The pellet cycle of a mixed isotope tokamak plasma is successfully reproduced with reduced turbulent transport modelling within an integrated simulation framework. In JET tokamak experiments, deuterium pellets with reactor-relevant deposition characteristics were injected into a pure hydrogen plasma. Measurements of the isotope ratio profile inferred a fast deuterium penetration time comparable to the energy confinement time. The modelling recovered the fast deuterium penetration timescale. The results are encouraging with regard to reactor fuelling capability and burn control.