| Noble gases play an important role in nuclear arms control because of their relationships with nuclear reactions and their inert chemical property. After brief learning the authentication of the compliance to NPT by radioactive noble gas, this paper focused on stable isotopic ratios of noble gas in stack for the purpose of nuclear safeguards at reprocessing plant.41Ar, 85Kr & 133Xe are not suitable for the safeguards of operating reactors and reprocessing plants because single nuclide doesn't demonstrate the reactor power, the fissionable isotopes of fuel element, and the bumup of spent fuel. It is also difficult to discover the undeclared nuclear facilities by these activation or fission products because of their uncertain release into environment , and their quick diffusion amoung air. However, for those facilities, which has been shut down or waiting for decommission, the increasing of radioactivity of these nuclides in the air may indicate the restart of the facilities. Under the circumstances, the monitoring of noble gaseous nuclides is more effective and less intrusive.83Kr, 84Kr, 86Kr, 131Xe, 132Xe and 134Xe are fission-produced stable nuclides. Their yields depend on fissioning isotopes, reactor types and bumup. Therefore, the ratios 83Kr/86Kr, 84Kr/86Kr, 131Xe/134Xe, and132Xe /134Xe can be used to derive the burnup and also the ratio 240Pu/ Pu in spent fuel which is the major concern for the purpose of nuclear safeguards. The percent isotopic abundance of Kr and Xe in nature air was different from that produced by heavy nuclide fission. 82Kr and 129Xe, major from air and only ultra-trace from fission reaction, can serve to subtract the air component in the mixture from stack at reprocessing facilities to get fission-produced ratios for 83Kr/86Kr, 84Kr/86Kr, 131Xe/134Xe, and132Xe/134Xe.The ratios of 83Kr/86Kr, 84Kr/86Kr, 13lXe/134Xe and132Xe /134Xe in PWR fuel has been calculated by 0RIGEN2.1 code. The 235U abundance was 3.0%, 4.45% and 1.6%in fuel elements of pressurized-water reactor. The noble gas isotopic ratios were also correlatedwith burnup for the spent fuel irradiated by thermal neutron and in Canadian deuterium uranium (reactor) reactor.4 powder samples in quartz tubes ( 14mg U3O8, 235U 3.0%) were put at the core region of the research heavy water reactor at CIAE for 47 and 81 days respectively. The generated gas in the samples was diluted by air to simulate the real gas in stack. The isotopic ratios of Kr and Xe were measured by noble gas mass spectrometer. The fissiongenic ratios of 83Kr/86Kr, 84Kr/86Kr, 131Xe/134Xe and132Xe /134Xe, inferred after background subtraction, were compared to a database of calculated isotopic ratios to evaluate the specific fuel parameters (e.g., burnup, fuel type, etc.). It was learnt that the ratios of 83Kr/86Kr, 84Kr/86Kr, 13lXe/134Xe andl32Xe /134Xe within 3% STD for measurment, could be used to verify whether the burnup of spent fuel was much less than 30GWd/MTU. The burnups estimated from ratios of 131Xe/134Xe and132Xe /134Xe with 1.7% measurement precision indicated the burnup with 2.3GWd/MTU deviation from the true value.The fissiongenic noble gas is easily released into environment during reprocessing and not accumulated in the region of concerned facilities. Hence, the ratios of noble gas in stack just give enough information for verifying independently what is being reprocessed but not divulging other sensitive information. Moreover, the gas sampling at stack is far away from the center of nuclear facilities, and it doesn't interfere the reprocessing plant's running plan.
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