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New U-235 and U-238 evaluations [1-3] were undertaken within the OECD/NEA Data Bank CIELO Project [4] and were adopted for the ENDF/B-VIII.0 library [5], which was released in 2018. Since then, several reports and publications were released that showed serious discrepancies with the light water reactor (LWR) performance of the previous ENDF/B-VII.1 library [6] in criticality studies as the function of the burnup, e.g., see Ref. [7]. A slight increase of the LWR reactivity was observed at the Beginning of Cycle (BOC) with a severe loss of reactivity at large burnup observed for the ENDF/B-VIII.0 library. Sensitivity studies showed some compensation effects at the BOC, but uniquely identified the U-238 evaluation as the responsible for the reactivity loss [7].
Recent changes in the U-235 resonance evaluation [8] slightly reduced the U-235 capture cross section below 20eV, but these changes are not expected to have an impact on reactivity trends with burnup. However, slight positive criticality bias may be expected due to U-235 changes.
In this work we focused on studying changes in resonance cross sections of U-238 that may explain the observed trend as a function of burnup. It was found that capture cross section from 0.1eV up to 10eV was reduced in ENDF/B-VIII.0 evaluation by about 2% [,4,5,9] compared to the ENDF/B-VII.1 evaluation [6,10] as shown in Figure 1.
(see attached PDF)
Figure 1. ENDF/B-VII.1 to ENDF/B-VIII.0 capture cross-section ratio in the resonance region.
The observed changes may explain the observed burnup trend as lower capture cross sections in U-238 around 1eV lead to increased criticality at the BOC, but results in lower criticality at higher burnup due to the reduced production of Pu-239 due to the lower capture cross section. This hypothesis was tested in deterministic calculations and confirmed in our preliminary studies.
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Additional changes in ENDF/B-VIII.0 evaluation in the resonance region [9] are on-going to try to preserve the capture cross section below 10eV reproducing at the same time the existing transmission experiments.
[1] R. Capote, A. Trkov, M. Sin, M. Herman, A. Daskalakis, Y. Danon, Physics of neutron interactions with 238U: new developments and challenges, Nucl. Data Sheets 118, 26(2014)
[2] R. Capote, A. Trkov, M. Sin et al., IAEA CIELO evaluation of neutron-induced reactions on 235U and 238U targets, Nucl. Data Sheets 148, 254 (2018)
[3] R. Capote, A. Trkov (coordinators), IAEA CIELO Data Development Project within the International Pilot Project of the OECD/NEA [1], 235U and 238U files released December 1st, 2017, https://www-nds.iaea.org/CIELO/
[4] M.B. Chadwick et al.: “CIELO Collaboration Summary Results: International Evaluations of Neutron Reactions on Uranium, Plutonium, Iron, Oxygen and Hydrogen”, Nuclear Data Sheets 148 (2018) 189–213.
[5] D.A. Brown et al.: “ENDF/B-VIII.0: The 8th Major Release of the Nuclear Reaction Data Library with CIELO-project Cross Sections, New Standards and Thermal Scattering Data”, Nuclear Data Sheets 148 (2018) 1–142.
[6] M.B. Chadwick, M.W. Herman, P. Oblozinsky et al., ENDF/B-VII.1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data, Nucl. Data Sheets 112, 2887 (2012)
[7] Kang-Seog Kim and William A. Wieselquist, “Neutronic Characteristics of ENDF/B-VIII.0 Compared to ENDF/B-VII.1 for Light-Water Reactor Analysis”, J. Nucl. Eng. 2 (2021) 318–335. https://doi.org/10.3390/jne2040026
[8] M.T. Pigni, R. Capote, A. Trkov, Y. Danon. “Updates of the U-235 resonance parameters below 20 eV”, INDEN collaboration, IAEA 2020. See U-235 tab at https://www-nds.iaea.org/INDEN/.
[9] H.I. Kim, C. Paradela, I. Sirakov et al., Neutron capture cross section measurements for 238U in the resonance region at GELINA, Eur. Phys. J. A 52, 170 (2016)
[10] H. Derrien, L.C. Leal, N.M. Larson, and A. Courcelle, Report ORNL/TM-2005/241 (2005). “Neutron Resonance Parameters of 238U and the Calculated Cross Sections from the Reich-Moore Analysis of Experimental Data in the Neutron Energy Range from 0 keV to 20 keV”.