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One of the main problems in accounting for and monitoring plutonium is measuring periodically its mass and isotopic composition by nondestructive means. The only nondestructive method determining the isotopic composition of plutonium is γ spectrometry using high-resolution semiconductor detectors. The isotopic composition is calculated using programs for analyzing the γ spectra, such as FRAM (USA) or MGA (USA). The plutonium mass is evaluated using a combination of two methods: counting neutron coincidences + γ spectrometry or calorimetry + γ spectrometry. When neutron coincidences are counted, data on the isotopic composition are needed to calculate the effective 240 Pu fraction ( 240 Pu eff ): 240 Pu eff = 2.52 238 Pu + 240 Pu + 1.68 242 Pu,

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