Physics Study of an Ultra-Long-Life MMLFR (Micro Modular Lead-cooled Fast Reactor) with Low Enriched Uranium Nitride Fuels

Abstract

The small modular reactors (SMR) including micro reactors have many desirable features such as low investment risk, factory-fabrication including on-site installation, high transportability, and efficient supply of electricity and heat in remote area. In particular, the micro reactors can operate as a part of the electric grid, indepnedently from the electric grid, or as part of a microgrid to generate up to 20MWt. They are characterized by the factory fabricated, transportable, and self-adjusting features. The last feature does not require a large number of specialized operators over a long operation cycle, which can be supported by passive safety and ultra-long-life cycle with small excess reactivity to minimize the operations related to reactor control. The liquid metal cooled SMRs can be designed to have an ultra long operation cycle due to the breeding resulted from fast neutron spectra. Proviously we designed lead cooled SMR reactor cores of 60MWt, loaded with the binary uranium metal fuel of U-10Zr. These cores were designed to have ultra-long cycles of 20~35 EFPYs (Effective Full Power Year) and very small burnup reactivity swing less than 1$, which can minimize the movement of the control rods for reactivity control.

The objective of this work is to design a MMLFR (Micro Modular Lead-Cooled Fast Reactor) using uranium-nitride fuel enabling to be operated over ultra-long life longer than 20 EFPYs with a very small excess reactivity (<1$) without refueling through breeding. The current thermal output of 35 MWt is slightly higher than the usual upper bound of micro reactor (i.e., 20 MWt). In particular, new design concepts of MMR cores having blankets of slightly enriched uranium are suggested to increase the operation cycle by enhancing breeding under the reduced core size. We designed the different cores having different number of the radial blanket rods placed in the central regions. The effects of the blankets on the core performances including the reactivity change over cycle, cycle length, burnup reactivity swing, burnup, power istribution, burnup distribution, and heavy metal inventories are analyzed in detail. Also, various reactivity coefficients and the shutdown margins of the primary and secondary control assemblies are analyzed.