1 GeV. We note that the high energy tail plays an important role in neutron multiplication. The spallation neutron spectrum peaks at approximately 1 MeV. The maximum number of fission events per proton in 232Th, 238U, and natural uranium will be 2.754, 11.446, and 17.888, respectively. The corresponding combined 233U and 239Pu production will be 48.357, 69.013, and 78.045 atoms per incident proton.