Experimental characterization of aerosol retention in the break stage of a dry steam generator in SGTR sequences.

Abstract

Severe accident Steam Generator Tube Rupture (SGTR) sequences are identified as major contributors to risk of Pressurized Water Reactors (PWR). Their relevance lies in the potential radioactive release, in form of aerosol, from reactor coolant system to the environment. However, radioactive particles could be partially retained in the secondary side of the steam generator, even in the absence of water. Lack of knowledge on the source term attenuation capability of the steam generator has avoided its consideration in probabilistic safety studies and severe accident management guidelines. As a consequence, the steam generator filtering capability is not usually taken into account either in the probabilistic risk assessment of nuclear safety or in the severe accident management guidelines.This thesis describes the main activities and results of a bench-scale experimental program focused on getting insights into the aerosol retention in the break stage of the secondary side of a dry steam generator. The thesis is framed in the CIEMAT contribution to the ARTIST project (2003)(2004)(2005)(2006)(2007)(2008) which was supported by te Spanish Nuclear Regulatory Commission (CSN). The general objective of the work was to generate a comprehensive database on fission product retention in the break stage of a dry steam generator during a severe accident SGTR sequence. The specific objectives were to assess the influence of aerodynamic flow field as well as the influence of particle nature on aerosol deposition in the tube bundle. To do so, a scaled-down mock-up with representative dimensions of a real SG was built. The aerodynamic characterization of the flow field within the break stage of the bundle was done via 2D PIV (particle image velocimetry) technique. The particle nature influence on retention was characterized through aerosol retention experiments in the tube bundle mock-up. The major variables investigated were the type of breach (guillotine vs fish-mouth), the inlet gas mass flow rate (75-250 kg/h) and the particle type (polidispersed TiO 2 agglomerates vs. solid, monodisperse SiO 2 spheres).The aerodynamic campaign permitted to characterize the flow field close to the breach for both type of breaches and to asses their similarities. Results showed that the jet flows within the tube bundle following a generic quasi-parabolic trajectory evolving from an oblique cross flow configuration to an axial one. Mean flow field near the breach is substantially affected by the entrainment of initially stagnant gas into the jet. This effect is fostered by the presence of tubes and their tight packing. Jet penetration and turbulence intensity are considerably enhanced when increasing inlet gas mass flow rate.The results of the aerosol campaign showed that particle nature substantially affects retention in the tube bundle: mass retention was low for TiO 2 agglomerates (less than 30%) whereas it was much higher for SiO 2 particles (around 85%). Collection efficiency is also affected by gas mass flow rate: its sensitivity ...