Micro-Heterogeneous UO{sub 2}-ThO{sub 2} Fuel Designs Evaluation
- Nuclear Engineering Department, Massachusetts Institute of Technology, Cambridge, MA (United States)
The thorium based fuel cycle applied to existing Light Water Reactors will reduce the plutonium generation rate and enhance the proliferation resistance of the spent fuel. Spatial separation of the uranium and thorium parts of the fuel can improve achievable burnup capabilities of the thorium based fuel designs through more effective breeding of {sup 233}U from {sup 232}Th. This study was focused on micro-heterogeneous fuel designs where the spatial separation of uranium and thorium is on the order of a few centimeters. Two conceptually different approaches were investigated: axially and radially micro-heterogeneous fuel geometry configurations. In the axial approach, each fuel rod consists of axially alternating stacks of uranium and thorium pellets, while the radial approach implies that the uranium and thorium fuel rods are arranged in checkerboard arrays within the fuel assembly. A special effort was made to understand the underlying reactor physics mechanisms responsible for the burnup enhancement. Two major phenomena were analyzed: the spectral shift effect and mutual and self-shielding of the uranium and thorium isotopes. The spectral shift effect was identified as a primary reason for the enhancement of burnup capabilities. Mutual resonance shielding of uranium and thorium is also a factor although it is small in magnitude. In particular, the absorption reaction rate in {sup 233}U in the resonance energy region is increased by less than 2% when the {sup 233}U is spatially separated from {sup 238}U. A reduction of plutonium generation by a factor of 3 in comparison with all-uranium PWR fuel using the same initial {sup 235}U content was estimated. It is shown that the micro-heterogeneous fuel can achieve higher burnup by up to 15% in comparison with the reference all-U fuel. However, denaturing of thorium pellets by small amounts of uranium significantly impairs this enhancement. The denaturing may be necessary in order to meet thermal limits by improving the power share of the thorium region at the beginning of the fuel irradiation as well as to dilute bred {sup 233}U in the discharged fuel. (authors)
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
- OSTI ID:
- 21167908
- Country of Publication:
- United States
- Language:
- English
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