Delayed neutron measurements of 232 Th neutron-induced fission

International audienceDelayed neutrons (DN) play an important role in nuclear reactor physics. Innovative critical reactor stud-ies bring to light the need of new DN yields data. For the thorium fuel cycle DN data for 232 Th is needed. In the literature, significant discrepancies were observed for energies below 4 MeV and data are dispersed around 14 MeV. Therefore DN absolute yields from 232 Th fission have been determined at the PTB Ion Accelerator Facility in Braunschweig. A consistent set of data has been measured for incident neutrons with energies of 2, 3, 4, 6, 7, 10 and 16 MeV. Ó 2014 Elsevier Ltd. All rights reserved. 1. Motivations Delayed neutrons (DN) emitted after fission are of prime inter-est for several topics. The DN detection is involved in some non-destructive interrogation techniques. These techniques are used to detect illicit material especially at the borders or to detect actin-ides in nuclear waste barrels. The active interrogation by neutrons followed by the detection of delayed neutrons or gamma is one of these techniques. In order to identify and quantify the potential presence of actinides the DN yields have to be known in the 1– 14 MeV energy range. Delayed neutrons have also an important role for the control of nuclear reactor. With the development of a new generation of reactors and of new fuels, additional informa-tion is required for DN yields. For example data for an enlarged energy range and more accurate measurements of existing data are needed. Today some pressurized water reactors (PWR) are loaded with Th-U or Th-Pu fuel. The proportion of fissions induced in thorium is quite small but the Thorium DN yield is almost 10 times larger than Uranium or Plutonium DN yield. Furthermore, for innovative thorium cycles, data on the DN yield of 232 Th are obviously important. The uncertainty and the correlations between measured DN data are essential in order to evaluate the uncer-tainty on the global reactivity of the reactor. Absolute DN yields (total number of DNs emitted per fission) available in the literature (see Fig. 1) show remarkable discrepancies at low energies and around 14 MeV. Evaluated data from ENDF/B-VI and JENDL 3.3 show the general trend of the energy dependence of DN yields. The two plateaus at low (up to 4 MeV) and high energies (above 7 MeV) are joined using a linear interpolation. However, this energy dependence is not confirmed experimentally since there are no data between 5 MeV and 14 MeV. Moreover, within a recent ISTC project, results obtained between 3 MeV and 5 MeV seem to indicate a quasi-constant yield in this energy region that deviates from the ENDF and JENDL evaluations. These observations have motivated an experimental campaign for measurements of DN yields of 232 Th for the incident neutron energy range from 2 MeV up to 16 MeV. The methodology is presented in Section 2, the neu-tron production facility and the detection set up are described in the Section 3. In Section 4, analysis and simulations are presented. Results are shown in Section 5. 2. Measurement strategy Emission of delayed neutrons (DN) follows the beta decay of some fission fragments, which are called precursors. Since there are more than 200 precursors, they are generally lumped in six groups according to their half-lives (Keepin, 1957). The time dependence of DN rate (s DN (t)) after an irradiation duration t irr , can be represented as a sum of six exponentials: s DN ðtÞ ¼ n f m d X i¼6 i¼1 a i e Àk i t ð1 À e Àk i t irr Þ: ð1Þ Here, m d is the total number of delayed neutrons per fission