Autocatalysis during fast reactor disassembly

Secondary criticality resulting from massive core rearrangement is discussed where the material dynamics results from either mild or strong initial excursions. Diffusion theory is used to study the effect of moving boundaries on the flux, and to study the effect of internal fuel rearrangement on keff. Numerical transport methods are used to make accurate determinations of keff for complicated fuel configurations arising from disassembly calculations. It is concluded that secondary criticality may occur from fuel rearrangement, but that it is unlikely in most cases. Variations in core height/diameter ratio, zonal enrichment, and overall size can change the tendency for autocatalysis significantly.