1977•Natl. Bur. Stand. (U.S.), Spec. Publ.; (United States)Requires access
Propagation of uncertainties in fission cross section standards in the interpretation and utilization of critical benchmark measurements
Charles R. Weisbin, Robert W. Peelle
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Abstract
This work explores the constraints imposed on the /sup 235/U(n,f) standard (proposed ENDF/B version V) by information deduced from clean integral measurements and demonstrates how uncertainties in fission cross section standards propagate in an uncertainty analysis and interpretation of those experiments. The question of what a significant improvement in the accuracy of the /sup 235/U(n,f) standard would accomplish is addressed in the limited context of analyses of GODIVA and JEZEBEL measurements. The CSEWG integral benchmark results and uncertainties were updated in accordance with more recent information. Sensitivity coefficients were developed and used to estimate calculated results which should be obtained using the subsequent release of /sup 238/U(n,f), /sup 235/U(n,f), and /sup 239/Pu(n,f) at version V status. Covariance files were evaluated and processed for all important cross sections except inelastic scattering. Uncertainties due to the /sup 235/U(n,f) standard were estimated to comprise more than half of the calculated uncertainty for criticality and (/sup 28/sigma/sub f///sup 25/sigma/sub f/) /sub c/spectral index in JEZEBEL as well as GODIVA, although the JEZEBEL assembly contained no /sup 235/U. It is not possible at this time to predict criticality or (/sup 28/sigma/sub c///sup 28/sigma/sub f)/sub c/ to anywhere near the accuracy obtained by direct measurements, andmore » therefore the integral results are significant to the analysis capability. Inclusion of integral information from GODIVA and JEZEBEL in an adjustment procedure was effective in reconciling all parameters other than (/sup 28/sigma/sub f///sup 25/sigma/sub f/)/sub c/ measurement in JEZEBEL. The adjustment procedure made changes of less than one standard deviation in the cross sections for /sup 235/U(n,f), /sup 235/U(n,..gamma..), /sup 238/U(n,f), /sup 238/U(n,..gamma..), and /sup 239/Pu(n,f) including an increase of approximately 1.5 percent for the /sup 235/U(n,f) cross section above 1.3 MeV.« less