Gas release from sludge

Abstract

Nuclear waste in the form of sludge of rather uncertain composition is held in 177 cylindrical tanks, typically 23m in diameter and 10m high. The radioactivity splits some water molecules into free radicals, which are thought to recombine on suitable catalytic sites to form hydrogen gas. The rate of production of gas (hydrogen and other species) is typically about 4m^3/day in the SY-101 tanks. When the concentration of hydrogen in the roof of the tank exceeds 4%, there is a risk of deflagration. The study group was asked to consider the mode in which the gas is released, whether as dangerous large eruptions or as acceptable continuous background release. We were also asked how much gas was stored in the sludge, where and in what form. The sludge is neither solid nor fluid, but rather a yield material with a yield stress of about 103 Pa.

The study group based its approach on a video produced by Pacific Northwest of a laboratory experiment in which gas was generated in a yield material. In these experiments oxygen was produced from hydrogen peroxide, and the material was a clay suspension. The video showed bubbles growing in the material, and the height of the sample rising, rather like baking bread. After some time, some bubbles were large enough to overlap, and they merged. The result of several mergers was to form cracks, fairly horizontal, which grew by being inflated by gas and then breaking sideways into a nearby bubble. A model of this crack growth is given in section 3.1. Gas was released to the surface from the network of cracks.