Water-cooled pebble-bed (WCPB) blanket, in which beryllium/beryllide in a pebble form is used as neutron multiplier, is one of blanket concepts based on conventional or near-future technology for fusion DEMO. Combination of water, as coolant, and beryllium/beryllide, however, may pose a critical safety problem, i.e. the chemical reactivity of the beryllium/beryllide pebble and hydrogen generation. We present a new phenomenological model of the reaction behavior of the beryllium/beryllide pebble with the steam. The model consists of the equations of the transients of (i) the radius of the unreacted part and (ii) the temperature of the pebble. We have developed a code PSYCHE to numerically solve the model equations. It has been found that the amount of the reaction-produced hydrogen obtained by the numerical simulation agree well with the experimental observations. We also show an application of the code to safety analysis of the transient behaviors of the Be and beryllide Be12Ti pebbles in an in-box LOCA, i.e. loss-of-coolant accident in a blanket box. The model simulation presents the better thermal stability of the Be12Ti pebble, compared to the Be pebble, in the in-box LOCA condition expected in a WCPB DEMO blanket.