Nuclear Data for ²³⁵U, ²³⁸U, and ²³⁹Pu in the Unresolved Resonance Region

Transmission and fission self-indication were measured for the isotopes ²³⁵U, 238U, and ²³⁹Pu in the unresolved resonance region for a wide range of material thicknesses. The fission cross section of 235U serves as standard. The other isotopes are the basic fuel components in plutonium fast breeder reactors., The experimental results obtained were analyzed together with average cross-section data using the multilevel R-matrix formalism. The objective of the consistent analysis was to estimate sets of mean resonance parameters. For this purpose, a new multilevel ladder technique was developed (Monte Carlo method) that enabled calculation of all the functionals of interest including their sensitivity coefficients related to the mean resonance parameters. Hence, the perturbation approach and the correlated random sampling technique were employed., The method of evaluation was strictly statistical, allowing a priori information on the parameters to be used. Thus, a new consistent evaluation of the mean resonance parameters under consideration and of the whole table of group constants requested was performed., The results are discussed with respect to their reliability in practical applications.