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The internal structure of ultra‐high performance concrete (UHPC) is very dense and prone to flaking in high‐temperature environments, thereby limiting its application in high‐temperature environments. In this study, polypropylene (PP) fibers were used in partially replacing steel fibers to prepare radiation‐protected UHPC‐containing hybrid fibers. The working performance, spalling behavior, mass loss, mechanical properties, γ‐ray shielding performance, and microstructure after exposure to different target temperatures (25°C, 200°C, 400°C, 600°C, and 800°C) of the radiation‐protected UHPC containing hybrid fibers were investigated. Results showed that the increase of PP fiber admixture did not have a significant negative effect on the flowability of UHPC mixes. The combination of steel fiber and PP fiber can effectively inhibit the spalling of UHPC at a high temperature. With the increase of temperature, the compressive strength and splitting tensile strength of UHPC showed a trend of first increasing and then decreasing, and the γ‐ray shielding performance gradually decreased. Compared with the normal temperature, the linear attenuation coefficient (μ) of UHPC at 800°C decreased by 12.6%, HVL and TVL decreased by 14.4%. Microstructural analysis showed that the porosity of UHPC increased with the increase of temperature, and the proportion of harmless pores showed a trend of increasing and then decreasing. Moreover, the high temperature led to the deterioration in the microscopic morphology of UHPC and the weakening of the bond between the steel fiber and the matrix. These findings revealed the reason for the decrease in mechanical properties of UHPC at high temperatures.

Nuclear technology benefits humans, but it also produces nuclear radiation that harms human health and the environment. Based on the modified Andreasen and Andersen particle packing model for achieving a densely compacted cementitious matrix, a new magnetite ultra-high-performance concrete (MUHPC) was designed using magnetite fine aggregate as a substitute for river sands with 0%, 20%, 40%, 60%, 80%, and 100% replacement ratios. The comprehensive properties of the developed MUHPC were tested and evaluated. These properties were fluidity, static and dynamic compressive strengths, high-temperature performance, antiradiation behaviors, hydration products, and micropore structures. Experimental results indicate that the developed MUHPC has high work performance and static and dynamic mechanical properties. The gamma ray shielding performance of MUHPC substantially improves with increased magnetite fine aggregate. Corresponding with 100% magnetite fine aggregate substitution, the linear attenuation coefficient of MUHPC is enhanced by 56.8% compared with that of ordinary concrete. Magnetite addition does not change the type of cement hydration products but improves the micropore structures of MUHPC and effectively reduces its total porosity and average pore diameter, thereby contributing to its mechanical and radiation shielding properties. The compressive strength and linear attenuation coefficient of the MUHPC can reach 150 MPa and 0.2 cm−1, respectively. In addition, the MUHPC also exhibits superior mechanical and radiation shielding performance at elevated temperatures (<400 °C). Finally, high strength and antiradiation performance support the use of MUHPC in radiation protection materials in the future.