Characterising the energy released by the disintegration of the radionuclides present in spent nuclear fuel is essential for the design, safety and analysis of storage, transport and disposal systems. Few measurements of this decay heat are available today. In addition, the available experimental values do not cover the wide spectrum of possible combinations between parameters such as discharge burn-up rate, 235U enrichment, cooling time, fuel design parameters, or operating conditions. The estimation of decay heat is therefore mainly based on calculation codes.
The evaluation of the uncertainty associated with the estimation of decay heat is important to achieve reliable predictions. Many efforts have been made to properly evaluate biases and uncertainties coming from nuclear data such as cross sections. The number of studies concerning uncertainties of an epistemic nature (uncertainty in the manufacture of some components, error in reading or adjusting mobile structures, etc…) is comparatively small. Among the latter, while the treatment of complex dependencies of scalar input parameters is well taken into account today, functional-type dependencies, i.e., expressed in the form of a function, are very little explored.
While uncertainties arising from the processing of fixed input parameters, such as fuel manufacturing parameters, independent of time, are quite well covered, the uncertainties coming from the processing of variable (or functional) parameters, such as operating history, evolving during reactor operations, are not. Irradiation history actually brings together several inter-correlated quantities (operating power, absorber movements, core evolution …), subject to modifications over time and influencing the value of numerous observables of interest, including decay heat. The models used today in industrial simulation tools do not make it possible to estimate this impact and to infer a validated uncertainty.
This research work will investigate the impact on decay heat of the uncertainties associated with input parameters having functional dependencies. We will particularly focus on the irradiation history of the reactors (PWR type). A first part of the work will be dedicated to the development of a substitution model for decay heat estimation and quantification of uncertainties of a functional nature. The second part will be devoted to the development of a sensitivity analysis method. Finally, a third part will concern the development of an inverse method for quantifying the uncertainties coming from irradiation modelling.
The doctoral student will be hosted in a reactor physics research unit of the CEA IRESNE located in Cadarache where he will collaborate with other doctoral students and specialists in the field.