Particle deposition and plugging in tubes and cracks (with special reference to fission product retention)

Abstract

The main function of a containment structure enclosing a nuclear reactor is to retain any accidental release of radioactive materials. However, if the containment has leak paths due, for example, to cracks or failed seals, then there is the possibility of particulate matter and gas passing through the opening due to the pressure difference. The work presented in this paper describes in some detail the three main steps reqired to assess the amount of gas and particulate matter that pass into the environment. Step one involves calculation of the gas flow through a tube of non-uniform cross sectional area, step two is concerned with the deposition of small solid particles suspended in the host gas, onto the walls of the tube and, in step three, the rate of buildup of deposit on the tube wall is calculated with the aim of determining the rapidity with which the tube is plugged. We recast the gas dynamics equation into a form suitable for calculating gas flow in non-uniform tubes in the laminar and turbulent flow regimes, from low speed up to critical flow conditions. The results of these calculations are compared with an extensive set of experiments and excellent agreement is found. We briefly outline the various deposition processes that particles can undergo on surfaces and show how the particle concentration in the tube may be calculated. Finally, an equation that determines the shape and rate of formation of a plug is developed. This equation is non-linear and integro-differential in nature, and in order to solve it we develope a method which involves prescibing a geometrical plug shape but allowing the parameters within it to be time dependent. Plugging times and the amounts of particulate and gas that have escaped prior to plugging are calculated. Comparison with experiment is made and reasonable agreement found. A general discussion of the shortcomings of the approach is given together with suggestions for further work.