Measuring the delayed neutrons multiplicity and kinetic parameters for the thermal induced fission of 235U, 239Pu and 233U
CEA, DES, IRESNE, DER, SPESI, Cadarache, F-13108 Saint Paul Lez Durance, France
2 CEA, DES, IRESNE, DTN, SMTA, Cadarache, F-13108 Saint Paul Lez Durance, France
3 CEA, DES, IRESNE, DER, SPRC, Cadarache, F-13108 Saint Paul Lez Durance, France
4 CEA DRF, IRFU, DPhN/LEARN, Saclay, France
5 CNRS IN2P3 LPSC, Grenoble, France
6 Institut Laue-Langevin, Grenoble, France
Published online: 19 November 2021
In the normal operation of nuclear reactors, the kinetic behavior of the neutron population in the core is driven by the so-called delayed neutrons (DN). The DN yield per fission, their average lifetime and their groups’ abundances are the main physical parameters used to predict the kinetic behavior of the reactor and its reactivity. The overall uncertainty associated to reactivity predictions, which is linked directly to the nuclear reactor safety margins, is thus closely dependent on a few parameters associated with DN.
Depending on the nuclear data library, DN kinetic parameters present significant discrepancies, even for major fissile isotopes such as 235U or 239Pu. In this context, CEA has long been working for improving DN nuclear data. In 2018, CEA launched the ALDEN project (Average Lifetime of DElayed Neutrons) in the framework of a collaboration between CEA/DES, CEA/DRF, CNRS/IN2P3 (LPSC, CENBG, LPC), ENSICAEN and Caen University. This program aims at measuring the kinetics of the delayed neutrons to derive the DN yield, their average lifetime and abundances.
Two experimental campaigns focusing on the thermal fission of 235U were conducted in 2018 and 2019. They demonstrated the concept feasibility and gave high quality estimations of the DN multiplicity (1.631 ± 0.014 %) and average lifetime (8.82 ± 0.6 s) for 235U. More recently in March 2021, a more ambitious irradiation campaign was conducted on 239Pu and 233U. This paper details the new experimental setup, which was upgraded to meet ILL safety requirements for handling plutonium. The data analysis process is presented, with a focus on the problem of dead time correction. Finally, some preliminary delayed neutron decay curves are showed and discussed.
Key words: Delayed neutrons / fission chambers / 3He proportional counters / fissile isotopes / uranium 235 / uranium 233 / plutonium 239
© The Authors, published by EDP Sciences, 2021
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