Weapons-grade substance is a material with level of enrichment or with characteristic suitable for nuclear bombs production. The most commonly used weapon-grade isotopes in different experiments are uranium-235 and plutonium- 239. It is known that isotopes like uranium-233, neptunium-237 and some isotopes of americium have also been used for the purposes of experimental researches, but by now there is no information they have been used for a real nuclear weàpons production. The goal of the present analysis is to be examined if the spent nuclear fuel of the most common nuclear reactors for electricity production, viz. pressurized water reactors, can be classified as practically unusable or not for nuclear explosive made-up. The possibilities for nuclear materials producement are analyzed and the concentration levels of the main isotopes (plutonium-239, neptunium-237, americium-241, americium-242m and americium-243) are examined and estimated. The significant quantities necessary for nuclear bombs production according to International Atomic Energy Agency are also estimated by comparing them with the obtained results. The results of this analysis may be useful in specifying the technical and organization actions which can increase the proliferation resistance of the fissile materials.