Boundary conditions and uncertainty evaluations in system thermalhydraulics

In the frame of the application of a methodology for estimating the uncertainties in code calculations of nuclear reactor transient behaviour (UMAE = Uncertainty Methodology based on Accuracy Extrapolation), the problem of phenomena whose occurrence leads to “cliff edges” in the transient is encountered. In the present paper, after a short summary of the UMAE characteristics, a possible way to cope with the evaluation of uncertainty resulting from the occurrence of boundary-condition-driven bifurcation is proposed. To this aim, reference is made to specific transient condition measured in downscaled integral facilities.