In a fusion reactor with high energy-gain, the fusion power will be mainly thermo-Nuclear (THN). Measurements of the THN neutron rate are a good performance indicator of a fusion plasma, requiring Neutron Emission Spectroscopy (NES) measurements to distinguish thermal and non-thermal contributions. We report here on recent NES results from JET high-performance plasmas with high fractions (about 65%) of THN emission. The analysis is made with a framework for analyzing NES data, taking into account THN reactions and beam-target reactions. The results are used to extrapolate to the equivalent DT rates, Finally, we discuss the applicability of using NES in the deuterium phase of ITER, both for the extrapolations to ITERs future DT performance as well as for measurements of confined energetic ions.