Nuclear Technology / Volume 158 / Number 1 / April 2007 / Pages 83-93
Technical Paper / Reactor Safety
Postulated accident sequences of a pressurized water reactor, consisting of steam generator tube ruptures (SGTRs) in combination with a melting core, have been demonstrated to represent a dominant contribution to the overall public risk. However, it should be expected that even in the absence of any water in the secondary side of the steam generator ("dry" SGTR scenario), some radioactivity retention takes place as a result of the interaction of the carrier gas with internal structures. The region near the tube breach becomes a key region because it behaves as a sink for the radioactive particles entering the secondary side, and consequently, it changes size distribution of aerosols flowing toward upper structures.
This paper identifies major issues that should be addressed to accurately estimate aerosol retention in the field near a tube breach during dry SGTR scenarios. By developing a simple Lagrangian model based on the filter-concept approach (ARISG-I), the specific aspects of fluid dynamics and aerosol physics involved have been explored and the major knowledge gaps highlighted.
Inertial impaction and turbulent deposition have been demonstrated to be major particle removal mechanisms. Their respective collection efficiencies have been derived by gathering and correlating separate effect data on particle deposition on cylinders in a crossflow configuration. Comparisons of model predictions to experimental data taken in a mock-up facility of the break stage under similar conditions to those anticipated in dry SGTR scenarios have been set. The substantial discrepancies found and their analysis have provided insights into the significance of drawbacks of model fundamentals, the inaccuracy of specific equations of deposition mechanisms, and most importantly, the lack of consideration of key phenomena that hinder aerosol retention.
According to this analysis the main areas where research is needed are: gas jet behavior across the tube bank; particle resuspension, erosion, and/or bouncing; and particle inertial impaction and turbulent deposition under foreseen conditions.