An Integrated Model for Analyzing Disruptive Accidents in Fast Reactors

A two-dimensional (R - Z) integral model for characterizing fast reactor excursions from accident inception through core disassembly is presented. For predisassembly calculations, a Eulerian geometric model is used and multichannel heat-transfer computations are performed. Reactivity feedback due to Doppler broadening, coolant density change and voiding, and fuel movement are taken into account. A Lagrangian coordinate system is used in the disassembly phase, wherein the neutronics balance consists of Doppler broadening and material motion. A unique feature of the model is the ability to accommodate a pointwise Energy-Density-Dependent Equation-of-State according to the local sodium inventory that actually exists at the time of disassembly. By providing a consistent basis for establishing the effective reactivity ramp rate, Doppler coefficient, appropriate Equation-of-State, and temperature distribution at the start of core disassembly, much of the arbitrariness normally associated with large accident analyses can be removed. For most accident analyses, this model predicts a significantly lower energy yield during a superprompt critical nuclear excursion than would be computed by using the conventional modified Bethe-Tait analysis.