Neutron evaluation and validation of lead isotopes for fast spectrum systems

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Brain, Peter, Jacob
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Electronic thesis
Nuclear engineering and science
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Lead-cooled fast reactors are being developed to address the current shortcomings of the conventionallight water reactor fleet, mainly passive safety elements and improved plant economics. Lead is an ideal coolant for fast reactors as it has a high thermal mass and boiling point which enable higher operating temperatures at atmospheric pressure than water. In addition, natural lead isotopes have very low neutron capture cross sections and are heavy nuclei causing little moderation of the neutron spectrum. However, when looking at neutron transport benchmark experiments that contain lead, there is no agreement between experimental and simulated values for various evaluated nuclear data libraries. The work detailed here are DOE-NEUP sponsored efforts to study benchmarks, find the discrepant nuclear data, and address them in evaluation efforts to improve simulations of lead-cooled fast spectrum systems. To this end, the three major isotopes of natural lead: 206Pb, 207Pb, and 208Pb were evaluated from thermal energies up to 20 MeV. The resolved resonance region (RRR) evaluations of the lead isotopes acted as the starting point for the evaluation as this energy region contains the lower energy limit for fast spectrum systems. Evaluating the RRR meant employing the Bayesian R-matrix code SAMMY for fitting experimental data. The outcomes of the RRR evaluations are updated thermal cross sections and capture cross sections. For the first time, quasi-differential scattering data was used in the RRR evaluation procedure to reinforce spin assignments for 208Pb and derive the uncertainties of elastic scattering angular distributions. After the RRR, fast region evaluations were performed on the same isotopes using both CoH3 and EMPIRE. The goal of these was to fit new inelastic gamma production data to constrain the inelastic channel. The Hauser-Feshbach theoretical cross sections were fit against total, double-differential scattering, and gamma-ray(from inelastic and 2n reactions) production cross sections. Uncertainty on the nuclear data is provided by both SAMMY and Kalman filtering. The outcome of these evaluation efforts is an improved scattering kernel which is seen in quasidifferential scattering data. Better scattering physics in turn addresses the poor predictions of fast critical assemblies and meets the goals of the DOE-NEUP sponsor.
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Rensselaer Polytechnic Institute, Troy, NY
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