A 232Th closed fuel cycle utilizing both a thermal and hybrid nuclear systems

• Description of a closed thorium fuel cycle with a fusion reactor for breeding U-233 and a thermal LWR to burn it.
• Two additional units to adjust U-233 concentration to these two reactors.
• The cycle can reach equilibrium of U-233 production/consumption when thermal reactor operates in enrichment range 1.5–2.25%.
• External thorium is between 0 and 30% of the total thorium in the cycle, depending on thermal reactor input enrichment.
• The maximum thorium utilization efficiency in the cycle is 45%.

Nuclear fuel cycles based on thorium are gaining close attention due to its higher availability and more homogenous geographical distribution. Thorium cycles, likely to be less problematic with regard to waste generation and weapons proliferation, will extend the availability of nuclear fuel by hundreds/thousands of years. The principal Th cycle involves the transmutation of the fertile 232Th isotope into the fissile isotope 233U by means of neutron capture. In the present study, the coupled operation of a hybrid fission/fusion system and a standard thermal reactor is analyzed. Using the MCNP neutronic transport code, the behavior for 233U consumption/generation in both systems as a function of Th/U feed ratio to each reactor is analyzed. The useful composition range for the feed to the thermal reactor was found to be between 1.7% and 2.25%; within these range, breeder input enrichment can be found which causes the rate of consumption and generation of 233U to be identical. Under this condition, from the point of view of the fissile isotope, the cycle is closed, with no net generation or consumption. The cycle requires a 232Th input to compensate the amount spent on breeding the fissile material; for the range of interest, this varies between 0 and 35% of the total mass flow in the breeder leg of the cycle.