Publications
Nuclear Fuel Cycle (NFC) Technology and Policy Program
Thermal and Economic Analysis of Thorium-Based Seed-Blanket Fuel Cycles for Nuclear Power Plants
M. Busse and M.S. Kazimi
MIT-NFC-TR-025 (August 2000)
Abstract
From the inception of nuclear power, thorium-based fuels have been of interest due to the abundance of thorium ore and to potential neutronic advantages in the efficiency of creating new fissile materials in the core. Early reviews of nuclear fuel cycles tended to conclude that the uranium cycle, currently used in nuclear power plants, was more preferable than the thorium cycle, but they considered cycles that involved highly enriched uranium, relatively low fuel burnup and fuel processing and recycling. The conditions of the nuclear industry have changed, focusing on high burnup once-through fuel cycles (no reprocessing) and U-235 <20% of U. This creates incentives for further analysis of the thorium-based cycle to assess its economic performance and safety margins given the expected benefits of reduced waste production and enhanced proliferation resistance.
This thesis analyzes alternative thorium-based fuel approaches from a thermal-hydraulics point of view. The proposed cycle’s performance is optimized given constraints that will facilitate the implementation of the concept in typical commercial power plants. The new designs are based on a seed and blanket configuration, where the seed region is rich in uranium fuel (U-235), thus is the supplier of neutrons, and the blanket region is mostly thorium, thus a net neutron absorber to generate new fuel (U-233) from thorium.
Two different designs are analyzed: the Seed and Blanket assembly as one Unit (SBU) and the Whole assembly as a Seed or Blanket (WASB). These designs are optimized from a thermal-hydraulic perspective and their economic performance is compared to the current fuel cycle. The optimization involves extracting the maximum energy without violating the limits on heat flux from fuel rods to the coolant. All calculations were performed using the subchannel analysis code VIPRE.
The proposed best approach for improving fuel thermal-hydraulics performance is based on the use of grids that allow differential loss coefficients between the seed and the blanket regions. This preferred approach to optimization was found to be more effective in the case of the WASB design because of its power distribution and the larger spatial separation between the two regions.
A very important factor in the economics of these designs is the achievable cycle length, which is a function of the neutronic design. An eighteen-month cycle is required in order to be economically comparable to the current fuel cycle. With comparable cycle length, the fuel cost per unit energy is found comparable to the current cycle, with slight benefits achieved for all-urania or thoria-based fuel depending on the cost of enrichment. However, waste performance and non-proliferation advantages may play an important role in fuel attractiveness to nuclear power plants and to the government and these were not factored in the economic analysis.
