Specimens of the European reference 9Cr–WTaV steel EUROFER97 alloyed with 0.081% and 0.114% B 10 (415 and 5800appm He after irradiation) were neutron-irradiated up to 16displacements per atom at temperatures in the range from 523K to 723K to study the influence of He concentration on its segregation and bubble formation. Microstructural investigations after tensile testing showed that round halos often formed around 10 B-containing precipitates as a result of the transmutation reaction 10 B(n,α) 7 Li. Based on their size and density, different layers were distinguished inside the halo. In general, transmission electron microscopy showed a high density of bubbles or cavities after 523K irradiation, even at the lower He concentration of 415appm. In addition, the influence of irradiation temperature and the average boron concentration in the specimen on the bubble size and density inside halos around precipitates was analyzed and correlated with the tensile properties and the fracture surface.