[en] Direct measurement of plutonium X-ray fluorescent radiation by semiconductor detector at simultaneous action of gamma radiation of fission products, which is more intensive by several orders than plutonium radiation can be possible only after decay of short lived isotopes and demands a long period of time. Developed is a method in which interfering gamma radiation separated by dispersion from plutonium (and uranium) X-ray fluorescent radiation determined then by semiconductor detector. Preliminary experiments have shown, that the method is specifically applicable for determination of plutonium/uranium ratio in nuclear fuel assemblies after short decaying and may be for measurement of soft gamma lines of mixed gamma radiation of the nuclear fuel assemblies. Now the test of the method in practice with using reactor assemblies is preparing