Uncertainty and sensitivity evaluation of aerosol deposition in PHEBUS containment during FPT-2 experiment

Abstract

During a hypothetical severe accident at a nuclear power plant (NPP), the aerosols and radionuclides could be released from the reactor through the engineering systems to the surrounding atmosphere. The last barrier preventing release is the containment, which surrounds the reactor and the main engineering systems. Most of radioactivity is released in the form of particles (aerosols). Therefore, aerosol transport and deposition processes are very important for NPP safety. Description of these processes requires a lot of physical parameters and conditions. Naturally, these parameters bring their own uncertainties, and so uncertainty and sensitivity analyses are necessary for final assessment of the accuracy of results. This paper presents the uncertainty and sensitivity analysis of PHEBUS FPT-2 simulation performed with the SUSA software. The containment simulation results were received using the lumped-parameter code COCOSYS. This paper includes a description of the PHEBUS containment, nodalisation, initial and boundary conditions, results and conclusions. The uncertainty analysis was performed assuming 95/95 probability, and tolerance limits showed that the measured values of the aerosol mass suspended in the gas phase were within the calculated uncertainty limits. However, the deposition on the vertical containment walls is not within the calculated limits. The sensitivity analysis showed that the parameters having the largest influence on the investigated parameters (mass suspended in gas phase, deposition on containment elliptic floor, deposition on condensers, deposition on vertical walls) are the following: the Mass Median Diameter (MMD), the dynamic shape factor, the particle agglomeration factor, the water film thickness, the initial pressure, the Geometric Standard Deviation of particle diameter, and the thickness of diffusive boundary layer.