The nuclear nonproliferation is a common objective for the international society, of which one of the most important issues is the nonproliferation of weapon-grade nuclear material. Plutonium is a by-product when nuclear reactors are operated. If a commercial power nuclear reactor operates without counting its economic benefits, it is possible that weapon-grade plutonium (WGPu) would be produced in the nuclear reactor with using uranium as nuclear fuel. In the paper, we quantitatively study the plutonium isotopic composition and yield of the WGPu produced in a pressurized water reactor (PWR), and thereby investigate the proliferation risk of commercial nuclear reactors. The properties of plutonium produced in the PWR are calculated by MCORGS, which is developed by us to link MCNP and ORIGENS for calculating the transport-burnup. For evaluating the changing behavior of plutonium isotopic ratio dependent on the cooling time after being discharged from a PWR, we add the model of calculating the depletion and decay properties of nuclear fuel into the MCORGS code system. In order to calculate the yield of WGPu produced in the PWR, we carry out the neutron and burnup calculations by using five reactor models. The simulation models and operation history are based on the configuration and parameters of Japanese Takahama-3 unit. According to the positions and proportions of UO2 fuel rods, burnable poison rods and guide tubes in Takahama-3 PWR, we build a PWR model of an infinite heterogeneous 66 pin cell lattice, carry out simulation calculation and explore the condition for WGPu existing in the two kinds of fuel rods. When the burnup of a UO2 fuel rod is no more than 4.7 MWd/kgU, it contains WGPu. When the burnup of a burnable poison rod is no more than 2.7 MWd/kgU, it contains WGPu. Therefore, the issue of WGPu production in PWR is transformed into the research of the spatial distribution of PWR burnup. In order to obtain the axial PWR burnup, we build an infinite fuel pin cell model in which the PWR is divided into 20 equal zones in the axial direction, and calculate PWR axial burnup distribution when it is operated at 9 typical powers of Takahama-3 PWR. It is found that the burnup value of the two ends of 1/20 section is worth 1/3 of the two middle ones. Based on the principle of neutron leakage in a PWR and the simulation results of a fuel assembly, we build a special PWR mode, in which the PWR is divided into 10 zones in radial direction, and obtain the radial distribution of PWR burnup after the first, the second and the third fuel cycle. Based on the WGPu existing condition and the spatial distribution of a PWR burnup, in this paper we present the exact position of WGPu contained in PWR core and the yield of WGPu in UO2 fuel rods. The calculation results indicate that the spent nuclear fuel with low burnup brings huge proliferation risk, of which the supervision should be strengthened.