[en] To reuse fissile materials through fuel cycle in future nuclear energy system development and achieve safety, economics, optimization of spent fuel management, an information of isotopic fissile content(U235, Pu239, Pu241, MA) is required to be provided basically. LSDS technology development was done to assay isotopic fissile contents. LSDS system consists of spectrometer, source neutron, measurement, data process. In 1st phase (2012-2013), key measurement technology, neutron source technology, conceptual design of detection system and shielding design were done. In the spectrometer, neutron spectrum and resolution analysis, detector design and response, detection model were performed. The optimum shielding model was proposed by shielding dose simulation. In 2nd phase (2014-2016), key parameters were determined in optimized spectrometer, fission detection sensitivity, accelerator and beam dump design, target design and neutron yield analysis, cooling system, material activation, optimized shielding model were done, and finally, mathematical assay model was proposed with software development to isotopic fissile content. Additionally, correction methodology was established by fission signal analysis. Using real U235(4.8%) and Pu239(47g, 91g), fission data was obtained. In fissile assay using various fissile materials, the content of uranium and plutonium was assay with 1-3% error. In the unknown sample, the content of plutonium was analyzed in 1-2% uncertainty. The isotopic fissile content assay technology is required in the reuse by fuel cycle for success of future nuclear energy system development. Moreover, such an advanced new fissile assay technology will lead the international nuclear area and contribute to technology export.