Study of the Tritium Breeding Blanket in an Hybrid Fusion–Fission Transmutation System

Abstract

The reactor studied in this work is the hybrid fusion–fission transmutation system (FFTS), which is a fusion–fission hybrid reactor with a central compact fusion neutron source (CFNS). It is based on the Tokamak concept, and it is surrounded by a zone made of transuranic elements obtained from reprocessing and recycling of spent fuel of light water reactors. High-energy neutrons, of fourteen MeV, are generated in the CFNS; they are produced by the deuterium-tritium reaction. In this study, the MCNPX Monte Carlo code was used to build up a model of the FFTS for studying the tritium breeding capability of the system. Tritium is produced from neutron capture in lithium, which is located in blankets specifically designed for this purpose. The tritium breeding ratio (TBR) is defined as the average number of tritium atoms bred per tritium atom burnt in the deuterium-tritium reaction. We must have TBR>1, for a self-sustained fusion economy. In the first step of this work, the location of the lithium blankets was defined. Afterwards, different blanket materials were tested: natural lithium, enriched lithium in 6Li, different lithium alloys with neutron multipliers like lead and beryllium [Li4SiO4, LiTiO3, FLiNaBe, FLiBe, Pb-15.8Li (Li-6 at 90 %)]. Finally, a study was carried out to determine the relationship between the width of the blanket and the tritium breeding. Concerning the blanket locations, we defined four: one in the central column of the FFTS, one in the upper and one in the bottom part of the fusion region of the system, and the last one in the external part of the fission region. This means that the first three blankets use high-energy neutrons from the deuterium-tritium reaction, and the fourth blanket uses neutron leaking from the fission reactions. The principal results show that the best option is the blanket with Pb-15.8Li with lithium enriched at 90 % in lithium-6 with TBR = 1.09. It was found that the blanket at the external part of the fission region has the higher tritium breeding capability. Regarding the blanket width, it was observed that most of the tritium breeding is carried out in the first 5 cm of the blanket, and beyond this width breeding is minimal; therefore, for the blanket it is more important to have a high view factor to neutrons (i.e., a big surface exposed to neutrons) than a deep region. Finally, it is important to mention that the FFTS was critical during 1000 days that were simulated with MCNPX.