[en] Destructive assay is an effective method for the analysis of nuclear materials in nuclear fuels, but is not suitable for some aspects of nuclear fuel cycle, for example, for accounting and control or safeguard purposes. Isotopic Source Assay System was imported from Intelcom Rad. Tech. Company, and the results of assay of enriched uranium and plutonium sealed for passive and active assay by the system are presented. A ²⁵²Cf source is provided for the assay system. Assay of unknown samples by this system is carried out relatively to the measurement of known standards. Several known standards approximating the physical and chemical properties of unknown samples are prepared to make calibration curves. When one fission event occurs in a sample, a few neutrons (2.5 neutrons on the average) and gamma ray (about 7 photons) are emitted simultaneously. By three detector coincidence out of four, one count is registered by the assay system. First, statistical informations and geometry were examined. Then, three kinds of enriched uranium were measured to examine the measurement on ²³⁸U. Passive and active measurements were performed on 4.32 grams of PuO₂ during one month to know reproducibility. In conclusion of these tests, it was proved to be able to apply this system to the analysis of nuclear materials similar in enrichment or isotopic composition, and scraps and wastes containing known matrix materials. (Wakatsuki, Y.)