Applications to Reactor Neutrinos, Decay Heat and Summation Calculations

In a PWR-type reactor, the total number of fissions that occur as a function of time comes essentially from the fission of the 4 isotopes 235U, 238U initially present in the fuel and 239Pu and 241Pu produced by neutron capture and successive beta decay of 238U. Figure 5, which shows this evolution as a function of time (the reactor burn-up), shows that 235U is the main contributor to this fission rate. The increase in the production of 239Pu over time also strongly increases the associated fission rate. The resulting fission products, which are neutron-rich nuclei, are then de-excited by beta or beta-n decay (with the emission of a beta-delayed neutron) and are therefore the source of the considerable flux of antineutrinos that comes out of nuclear reactors.

Figure 5 : Evolution of the fission rate per second in a PWR-type reactor.

The beta decay properties of neutron-rich nuclei, in particular of fission products play a major role in the estimation of observables important for nuclear reactor safety as well as neutrino physics, whether fundamental or applied. Recently, the reactor anomaly - a significant discrepancy between predictions made from recent estimates of antineutrino energy spectra and fluxes measured by neutrino experiments within 100m of a reactor core - has attracted the attention of the particle physics and nuclear physics communities [9]. This issue has led to many new neutrino physics experiments in the vicinity of research reactors around the world including the Solid experiment in which the group is involved.

A better knowledge of the spectrum of reactor antineutrinos is required, and for this, a mastery of the underlying nuclear physics is essential. Nuclear physics plays a major role in the two spectrum calculation methods currently under study and the Subatech team is an international leader in the prediction of antineutrino spectra based on the summation method from nuclear data [7,8] in a highly competitive context. For a given isotope, the summation method makes it possible to calculate the expected total energy spectrum of antineutrinos produced in a nuclear reactor. Based on the knowledge of the decay data of individual nuclei, one can first calculate the expected individual antineutrino spectra for the approximately 800 fission products of a given isotope using Fermi theory. The summation method then consists of summing all these individual energy spectra weighted by their fission yield from the corresponding nuclear databases.

Our most recent predictions, of unequalled quality thanks to Total Absorption Gamma-ray Spectroscopy (TAGS) measurements carried out over the last decade, have been published in a [high-impact journal](/en/recherche/equipes/sen/actualites/une-nouvelle-prediction-des-spectres-en-energie-des-antineutrinos-des-reacteurs-de-moins-en-moins-compatible-avec-l-existence-de-neutrinos-steriles [8], a viewpoint on a PRL of the Daya Bay collaboration on the topic [10] and in an article in the CNRS/in2p3 newsletter ("Deficit of antineutrinos produced in reactors: nuclear physics provides elements of an answer", with Muriel Fallot's team). These predictions have recently been taken up again in a new publication [11] to study their global impact on neutrino physics.

The aim of our TAGS experiments and instrumental projects (e-Shape, (NA)2STARS) is to provide a definitive answer to the questions associated with the emission of antineutrinos from reactors. Finally, a common issue with the study of antineutrinos is the study of fission products at the origin of the decay heat of the reactors after their shutdown. This decay heat represents about 7%!o(MISSING)f the nominal power of a reactor and must be evacuated or else the fuel cladding will melt (cf. the Fukushima accident in 2011). To date, significant differences remain between integral measurements made in experiments within reactors and calculations using nuclear data, even for the decay heat associated with the most common fissile isotopes such as 235U and 239Pu. Our TAGS experiments are aimed to improve significantly this situation in order to reduce the actual uncertainties and obtain reliable decay heat predictions for innovative reactors.

Bibliographie

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[4] A.-A. Zakari-Issofou et al., Total Absorption Spectroscopy Study of 92Rb Decay: A Major Contributor to Reactor Antineutrino Spectrum Shape, Phys. Rev. Lett. 115, 102503 (2015);

[5] V. Guadilla et al., Total absorption γ-ray spectroscopy of the β-delayed neutron emitters 137I and 95Rb, Phys. Rev. C 100 (4) (2019) 044305.

[6] V. Guadilla et al., Large Impact of the Decay of Niobium Isomers on the Reactor ¯νe Summation Calculations, Phys. Rev. Lett. 122 (2019) 042502.

[7] M. Fallot et al., New Antineutrino Energy Spectra Predictions from the Summation of Beta Decay Branches of the Fission Products, Phys. Rev. Lett. 109, 202504 (2012).

[8] M. Estienne et al., Updated Summation Model: An Improved Agreement with the Daya Bay Antineutrino Fluxes, Phys. Rev. Lett. 123, (2019) 022502.

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[10] M. Fallot, Viewpoint: Getting to the Bottom of an Antineutrino Anomaly, Physics 10, 66 (2017).

[11] J. Berryman and P. Huber, Reevaluating Reactor Antineutrino Anomalies with Updated Flux Predictions, Phys. Rev. D 101 (2020) 015008, arXiv:1909.09267.