A simplified mass ablation and relocation treatment ofhigh-field-side/low-field-side pellet injection is proposed. It includesa model of pellet ablation, parallel flow model for the attachedablation channel and a simplified magnetohydrodynamic pelletdisplacement model. This treatment reveals satisfactory agreement withthe present-day experiments on ASDEX-U and DIII-D tokamaks. The modelprovides the optimization directions for deep plasma fuelling inexperiments.