Fission Fragments of Th-232 and U-238
Measurements of Mass, Energy and Angular Distributions of Fragments from Neutron-Induced Fission of Th-232 and U-238 and Creation of Intermediate Energy Nuclear Data Files for ADS Research
Tech Area / Field
- PHY-ANU/Atomic and Nuclear Physics/Physics
- FIR-NOT/Nuclear and Other Technical Data/Fission Reactors
- FIR-REA/Reactor Concept/Fission Reactors
8 Project completed
Senior Project Manager
Tocheny L V
Khlopin Radium Institute, Russia, St Petersburg
- Nuclear Physics Institute, Russia, Leningrad reg., Gatchina
- University of Uppsala / Svedberg Laboratory, Sweden, Uppsala\nUniversite catholique de Louvain/Institut de physique nucleaire, Belgium, Louvain-la-Neuve\nUniversität Hannover / Zentrum für Strahlenschutz und Radioekologie (ZSR), Germany, Hannover\nNRG an ECN KEMA company, The Netherlands, Petten\nUniversity of Uppsala / Department of Neutron Research, Sweden, Uppsala
Project summaryNowadays the accelerator-driven systems (ADS) have gained the worldwide attention as a line of attack on the problem of transmutation of high-level nuclear waste. The ADS is a subcritical system driven by extra neutrons coming from a spallation target irradiated by energetic charged particles. In such a system, the neutron spectrum is stretched from thermal energy up to the energy of the incident beam. For this reason, conceptual ADS studies require nuclear data on neutron-induced reactions within a wide incident energy range including the so-called intermediate energies, i. e. between 20 and 200 MeV.
Intermediate energy nuclear data needs for ADS have been defined in a report by Working Party on International Evaluation Co-operation (WPEC) . The high-priority request list presented by WPEC contains, along with structural, spallation target and other materials, two “fuel” nuclides 232Th and 238U for which data on (n, f) reactions are requested. The role of these fission reactions in the ADS performance is not yet understood sufficiently because the number and spectrum of neutrons entering the reactor core depend on the ADS design. It is clear, however, that under specific conditions actinide fission at intermediate energies may result in intensive production of long-lived fission products, e.g. 126Sn (T1/2 ≈105 y), which are close in mass to symmetric part of relevant fragment mass distributions and which are present in relatively small quantities in conventional nuclear waste.
The demands for intermediate energy neutron data stimulate a comprehensive research in this field. In recent years, a growing number of experimental and theoretical studies of neutron-induced reactions at intermediate energies are being performed around the world. Furthermore, the development of new neutron beam facilities covering the intermediate energy range is under way as well as the upgrading of existing ones .
Investigation of neutron-induced actinide fission at intermediate energies is a major challenge for both theory and experiment. As one passes from low- to intermediate incoming energies, a number of factors complicating the theoretical description of fission process come into play. The pre-equilibrium emission, the multi-chance fission and the dependence of fissioning nuclides characteristics on excitation energy are the main of them. The theory runs into problems with consistent description of integral and differential fission observables, although it is possible to reproduce both of them with input parameter sets that differ from each other. For example, the adequate prediction of (n, f) cross section and fragment mass distributions has been attained by Duijvestijn et al.  for the 238U(n, f) reaction at different values of the saddle-point to ground-state level density ratios. Recent calculations of fission fragment angular distributions for the 232Th(n, f) and 238U(n, f) reactions [4, 5] have also revealed a difficulty in consistent description of (n, f) cross sections and fragment angular anisotropy  at intermediate energies.
Further refinements in theoretical description of actinide fission at intermediate energies are impossible without appropriate experiments in this field. For some actinides, including 232Th and 238U, there are quite accurate data on (n, f) cross sections at incident energies up to 200 MeV, but there is a lack of data on differential fission observables. At present, only one experiment is known in which fragment kinetic energies and mass yield distributions have been measured for the 238U(n, f) reaction as a function of neutron energy from the threshold up to 500 MeV . The experiment was performed using the LANSCE spallation neutron source at LANL. However, in the intermediate energy range the measured distributions have poor statistics that complicates comparison of the data with theoretical predictions.
