Combustion within Porous Waste

Abstract

Flammable gases (primarily hydrogen and nitrous oxide but also ammonia and methane) are continuously being generated within the waste contained in the tank farms at Hanford Site. Some portions of the waste are porous and conceivably, a combustion event could occur within the waste due to accidental ignition. This has been postulated as a potential hazard since deflagrations and detonations are observed in laboratory experiments to propagate through combustible gases in porous materials, or through interconnected flammable gas voids. The waste in Hanford storage tanks are mainly in three different forms, a: salt cake, b: sludge, c: supernatant. Formation of a crust layer on the top of the waste is also observed in some tanks. The salt cake waste and crust resemble porous materials while sludge and supernatant looks like highly viscous fluids retaining flammable gas as bubbles or inclusions. Although laboratory experiments showed the possibility of propagation of deflagration or detonation in waste-like porous materials filled by flammable gases, the relevance of this issue to safety evaluations at Hanford is a matter of contention. In order to clarify this issue, we have reviewed the relevant data on laboratory experiments related to combustion in porous material. in doing this, we have concentrated on the flame literature rather than the detonation literature, since Makris et al. (1995) have already examined that. Further, significant mechanisms for the initiation of detonation (i.e., geometries resulting in strong flame acceleration within the dome) have not been identified therefore making flames a much more likely outcome of accidental ignition than detonation. ignition of flammable waste gases in the waste or the dome space of a tank can occur during intrusive operations into the waste or dome. External events which are not foreseeable such as lightning can also ignite the flammable gas retained in the waste. The present report only examines the basic issues in propagation of deflagration or detonation within waste. The process or probability of combustion ignition and other combustion events such as burns in the dome are not considered. After our review of the literature, some simple estimates of the potential for flame and detonation propagation are given. We conclude with a discussion of the uncertainties and measurements required to resolve this issue.