Neutron Balance Analysis for Sustainability of Breed-and-Burn Reactors

One objective of the present work is to determine the minimum burnup (BU) required to sustain a breed-and-burn (B&B) mode of operation in a large 3000-MW(thermal) sodium-cooled fast reactor core fed with depleted uranium-based metallic fuel. Another objective is to assess the feasibility of using the fuel discharged at the minimum required BU for fabricating the starter of an additional B&B core without separation of actinides and most of the solid fission products. A melt-refining process is used to remove gaseous and volatile fission products and to replace the cladding when it reaches its 200 displacements per atom radiation damage limit. Additional objectives are to assess the validity of a simplified zero-dimensional (0-D) neutron balance analysis for determination of the minimum BU required and the maximum BU attainable in a B&B mode of operation and to apply this 0-D methodology to assess the feasibility of establishing a B&B mode of operation in fast reactor cores made of different combinations of fuels, coolants, and structural materials.

It is found that the minimum BU required to sustain the B&B mode in the referenced depleted uranium-fueled B&B reactor is 19.4% FIMA. The number of excess neutrons that can be generated by the fuel discharged at 19.4% FIMA is found sufficient to establish the B&B mode in another B&B core. The net doubling time for starting new B&B reactors with fuel discharged from operating B&B reactors is 12.3 yr.

The minimum BU required to sustain the B&B mode of operation in alternative core designs was found to be 29% FIMA when using Pb-Bi coolant with metallic uranium fuel and 40% FIMA when using nitride fuel with sodium coolant. The B&B mode of operation cannot be established using thorium fuel and liquid-metal coolant.

The results derived from the neutron balance analysis strongly depend on the value of the estimated neutron leakage probability and the fraction of neutrons lost in the reactivity control systems. A neutron balance performed using a simplified 0-D core model, although not accurate due to, primarily, inaccurate spectra predictions, provides reasonable estimates of the minimum required and the maximum attainable BUs despite the fact that its k evolution prediction is inaccurate. The 0-D approach can save much computational effort and time and is found to be useful for scoping analysis.