Abstract
There exists experimental evidence for strong fluctuations of the average neutron multiplicity from resonance to resonance in 239Pu(n,f). These fluctuations have been shown to impact nuclear reactor benchmarks by reducing the criticality. The fluctuating neutron multiplicity can be explained as a consequence of the competition between direct fission and the (n,γf) process. However, there is also evidence for fluctuations of the fission fragment mass yields from resonance to resonance. The mass yield fluctuations may also contribute to fluctuations of the neutron multiplicity averaged over all fission fragment masses. In order to model the contribution to the neutron multiplicity fluctuations by the fission fragment mass yield fluctuations new data on the correlations between fission fragment properties and neutron multiplicities are in need. We present experiments carried out to determine prompt neutron multiplicity correlations with fission fragment masses and total kinetic energies in the reaction 239Pu(n,f). The experiment has been performed at the GELINA facility at JRC-Geel. A twin position-sensitive Frisch-grid ionization chamber is used for fission fragment identification via the double kinetic energy technique. An array of scintillation detectors is employed for neutron counting. Correlations between average neutron multiplicities and fission fragment properties have been measured with improved resolution in both mass and TKE, compared to data from the literature.
Highlights
Fluctuations of the average prompt neutron multiplicity from the reaction 239Pu(n,f) in the incident neutron energy range of the resolved resonances have been observed experimentally [1,2,3]
A new evaluation of the prompt fission neutron spectrum (PFNS) in the thermal energy range has determined a lower value of the average neutron energy than that reported in the existing evaluated nuclear data libraries [5]
A number of thermal-solution benchmarks have shown that the use of a softer prompt fission neutron spectrum at thermal energy, combined with new thermal neutron constants, yields k-eff values that are larger than measurements by a margin that increases as the above-thermal-leakage fraction increases [7]
Summary
Fluctuations of the average prompt neutron multiplicity (νp) from the reaction 239Pu(n,f) in the incident neutron energy range of the resolved resonances have been observed experimentally [1,2,3]. A new evaluation of the prompt fission neutron spectrum (PFNS) in the thermal energy range has determined a lower value of the average neutron energy than that reported in the existing evaluated nuclear data libraries [5]. A number of thermal-solution benchmarks have shown that the use of a softer prompt fission neutron spectrum at thermal energy, combined with new thermal neutron constants (adapted to fit with the IAEA standards), yields k-eff values that are larger than measurements by a margin that increases as the above-thermal-leakage fraction increases [7].
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