Deeper insight into the features of a reactor as a source of antineutrinos is required for making further advances in studying the fundamental properties of the neutrino. The relationship between the thermal power of a reactor and the rate of the chain fission reaction in its core is analyzed.

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Newνe, e scattering experiments aimed for sensitive searches of the νe magnetic moment and projects to explore small mixing angle oscillations at reactors call for a better understanding of the reactor antineutrino spectrum. Here we consider six components, which contribute to the total νe spectrum generated in nuclear reactor. They are: beta decay of the fission fragments of 235 U, 239 Pu, 238 U and 241 Pu, decay of beta-emitters produced as a result of neutron capture in 238 U and also due to neutron capture in accumulated fission fragments which perturbs the spectrum. For antineutrino energies less than 3.5 MeV we tabulate evolution ofνe spectra corresponding to each of the four fissile isotopes vs fuel irradiation time and their decay after the irradiation is stopped and also estimate relevant uncertainties. Small corrections to the ILL spectra are considered.1 Here F N ≈ 5.5ν e /fission represents summed contribution from beta decays of fission fragments of four fissile isotopes 235 U, 239 Pu, 238 U and 241 Pu, undistorted by their interaction with reactor neutrons, U N ≈ 1.2ν e /fission comes from beta decay of 239 U ⇒ 239 Np ⇒ 239 Pu chain produced via neutron radiative capture in 238 U and δ F N < 0.03ν e /fission originate from neutron capture in accumulated fission fragments and give small but not negligible local distortions of the total energy spectrum of the reactorν e .Plan of this report is as follows: First, we present a short (and incomplete) overview of a half a century long history, which has led to the present understanding of the reactor antineutrinos.Second, we give new results on the computed evolution ofν e energy spectra corresponding to four fissile isotopes vs fuel irradiation time and their decay after the end of the irradiation; we compare all available data and estimate relevant uncertainties.After these data are presented on antineutrinos due to neutron radiative capture in 238 U and in accumulated fission fragments.Finally we consider small corrections to the ILL spectra.

The question of using neutrino radiation from reactors was first posed in the mid-1970s [1, 2]. The first demonstration measurements were performed when the neutrino laboratory was set up at the Rovno nuclear power plant and it was shown, for a small statistical sample, that neutrino radiation can, in principle, be used in practice [3, 4].The inadequacy of the statistics and the high background of extraneous radiation have always been the main obstacle to recording reactor antineutrinos (~e)' The apparatus presented in the present paper -the Rovno neutrino spectrometer [5] -has partially solved these problems. Installed in the neutrino laboratory, it is intended for solving both fundamental and applied problems. In the present paper it is presented as an apparatus for measuring the power output of a reactor and the fuel burnup from the total recorded flux and spectrum of antineutrinos.The neutrino flux is directly proportional to the reactor power with constant fuel composition. The fuel composition changes during reactor operation, and as a result the proportionality coefficient decreases by 5-7%. Until recently these changes were calculated theoretically. In the present work we observed experimentally the decrease, associated with uranium burnup and plutonium accumulation in the fuel core of the reactor, of the neutrino flux and we have measured for the first time the distortion of the antineutrino spectrum as a result of this effect.Thus this work is self-consistent. All necessary corrections can be extracted directly from the measurements and are not introduced by a computational method.Neutrino Detector and Recording of Neutrinos. The detector consists of a rectangular tank, made of transparent plexiglass and filled with a hydrogen-containing scintillator (1050 liters) containing gadolinium. The volume of the detector is viewed, through transparent lightguides, on both sides, with 84 FI~U-125 photomultipliers.Antineutrinos are recorded by the inverse/3-decay reaction on hydrogen nuclei ~e+p-*e++ nby detecting the positron together with the annihilation 3'-rays (first event), and then the neutron (second event) after it is moderated and captured by gadolinium. The detector is divided into two parts by light-reflecting surfaces. The measurements were conducted in the central volume (510 liters), and the annular volume of 540 liters was used to increase the detection efficiency of the annihilation 3' rays and the 3' rays from capture of neutrons by the gadolinium and as shielding from extraneous radiation.The apparatus described above is installed under the VVI~R-440 reactor, 18 m from its fuel core in a shielding steel housing.The measurements were performed in the same manner for a period of three years: 3-4 months before the reactor was shut down for planned preventive maintenance and partial reloading of fuel, one month of measurements of the correlated background during the maintenance, and once again 3-4 months of power measurements after maintenance. The background of random events, imitating the reactio...