The prime objectives of the present project are the experimental study of differential characteristics of intermediate energy neutron-induced fission of actinides in the context of the nuclear data needs for ADS and creation of appropriate nuclear data files.
It is expected that fission fragment mass, energy and angular (MEA) distributions will be measured for the reactions 232Th(n, f) and 238U(n, f) in the incident neutron energy range 10-175 MeV. The measurements will be performed using the quasi-monoenergetic neutron beams at UCL and TSL. The fission fragment detectors will be developed at KRI and PNPI. The experiments at the UCL will be carried out at the reduced beam pulse repetition frequency that permits time-of-flight (TOF) measurements without unwanted frame-overlapping of neutron TOF spectra. The MEA distributions will be measured at UCL in the neutron energy interval from 10 to 60 MeV with a step of 5-10 MeV. To make up for the reduction of the neutron beam intensity it is planned to use a multi-section Frisch-gridded ionization chamber (MFGIC) developed at KRI in framework of the ISTC project #1309. The MFGIC (see Fig. 1) consists of a stack of twin ionization chambers that makes it possible to measure the MEA distributions for seven fissile targets. The estimated statistics of fissions per neutron energy bin of 1 MeV is an order of magnitude larger than in the experiment performed at LANL .
The TSL neutron beam facility is one of the few in the world that can deliver quasi-monoenergetic neutrons with energy up to 175 MeV. The experiments at the TSL will be performed in the 20-175 MeV neutron energy range with about 30-40 MeV step. Fission fragment mass and energy distributions will be measured with the MFGIC and the novel detection technique (PPAC + combined Bragg-PPAC) developed by the PNPI group. The latter implies the measurement of kinetic energy of the fragment and its TOF between start and stop fission fragment detectors (see Fig. 2). One of the fragments from a binary neutron-induced fission act will be instantly detected by the PPAC, thus producing the start timing mark with time resolution of about 300 ps. The other complementary fission fragment will be allowed to travel in vacuum and to arrive at the Bragg-PPAC detector [8, 9], which will provide the corresponding stop timing mark and the energy of the second fragment. The Bragg-PPAC hybrid detector is a combination of a wide-aperture ionization chamber with a thin PPAC in the same gas-filled volume. In addition to fragment TOF measurements, the timing mark from the start PPAC together with RF signal of cyclotron will be used for TOF measurements of incident neutrons that will allow us to separate fission events induced by high-energy peak neutrons from the ones due to the low-energy tail. Partly, this experimental technique has been successfully used for studies of proton-induced fission in framework of the ISTC Project #1405 .
The major part of the proposed work will be performed in Russia, including development of the detectors, as well as designing, manufacturing and testing the new reaction chambers with vacuum and gas-filling systems. The experiments at neutron beam facilities in Louvain-la-Neuve and Uppsala will be done with the participation of scientific groups from the respective institutions.
The experimental part of the project will be supplemented with the theoretical analysis of differential fission observables to be obtained. The calculations will be done using the models and codes which are in operation at the NRG  and KRI-PNPI [5, 11]. In addition, comparative analysis of data on mass distributions of fission fragments will be performed in collaboration with ZSR group, which performs studies of neutron-induced radioisotope production with the activation techniques .
Deliverables of the proposed project will consist of tabulated fission fragment distributions for 232Th and 238U nuclei in about 10 energy points in the 10-175 MeV energy range, as well as of corresponding precompiled EXFOR database entries. Besides, the ENDF-formatted nuclear data files will be created for the product yields from fission of actinides at intermediate incident neutron energies. The deliverables will be made available for the scientific/technology community through the NEA Data Bank.
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