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• The size of the gaps around the ITER equatorial and upper ports has associated a certain tolerance due to manufacturing and installation. • Gap size variability has an impact on the nuclear responses. • MCNP simulations show the extent of the impact of the gap variability. • A novel modelling approach, which was developed ad-hoc and used to manipulate the size of the port gaps via displacements of the geometry universes, is described. The size of the gaps around the ITER equatorial and upper ports has associated a certain tolerance due to manufacturing and installation. This variability has an impact on the nuclear responses at magnets and port interspace maintenance locations. This effect is not usually considered as the ITER neutronic reference models represent the nominal gaps configuration. This paper aims to estimate the effect of the gap size variability on nuclear heating in the magnets during plasma operation and shut-down dose rate at the port interspace maintenance location taking advantage of a set of MCNP simulations with different gap configurations. The effect has been shown to produce increases of up to 30% in the worst-case configurations. The novel modelling approach, which was developed ad-hoc and used to manipulate the size of the port gaps via displacements of the geometry universes, is also described here. This approach led to important time savings, and it may be used in future studies with similar requirements. [ABSTRACT FROM AUTHOR]