A high-temperature reaction vessel system was set up to study the wall deformation during cyclic hydriding/dehydriding reactions of Mg(2)Ni powders at 255 degrees C. Effects of packing fraction and initial particle size of Mg(2)Ni powders were characterized. Results indicated that a strain accumulation was found on the wall surface with progressive cycles of hydriding/dehydriding reactions. The wall strain varied with position, as a lower position had a larger deformation. Such an accumulation and position-dependence of wall strain in the vertical hydride storage vessel could be attributed to a pulverization-densification mechanism. At a given vessel position, the hoop strain was increased with increasing packing fraction from 50 to 70 vol%. For a 50-vol% packing fraction, the wall deformation was enlarged when the initial powder size was increased from 200 mesh (74 mu m) to 100 mesh (149 mu m). After 50 cycles of hydriding/dehydriding reactions, the particle size was significantly reduced to about 1 mu m. (C) 2011 Elsevier B. V. All rights reserved.