Pressurization as a result of hydrogen combustion has long been recognized as a threat to containment integrity under the conditions of reactor accidents [3]. The threat was highlighted by the hydrogen combustion event during the accident at Three Mile Island (see Fig. 7.1). Of course, the far more damaging hydrogen combustion events during the reactor accidents at Fukushima Daiichi have raised awareness of hydrogen combustion even more.

Hydrogen Sources

Under reactor accident conditions, the predominant source of hydrogen comes from the steam oxidation of zirconium alloy cladding on the reactor fuel:

The rate of this reaction is limited by the formation of the oxide product and follows parabolic kinetics [4]. The rate does increase exponentially with temperature up to the point it is limited by the mass transport of steam to the cladding. In typical predictions of reactor accidents, the mass transport of steam becomes

FIG. 7.1


limiting when peak fuel cladding temperatures are about 2100 K. That is, vigorous hydrogen production can take place before there is substantial fuel degradation and melting within the reactor core.

Oxidation of other metals such as stainless steel within the reactor core and especially structures above the reactor core can augment hydrogen production [5]. Water corrosion of aluminum and zinc alloys in containment and water radiolysis are not significant sources of hydrogen in comparison to those coming from steam oxidation of zirconium alloy and stainless steel during core degradation [6].

Steam oxidation of fuel cladding and other metals within the reactor containment is often predicted in severe accident analyses to be incomplete by the time core debris has relocated to the lower plenum of a reactor vessel. Penetration of the reactor vessel by the core debris can lead to additional phases of combustible gas generation during core debris interactions with water and structural concrete in the containment. In the case of core debris interactions with concrete, combustible carbon monoxide, CO, is produced along with hydrogen. These additional sources of combustible gas released to the reactor containment will be discussed further in later sections of this chapter. Here the focus is on the modes of hydrogen combustion and the loads combustion can impose on reactor containments.

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