Evaluation of the water spray impact on premixed hydrogen-air-steam flames propagation

ANS Conference / 13-17 juin 2010, San Diego (Etats-Unis)

In the hypothetical case of a severe accident in a nuclear reactor with core meltdown, the interaction of the hot core with the cooling water can generate large amounts of hydrogen. It can also result due to oxidation of metals present in the corium pool or in the basemat during the molten corium-concrete interaction phase. This
hydrogen is transferred into the containment (and transported therein) by convection loops arising essentially from condensation of steam released via the
RCS break or during corium-concrete interaction. Depending on mixing in the containment atmosphere, the distribution of hydrogen is more or less homogeneous. If considerable hydrogen stratification exists, then local concentration of hydrogen may become substantial, and may exceed the lower flammability limit. In case of ignition, the subsequent pressure loads may adversely affect the containment.
To preserve the containment integrity, several safety systems could be used. Among them, spray systems and hydrogen Passive Autocatalytic Recombiners (PARs) have been extensively studied at Institut de Radioprotection et de Sûreté Nucleaire (IRSN) in France.
The spray primary function is to remove heat and condense steam in order to reduce pressure and temperature in the containment building. In case of hydrogen release inside the containment, sprays homogenize the hydrogen distribution and may lead to
“de-inertization” of the mixture through the condensation of steam on water droplets.
To keep the containment atmosphere inert during the in-vessel hydrogen production phase, Severe Accident Management Guidelines (SAMG) recommend postponing the spray system activation at least 6 hours after the beginning of core degradation. During this time, hydrogen concentration would be reduced by recombination.
In case of ignition, the water sprays can affect the flame propagation. Indeed, the role of water sprays on premixed flame propagation is complex and depends strongly on several parameters such as the liquid water fraction, droplets distribution, droplets size.
The aim of the present work is to bring preliminary results on the effect that the water spray could have on premixed hydrogen-air-steam flame behavior in case of combustion.