Two different experimental techniques, namely positron lifetime spectroscopy (PALS) and electron energy loss spectroscopy (EELS), were used in a complementary fashion to provide actual helium concentration in a range of cavities induced by irradiation in spallation conditions. Results of the experiments on ferritic/martensitic steels with 450 - 1800appm of helium show helium-to-vacancy ratios 0.2 - 1, inversely proportional to the sizes of the vacancy clusters. While in low-dpa sample, the helium is predominantly confined in small vacancy clusters (He-to-vacancy ratio ∼ 1), high-dpa sample contains a high number density of helium-filled bubbles. The obtained results are in a good agreement with the molecular dynamics simulations published in the literature.