Neutron capture and the antineutrino yield from nuclear reactors

TL;DR

This paper identifies a flux-dependent correction to the antineutrino spectrum from nuclear reactors caused by nonlinear nuclides, which affects low-energy antineutrino flux predictions and varies with reactor type and neutron flux.

Contribution

It introduces four nonlinear nuclides affecting antineutrino spectra and develops an analytic model to quantify their impact across different reactor types.

Findings

Correction can reach 0.9% in pressurized water reactors

Naval reactors may experience up to 10% correction

The correction depends on neutron flux and reactor operation

Abstract

We identify a new, flux-dependent correction to the antineutrino spectrum as produced in nuclear reactors. The abundance of certain nuclides, whose decay chains produce antineutrinos above the threshold for inverse beta decay, has a nonlinear dependence on the neutron flux, unlike the vast majority of antineutrino producing nuclides, whose decay rate is directly related to the fission rate. We have identified four of these so-called nonlinear nuclides and determined that they result in an antineutrino excess at low-energies below 3.2MeV, dependent on the reactor thermal neutron flux. We develop an analytic model for the size of the correction and compare it to the results of detailed reactor simulations for various real existing reactors, spanning 3 orders of magnitude in neutron flux. In a typical pressurized water reactor the resulting correction can reach 0.9% of the low energy flux which is comparable in size to other, known low-energy corrections from spent nuclear fuel and the non-equilibrium correction. For naval reactors the nonlinear correction may reach the 10% level.