Adjusting JEFF-3.3 actinide data using a new, dedicated methodology for selecting suited assimilation database parameters

• Nuclear data adjustment. • New universal assimilation database selection algorithm. • Application to fast, hard spectrum systems using JEFF-3.3 data. • Adjusting data for U-235, U-238, Pu-239 and Pu-240 above the resonance region. • Overall performance of the adjusted data much improved as compared to prior data. A general method is proposed to choose, among several benchmarks, suitable assimilation database parameters to be used in adjustments. All these benchmarks are predetermined keeping in mind specific goals of the data assimilation. The main principle of the novel methodology is that of excluding groups of strongly cross-correlated computed parameters by estimating their posterior values of χ 2 . The assimilation tries minimizing the mean χ 2 for the entirety of the considered benchmarks, including those target parameters which are not part of the selected assimilation database. The methodology currently applied to the JEFF-3.3 library, is demonstrated by adjusting data for 235 U, 238 U, 239 Pu and 240 Pu in conjunction with a class of integral parameters (effective multiplication factors) of fast criticalities including the Godiva and Jezebel spheres, for which the effect of varying resonance parameters in the calculations may be largely neglected. The computed mean χ 2 of 36 values in total, among which seven refer to the assimilation database, is halved, and the variance of the individual χ 2 -values decreases by a factor of 6 with respect to the prior values. More explicitly, the posterior values of respectively 1.10 and 3.77 for χ 2 and the variance of the individual χ 2 -values are much smaller than the corresponding prior values of 2.32 and 22.38, which is significant. As regards the adjustment, for 235 U, it is found that μ ¯ and the inelastic scattering cross-section respectively decrease by maximum 3% and 2.6%; ν ¯ is uniformly decreased by maximum 0.053% and the fission spectrum is somewhat harder. For 238 U, μ ¯ increases by maximum 5.4% and the inelastic scattering cross-section is lowered by 1%. For 239 Pu, ν ¯ is uniformly increased by maximum 0.027% and the fission spectrum is slightly softer. For 240 Pu, the fission cross-section is decreased by maximum 3.9%. The proposed adjustments are well within the uncertainty limit of one standard deviation for the nuclear data.