Non-equilibrium Antineutrino spectrum from a Nuclear reactor We consider the evolution of the reactor antineutrino energy spectrum during the periods of reactor ON and OFF. The calculations show that antineutrino spectrum never comes to equilibrium state during reactor operation run. The existing method of experimental data analysis can be improved to increase an accuracy of predicted values in an experiment.

Antineutrino energy spectrum Magnetic moment ~70% ~30%

Soft part of antineutrino spectrum E < 1.5 MeV 238 U + n  239 U  239 Np  239 Pu

Recoil electrons spectra for magnetic and weak scattering of reactor antineutrinos Digits are values of magnetic moment 1 2 3

Standard approach to get a reactor spectrum and a cross section for inverse beta-decay (E > 1.8 MeV) Traditionally assumed that energy spectrum comes to saturation in one day after reactor starts and falls down to zero in one day after reactor shuts down. In this approach the saturated spectrum is found as a sum of partial spectra  i  (E) of the four fission isotopes:  (E) =  i  i  (E), i labels all isotopes ( 235 U, 238 U, 239 Pu, 241 Pu) K.Schreckenbach et al., Phys.Lett.B160, 325 (1985); A.Hahn et al., Phys.Lett.B218, 365 (1989); P.Vogel et al., Phys.Rev. C24, 1543 (1981)  V-A =  i  i, where  i =   (E)  i  (E)dE,  i  is a part of fissions. The best measured cross section (Bugey, 1994 y.)  meas =    cm 2 /fiss.  (68% C.L.) with  5 = 0.538,  9 = 0.328,  8 = 0.078,  1 =  meas /  V-A =  (exp.)  (V-A)

Inclusion of non equilibrium effects In reality the spectrum does not reach the equilibrium due to two effects: Long lived fission fragments accumulating. Formation of beta-emitters due to neutron radiative capture in fission fragments in the reactor core.  (E,t) = F  (E,t) + C  (E,t) For calculation we used the data base including 571 isotopes with yields more than per fission.

Ratios of the current spectra to that at the end of the 330-day run for reactor ON and OFF periods 235 U

Ratios of the current recoil electron spectra to the spectrum at the end of the 330-day run for reactor ON and OFF periods 235 U Solid lines for weak, dashed for magnetic scattering

The time evolution of spectra during reactor ON period. 1 y. 3 y.

Residual Antineutrino emission during reactor OFF period 30 days 180 days 1 y.

Last isotope is 90 Y(T 1/2 = 64 h, E max = MeV), that is in equilibrium with its long lived predecessor 90 Sr (T 1/2 = 28.6 years) Fission fragment Fission nucleon 235 U 239 Pu 241 Pu 238 U 97 Zr I Y Ru Ce Sr Yields of long lived fragments (%) In first 10 days 97 Zr(E max = MeV), 132 I (E max = MeV) and 93 Y(E max = MeV) become in equilibrium and later increase will be defined by halflifes of 106 Ru and 144 Ce 106 Ru  Rh  Pd (stab.) 144 Ce  Pr  Nd (T 1/2 = 3  y.) Some of the Long lived fragments

The increase of inverse beta-decay cross sections for uranium and plutonium spectra during reactor ON period 235 U

The decrease of inverse beta-decay cross sections for uranium and plutonium antineutrino spectra during reactor OFF period (2 years of irradiation)

The increase of C  (E) component of antineutrino spectrum due to the capture of neutrons by fission fragments in PWR-1000 reactor. 103 Rh, 147 Pm 99 Tc 109 Ag 1 – the beginning, 2 – the middle and 3 – the end of reactor run.

Conclusion The antineutrino spectrum in the region E > 1.8 MeV has a non equilibrium component with relaxation time exceeding the reactor operational time. It is necessary to take into account the additional neutrino emission due to accumulation of long lived fragments and neutron capture by fragments in a reactor core. One needs to know the prehistory of the fuel to account precisely residual antineutrino emission. The resulting corrections are relatively small but not negligible. In the antineutrino energy range MeV the relative contribution of the additional radiation is about 2-4% that is greater than the ILL spectra uncertainty. In contrast to the standard approach :