[en] By comparing the ablation time of a hydrogen pellet in a tokamak discharge with the time required for the sublimation process, the vaporization of the pellet is shown to be a dynamic phase transition - i.e. the transport of heat is due to the propagation of an evaporation front. Based on this finding, an alternative boundary condition, consistent with the energy conservation law, is formulated. Computational results utilizing the new boundary condition indicate that the ablatant near the pellet surface is hotter and less dense compared with the results which make use of the previous condition of the vanishing flux. The discrepancy between the two solutions becomes less significant once the ablatant reaches the sonic radius. The scaling law of the pellet ablation rate is unaffected by this change of boundary condition. The present analysis shows that the validity of the neutral shielding model is based mainly on the existence of a thin envelope around the pellet where strong energy absorption occurs and is insensitive to the actual vaporization process occuring at the pellet surface. (Auth.)