Current work in Research Centre Rez focuses on the validation of potential neutron emission monitors of the volumetric D–T source based on the sealed tube design. The sealed tube source solution provides a relatively cheap and exact simulation of neutron energy and flux distribution, as expected in ITER, and is therefore convenient for neutron transport investigation through potential structural materials. The problem for absolute measurements is the inability to monitor the source emission precisely. It was proposed to use the 23Na(n,2n)22Na reaction which on the one hand produces 22Na with a 2.6 y half-life and on the other hand, possesses a validated cross section which has been cross-compared with standard 197Au(n, γ)198Au in a fission spectrum produced by the LR-0 reactor. In this paper, we show that the 23Na(n,2n)22Na reaction, despite its high threshold (12.96MeV) is sensitive enough to produce measurable activities under irradiation at low power levels even in a fission spectrum. Based on this experiment it is estimated that a similar solution could work as well in the case of emission monitoring from a relatively weak sealed tube source with a maximum intensity≈1×1010n/s. It can be stated that the results produced by calculation using different data libraries show large discrepancies with the experimentally determined reaction rates in the range up to 33.0%. The relatively large uncertainty in the nuclear data in a fission reactor spectrum could furnish an explanation, however the values of cross sections in a D–T spectrum have yet to be validated.