Analysis of hypothetical overpower accidents in the SNR-300 Mark 1A core with modeling of failure incoherence

Incoherency effects due to the statistical nature of fuel rod failure have been assessed for transient overpower (TOP) accidents with different initiating ramp rates. Characteristic single subassembly feedbacks from sodium voiding, fuel injection, and fuel sweep-out after failure were calculated and superimposed incoherently. Effective feedback functions were thus generated for characteristic subassembly groups (channels) and used as tabular reactivity input to corresponding multi-channel whole core TOP simulations using the HOPE computer model. Analyses were performed for the SNR-300 Mark 1A end of life (EOL) core with initiating ramp rates of 0.15, 0.5 and 5 S/sec. The results are compared with those from a calculation with coherent subassembly behavior in each channel. It is concluded for the mild transient that irrespective of the assumed axial failure location, negative fuel sweep-out reactivity from early failed subassemblies will prevent the accident from reaching a prompt critical stage and lead to a gradual shutdown if the assessed failure incoherencies are applied. For the higher ramp rates (>=0.5 S/sec) the assessed incoherencies will not always lead to an early shutdown, but they can considerably reduce the potential to accumulate high ramp rates in the initiating phase of TOP accidents. (Auth.)