Malfunction tests and vibration analysis of PWR internal structures

Abstract

To diagnose changes liable to occur in the vibration behavior of internals, it is important to understand the influence of changes in the mechanical properties of elements on the output signals obtained from neutron chambers placed out of core and accelerometers fixed to the reactor vessel. To do this, the effects of changes liable to occur in the hold-down springs and the flexures were simulated on the SAFRAN loop, using a representative hydroelastic mock-up (geometric scales 1/8). The results obtained experimentally on SAFRAN for different characteristics of the hold-down spring, which lies between the upper part of the core barrel and the vessel head, have been published. In this paper, we propose to present the results of the investigation of the fracture of one or more flexures which connect the cylindrical thermal shield to the core barrel. This work is in two parts: 1. a) Computation based on a hydroelastic model using the sub-structuration computer program TRISTANA of the CASTEM system. 2. b) Tests simulating flexure fracture: 1. 1 - in air, for an understanding of the mechanisms involved; 2. 2 - on the SAFRAN loop with a representative flow in order to estimate the strains liable to exist on the vibration signatures recorded on displacement transducers and accelerometers. Good agreement was observed between the computation results with the theoretical model employed and those obtained experimentally. In this presentation, we shall provide details showing that the detection of the occurrence of this process is very delicate. However, on the nuclear power spectral densities obtained on a power plant, it seems possible to detect a light frequency shift of the shell mode on the thermal shield in certain operating situations. This study, which required substantial experimental and computation resources, was conducted as a joint project between Commissariat à l'Energie Atomique, l'Electricité de France and Framatome.