© 2018 The Japan Society of Plasma Science and Nuclear Fusion Research. Currently, there is much demand for a non-destructive assay technique to quantify special nuclear materials of 235U and Pu isotopes in a field of nuclear nonproliferation. For this purpose, a compact NRTA system is under development. The performance of a proposed compact NRTA system was investigated by calculating a neutron transmission spectrum for a spent nuclear fuel. In this paper, we mainly evaluated how a transmission spectrum was affected by neutron pulse width and flight path length of the system and discuss the appropriate values from a viewpoint of measurement of special nuclear materials.