Sensitivity and uncertainty analysis for nuclear data of relevance in spent nuclear fuel characterization

For an adequate assessment of the spent nuclear fuel, it is necessary to consider all possible uncertainties that affect the design, performance and possible accidents in nuclear facilities environment. Nuclear data are a source of uncertainty that is involved in neutronics, fuel depletion and activation calculations. Methodologies for performing the uncertainty propagation calculations need to be implemented in order to analyze the impact of the nuclear data uncertainties on critical responses during operation and in the event of accident, such as decay heat or neutron multiplication factor. Nevertheless, it is necessary to understand the current status of nuclear data and their uncertainties, in order to be able to handle this type of data. For this purpose, the state-of-the-art of different nuclear data libraries was analyzed during the development of this thesis, focusing on two main kinds of data: radioactive decay and fission yield data. To process these data the SANDY code was used. In this nuclear data sampling code two methodologies were added to obtain perturbed decay data and fission yields. A revision of the state-of-the-art of fission yield data shows inconsistencies in uncertainty data. For this reason, the possibility to use the formulas of the Bayesian/Generalised Least Square (GLS) update technique was also inserted in SANDY. Another contribution in the development of SANDY is related to the sampling methodology. Indeed, the possibility to choose between different distributions to sample from was inserted. The three distributions analyzed are: Gaussian, lognormal and uniform. Then the Fission Pulse Decay Heat problem is tackled: first because of its importance during events after shutdown and because it is a clear exercise for showing the impact and importance of decay and fission yield data uncertainties, testing the implemented perturbation methodologies. In this frame, a sensitivity and uncertainty analysis was carried out to evaluate the final uncertainty of fission pulse decay heat due to the uncertainties on the mentioned input data, to study the individual contribution of each nuclide and to quantify the most relevant isotopes. Finally, the sampling methodology was used to test the new fission yield evaluations released by CEA and to produce several sets of samples for each input distribution analysed.