Study of a system for PWR fuel assembly characterization during refuelling operations
Document typeMaster thesis (pre-Bologna period)
Rights accessOpen Access
This final thesis project aims to define the main characteristics of an innovative out-core system to apply to a real PWR nuclear power plant during outages operations. Thanks to the combination of the video system with commercial detection equipment, the device will strongly limit the possibility of fuel assembly misloading and so the reactivity incidents caused by a wrong placement of a fuel assembly inside the core. Besides, it will support the in-core burn-up measurement, concerning profile and average values, without important influences on the loadingunloading refuelling schedule. Furthermore, the possibility to recognize the typology of fuel, irradiated or fresh, combined with the theoretical automatic capability of the system could also decrease the effective dose to the workers. In addition the possibility to determine the amount of Plutonium isotopes and the total fissile material will be a very important feature in order to avoid nuclear proliferations. The functions introduced in the device require the previous burn-up calculations, the determination of the passive neutron emission and a gamma ray spectrometry. For this reason, the project of this special passive device has required the utilization of a depletion code, ORIGEN-S, in which the data of a confidential spent fuel assembly database have been introduced, and of a Monte Carlo code, MCNP. The analysis of the huge number of ORIGEN-S simulations that has been conducted, has allowed understanding and analysing the sensitiveness of the burn-up and the main neutron emitters to some parameters. Thanks to a user made MatLab programme, the best fit has been implemented in the main correlations and a database has been automatically created with all ORGEN-S useful information, as for instance the fuel composition during the irradiation time and the gamma and neutron energy discretize spectrums for each simulation. The comparison with several technical publications has given good results. Furthermore, taking into account the characteristics of a refuelling operation and the results of the depletion code, it has been decided to place the device into the transfer channel. For this reason several components, as for example the detectors and the pre-amplifier, will require the project of a special waterproof box. Once the placement and the whole equipment has been selected, the components have been modelled in the Monte Carlo software also taking into consideration to realize them in the more detailed way considering also the computational time. Difficulties have been encountered due to the slow processing speed of the old hardware used (Pentium III) and the limitation of MCNP in very complex problems: gamma and neutron spectrum sources, very low detection efficiency and so big amount of particles simulated. However, the possibility of measuring the burn-up by the neutron flux has been tested and verified. No significant increment of refuelling operation time and the possibility of an automatic core loading verification allows increasing the availability of the nuclear power plant. In conclusion, despite the necessary future improvements that the project shows to require, it can be considered viable under all points under all aspects. As a matter of fact, with low necessary investments requested, the system allows environmental benefits, as the slightly decrease of nuclear waste amount.
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