Jason Hou (alumnus), Staffan Qvist (alumnus), and Ehud Greenspan
Breed-and-burn (B&B) reactors are a special class of fast reactors that have the potential of significantly improving the sustainability of the nuclear fuel cycle. They are designed to use low grade fuel (e.g. depleted uranium) without fuel reprocessing. One of the most challenging practical design feasibility issues of B&B reactors is the high level of radiation damage their fuel cladding must withstand—more than twice the maximum radiation damage cladding materials in fast reactors have previously been exposed to. This work investigates the feasibility of reducing the peak minimum required radiation damage level by introducing a three-dimensional (3D) in-core fuel management strategy.
A new conceptual design of a B&B core made of axially segmented fuel assemblies was adopted to facilitate the 3D shuffling. The assemblies of the 3D shuffled system are each made of two to four subassemblies that are axially stacked. The subassemblies can be disconnected from one another and then stacked together in a different order and/or combination to constitute new assemblies. Each subassembly is made of short vented fuel pins, requiring a few centimeters of fuel-free space on the top of the pins to accommodate the venting device and the axial fuel swelling.
A combinatorial search methodology has been developed and implemented for 3D shuffling pattern (SP) optimization based on the Simulated Annealing (SA) algorithm. The primary objective of the SA optimization is to minimize the peak radiation damage and its secondary objective is to minimize burnup reactivity swing, the radial power peaking factor, and the variation in power level experienced by a fuel assembly over the cycle.
Compared with the optimal conventional 2D fuel shuffling, the optimal 3D SP offers (1) a 34% reduction of the peak radiation damage level, down to ~350 dpa, (2) a 45% increase in the average fuel discharge burnup, and hence the uranium utilization, and (3) does not violate any major neutronics or thermal-hydraulics constraints. For the same peak dpa level, the average discharge burnup of the 3D shuffled core is 2.23 times that of the 2D shuffled core; this corresponds to a ~120% relative increase in the fuel utilization. These significant improvements may enable nearer-term commercialization of B&B reactors. In the long term, the successful deployment of the B&B core along with optimal 3D shuffling of fuel could provide at least a 30-fold increase in uranium utilization compared to current once-through LWRs, and hence significantly improve the sustainability of the once-through nuclear fuel cycle.