Neutron Multiplication Factors as a Function of Temperature: A Comparison of Calculated and Measured Values for Lattices Using ²³³U0₂-Th0₂ Fuel in Graphite

Neutron multiplication factors calculated as a function of temperature for three graphite-moderated ²³³ U02-Th02-fueled lattices are correlated with the values measured for these lattices in the high-tempera-ture lattice test reactor (HTLTR). The correlation analysis is accomplished by fitting calculated values of k^T) to the measured values using two least-squares-fitted correlation coefficients: (a) a normalization factor and (b) a temperature coefficient bias factor. These correlations indicate the existence of a negative (nonconservative) bias in temperature coefficients of reactivity calculated using ENDF/B-IV cross-section data. Use of an alternate cross-section data set for thorium, which has a smaller resonance integral than ENDF/B-IV data, improved the agreement between calculated and measured temperature coefficients of reactivity for the three experimental lattices. The results of the correlations are used to estimate the bias in the temperature coefficient of reactivity calculated for a lattice typical of fresh ²³³U recycle fuel for a high-temperature gas-cooled reactor (HTGR). This extrapolation to a lattice having a heavier fissile loading than the experimental lattices is accomplished using a sensitivity analysis of the estimated bias to alternate thorium cross-section data used in calculations of k_x(T). The envelope of uncertainty expected to contain the actual values for the temperature coefficient of the reactivity for the ²³³U-fueled HTGR lattice studied remains negative at 1600 K (1327°C). Although a broader base of experimental data with improved accuracy is always desirable, the existing data base provided by the HTLTR experiments is judged to be adequate for the verification of neutronic calculations for the HTGR containing ²³³ U fuel at its current state of development.