Abstract
The spent fuel of current nuclear reactors contains fissile plutonium isotopes that can be combined with uranium to make mixed oxide (MOX) fuel. In this way the Pu from spent fuel is used in a new reactor cycle, contributing to the long-term sustainability of nuclear energy. However, an extensive use of MOX fuels, in particular in fast reactors, requires more accurate capture and fission cross sections for some Pu isotopes. In the case of Pu242 there are sizable discrepancies among the existing capture cross-section measurements included in the evaluations (all from the 1970s) resulting in an uncertainty as high as 35% in the fast energy region. Moreover, postirradiation experiments evaluated with JEFF-3.1 indicate an overestimation of 14% in the capture cross section in the fast neutron energy region. In this context, the Nuclear Energy Agency (NEA) requested an accuracy of 8% in this cross section in the energy region between 500 meV and 500 keV. This paper presents a new time-of-flight capture measurement on Pu242 carried out at n-TOF-EAR1 (CERN), focusing on the analysis and statistical properties of the resonance region, below 4 keV. The Pu242(n,γ) reaction on a sample containing 95(4) mg enriched to 99.959% was measured with an array of four C6D6 detectors and applying the total energy detection technique. The high neutron energy resolution of n-TOF-EAR1 and the good statistics accumulated have allowed us to extend the resonance analysis up to 4 keV, obtaining new individual and average resonance parameters from a capture cross section featuring a systematic uncertainty of 5%, fulfilling the request of the NEA.
Highlights
The second requirement is not fulfilled by C6D6 detectors and a mathematical manipulation of the detector response is needed to achieve the proportionality between detection efficiency and γ -ray energy: the counts recorded at each deposited energy are weighted by a factor dependent on its energy, given by the so-called weighting function (WF)
An example of the mentioned distributions of the reduced χ 2 (i.e., χ 2 over the number of experimental points) as a function of the radiative and neutron width values is given in Fig. 6 for the two largest resonances, both showing a well-bound minimum. While this method worked for the majority of the fitted resonances, some SAMMY fits of resonances with n γ, featuring a very low sensitivity to n in the fit, converged after many iterations towards too large n values, inconsistent with the evaluated resonance parameters extracted from a multichannel analysis of all available measurements
The target consisted of seven thin layers of 242Pu enriched to 99.959%, each of 45 mm in diameter, with a total mass of 95(4) mg electrodeposited on thin aluminum backings
Summary
The future of nuclear energy points to the use of innovative nuclear systems such as accelerator driven systems and Gen-IV reactors aimed at the reduction of the nuclear waste. Aiming at improving the evaluation of the fast energy range in terms of average parameters, the NEA, in its Nuclear Data High Priority Request List (HPRL) [14], requests high-resolution capture measurements with improved accuracy below 2 keV (see Table I). In order to respond to the target accuracies required by the NEA in different energy ranges, listed, a new time-of-flight measurement of the capture cross section of 242Pu was proposed and approved by the CERN INTC [18] in 2013. Each of the detectors in the capture setup and the beam monitoring system is connected to one channel of the n_TOF Data acquisition system [36], which features 12-bit digitizers sampling at 900 MSamples/s during 100 ms following the arrival of the proton pulse to the spallation target (i.e., recording signals of reactions induced by neutrons for energies down to 18 meV). The full data movies are automatically transferred from the DAQ computers to the CERN Advanced Storage manager (CASTOR) for their long-term storage and off-line analysis
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