Measurement of the $^{233}$U neutron capture cross section at the n_TOF facility at CERN

Abstract

The Thorium-Uranium (Th-U) fuel cycle has been envisaged as an alternative to the Uranium-Plutonium (U-Pu) fuel cycle for electricity generation using nuclear power reactors. Indeed, thorium can be used as a nuclear fuel, and several studies and R&D programs seem to provide evidence on the sustainability of the Th-U fuel cycle, due to (i) the natural abundance of Thorium, (ii) the improved proliferation resistance offered by the Th-U fuel cycle relative to the U-Pu fuel cycle, (iii) the better neutronics performance of the Th-U fuel cycle throughout the whole neutron energy range compared to the U-Pu fuel cycle, (iv) the lower radiotoxicity of the generated spent fuel in reactors with Th-U fuel cycle and, consequently (v) better economics and public acceptance of the reactors operated using the Th-U fuel cycle compared to those using the U-Pu fuel cycle (prior to the Generation IV nuclear reactors). In a nuclear reactor operated using the Th-U fuel cycle, $^{233}$U is a key nuclide governing the neutronics performance of the system and consequently its economics, nuclear safety and proliferation resistance properties and characteristics. Therefore, the accurate knowledge of the neutron capture and neutron induced fission cross-sections in $^{233}$U are of paramount importance for the assessment of the sustainability of the Th-U fuel cycle and for the design of nuclear reactors using such fuel cycle. In this work, the neutron capture and neutron induced fission cross sections of $^{233}$U have been measured simultaneously at the neutron Time-Of-Flight facility (n_TOF) at European Organization for Nuclear Research (CERN) in the energy range from 1 eV to 1 keV using the 4$pi$BaF2 Total Absorption Calorimeter (TAC) as a detection device. The n_TOF facility is a premier neutron spectrometer worldwide, driven by a 20 GeV pulsed proton beam, operated with a low duty cycle and featuring high instantaneously neutron fluxes, excellent neutron energy resolution, low backgrounds and a Data Acquisition System supported by fast electronics. The measurement of the$^{233}$U capture cross section is a very challenging task requiring the discrimination of the neutron capture component from the neutron induced fission component. The neutron induced fission is the main competing reaction, whose cross-section is several times higher than the one for neutron capture. The total absorption technique has been employed together with the Calorimetric Shape Decomposition (CSD) method for discriminate between competing nuclear reactions for the first time in this work