Measurement of fission cross-section of actinides at n_TOF for advanced nuclear reactors

The subject of this thesis is the determination of high accuracy neutron-induced fission cross-sections of various isotopes - all of which radioactive - of interest for emerging nuclear technologies. The measurements had been performed at the CERN neutron time-of-flight facility n TOF. In particular, in this work, fission cross-sections on 233U, the main fissile isotope of the Th/U fuel cycle, and on the minor actinides 241Am, 243Am and 245Cm have been analyzed. Data on these isotopes are requested for the feasibility study of innovative nuclear systems (ADS and Generation IV reactors) currently being considered for energy production and radioactive waste transmutation. The measurements have been performed with a high performance Fast Ionization Chamber (FIC), in conjunction with an innovative data acquisition system based on Flash-ADCs. The first step in the analysis has been the reconstruction of the digitized signals, in order to extract the information required for the discrimination between fission fragments and the background, as well as for the determination of the neutrons’s energy from their time-of-flight. Fission cross-sections for the various isotopes have been determined relative to the 235U(n,f) reaction, which is considered a standard of measurement in a wide energy range. In order to minimize systematic uncertainties, this reaction has been measured with the same detector and at the same time of the reactions subject of this thesis. A fundamental part of the thesis work has been the analysis of the 235U(n,f) reaction, which has allowed to study the response of the fission chamber, thanks also to the use of detailed Monte Carlo simulations performed with state-of-the-art codes for neutron transport and interaction. Moreover, the analysis of the 235U(n,f) reaction has allowed the energy calibration of the neutron beam, the determination of the incident neutron flux and an accurate estimate of the background. In the present thesis the final results for the 233U(n,f) cross-section are shown, as well as the preliminary results for the 241Am(n,f), 243Am(n,f) and 245Cm(n,f) cross-sections. The characteristics of the n TOF neutron beam have allowed to obtain results in a wide energy range, from about 30 meV to 1 MeV, in a single measurement. For the 233U(n,f) case, the final uncertainties on the cross-section are slightly larger than 3%, a value required for the development of innovative nuclear systems. In order to reach such an accuracy, corrections for sample-dependent efficiencies, as well as for reaction-related dead-time effects, have been applied. The thesis is organized as follows: Chapter 1 contains the motivations for the request of accurate fission cross-sections on actinides and on isotopes of interest for the Th/U fuel cycle. The main characteristics of the n TOF facility, in particular those relevant to fission measurements, are presented in Chapter 2. Chapter 3 contains a detailed description of the experimental apparatus used for the fission measurements. The study of the detector response, in particular in terms of detection efficiency and beam attenuation, is also presented. In Chapter 4, the analysis procedure used for data reduction, starting from the signals reconstruction procedure, is presented. The results for the 235U(n,f) and 238U(n,f) reaction, typically used as reference for fission measurements, are presented and discussed. Chapter 5 is dedicated to the determination of the 233U(n,f) cross-section. In this chapter, the detailed procedures used for background minimization, subtraction and corrections of experimental effects (detection efficiency and dead-time) are described. The extracted cross-sections, characterized by a very high accuracy (3%) in the entire energy range (from 30 meV to 1 MeV), are then compared with previous measurements and with cross-sections tabulated in evaluated data libraries. The comparison clearly shows the need to update the evaluations, in order to increase the reliability of the data required for the feasibility study and design of innovative nuclear systems based on the Th/U cycle. Chapter 6 presents the results, in some cases still preliminary, of the 241Am(n,f), 243Am(n,f) and 245Cm(n,f) reaction cross-sections. In this case, the very high background, associated with the natural radioactivity of the s amples, and the contaminations from other isotopes complicate the data analysis and result in an increased uncertainty on the extracted cross-sections. For the 243Am(n,f) reaction it has been possible to obtain accurate cross-sections only above 350 keV. In the other two cases, on the contrary, cross-sections in the entire neutron energy range from 30 meV to 1 MeV have been determined. Nevertheless, since it was not possible, in this work, to estimate with good accuracy the detection efficiency, due to the high background caused by the radioactivity, the extracted cross-sections have been normalized to the results of previous measurements or to tabulated cross-sections in conveniently chosen energy ranges. For this reason, the cross-sections obtained so far are still preliminary and affected by an uncertainty higher than that required for the development of Generation IV reactors for energy production and nuclear waste transmutation. Nonetheless, the present results are among the best currently available. It is reasonable that additional refinements in the analysis procedures and, eventually, measurements dedicated to the normalization problem will allow to improve the accuracy of the data shown in this thesis, up to the point required by the applications in the field of nuclear energy.