Mechanical analysis of the equipment hatch behaviour for the French PWR 900 MWe under severe accident
Congress title :19th International Conference on Structural Mechanics In Reactor Technology (SMIRT 19)
Congress location :Toronto
Congress date :12/08/2007
This paper presents the mechanical analysis carried out by the French institute IRSN (Institute for Radiation Protection and Nuclear Safety) on the equipment hatch of the French PWR 900MWe containment building under severe accident, in order to quantify the possibility of failure due to pressurization effects cumulated with thermal effects. Calculations are carried out with CAST3M code using the finite element method and three-dimensional deterministic model. The equipment hatch is identified as the weakest part of structure in the global model of the French PWR 900MWe containment building shown in another paper of SMIRT 2007, presented by IRSN. Calculations are based on a multi-scale method using the results of the global model, so that each model is on the scale of the mechanical phenomena represented, by preserving a reasonable calculation cost. Incertitudes are quantified by sensibility studies. The scenario of severe accident is based on a progressive increase of pressure and temperature inside the containment building due a large break of primary circuit at full power with peak pressure due to hydrogen combustion and long-term loading due to Melt-Corium Concrete Interaction and inner containment water vaporization. Two kinds of failure may happen : a crack in the leaktight steel structure and a disjunction of the two flanges of the closing system when the internal pressure increase. A maximum gap of 40 µm leads to a leakage area of about 1 cm². Because of irradiation, the seal between the flanges may lose its elasticity and its capacity of leaktighness and is not taken into account in calculations. This is why detail models are needed to modelise the screws tightening the two flanges, with contact and shearing phenomena. The exact 3D geometry and the realistic boundary conditions constitute an innovative approach of the closing system : at conception, the equipment hatch has been assumed to support essentially axisymmetric efforts. The detailed models enable to understand for the first time how it is working and the main results is that there is a shear force applied to the screws due to the non-uniformity of efforts, deformations, contact and friction along the circumference of the two flanges and then, the necessity of providing new screws made of higher grade steel with larger diameter. This is why EDF is now changing the screws of the equipment hatch in all French PWR900 MWe.