Hydrogen/dust explosion hazard in ITER: Effect of nitrogen dilution on explosion behavior of hydrogen/tungsten dust/air mixtures

The work is aimed at supporting inert-gas dilution method proposed to mitigate hydrogen/dust explosion hazard in ITER in case of severe accidents. A standard method of 20-l sphere is used to study the effect of nitrogen dilution on the explosion behavior of 0.5-μm tungsten dust dispersed in hydrogen-containing air atmospheres. The oxygen content varied from normal 21 to 10 vol.%. The hydrogen concentration was varied from 7 to 18 vol.%; the tungsten dust cloud density of 2 kg/m3 was chosen to test as the optimal, i.e. the most dangerous dust concentration. The tested mixtures were formed in a spherical combustion chamber of 20-l volume at normal initial conditions and ignited at its center by a weak electric spark. In general, the tested dust/hydrogen mixtures explode more dangerously than hydrogen alone: they can generate higher explosion overpressures and explode faster than the corresponding hydrogen/air mixtures without dust. For all the tested mixtures the nitrogen dilution reduces both the explosion overpressure and pressure rise rate; however, its influence is more pronounced on how fast combined explosions proceed. In case of 7 vol.% H2, the explosion overpressure decreases from 4.5 to 3.5 bar at O2 decrease from 21 to 10 vol.%, while the pressure rise rate drops from 400 to 60 bar/s. In case of 18 vol.% of H2, the corresponding values are 6.2–2.6 bar explosion overpressure and 1370–170 bar/s pressure rise rate. An extrapolation of the obtained results to lower oxygen concentrations gives the value of limiting oxygen concentration, at which the combined explosions are to be suppressed, about 8–9 vol.%.