Attenuation of radioactive releases from NPP under accident conditions: An experimental research on risk-dominant scenarios

Abstract

Accident scenarios in which a nuclear power plant (NPP) containment is by-passed, may become a significant contribution to the NPP risk. The reason is the direct access to the environment that any radioactive material escaping from a degrading core would have. This is the case of steam generator tube rupture (SGTR) severe accident sequences in pressurized water reactors (PWRs), in which tube breaches in the steam generator communicate the primary system of the nuclear reactor and the environment.This paper presents the major results of an experimental investigation aimed to assess and characterize the mitigation of radioactive releases when flowing through the secondary side of a steam generator during SGTR severe accident sequences. A set of experiments performed in a mock-up tube bundle scaled from a real steam generator has demonstrated that particles are significantly retained in the region around the breach, even in the total absence of water. The tests explored the anticipated conditions in the scenario: gas jet injection, micron particles of different nature (dense spheres vs. aggregates) and tube breach types (guillotine vs. fish-mouth). The main variable affecting retention is the particle nature. The collection efficiency reached 75–95% in the case of dense spheres. The influence of natural aerosol mechanisms like inertial impaction, turbulent deposition, resuspension and fragmentation are discussed in the paper.