Using Fuel Performance to Determine Operating Limit Uncertainty Allowance

An evaluation of the relationship between operating limit uncertainty allowance and fuel performance has been made. A simple analytic relation between uncertainty allowance and the number of fuel rods exceeding fuel limits due to measurement uncertainties has been derived. An evaluation of this relation for selected and bounding power distributions indicates that the expected fraction of the core challenging fuel limits is strongly dependent on (a) the operating power distribution and (b) the estimates of measurement uncertainties used in determining the operating limit uncertainty allowance. Also, the various (1-sigma, 2-sigma, 95/95, etc.) criteria used in selecting uncertainty allowance are found to differ significantly in terms of the number of rods that exceed fuel limits.