Hydrodynamic simulations are used to calculate the identical pion HBT radii, as a function of the pair momentum $k_{rm T}$. This dependence is sensitive to the magnitude of the collective radial flow in the transverse plane, and thus comparison to ALICE data enables us to derive its magnitude. By using hydro solutions with variable initial parameters we conclude that in this case fireball explosions start with a very small initial size, well below 1 ${rm fm}$.
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