Abstract: A finite element model was established to examine the ultimate state of reinforced concrete beams in nuclear power plants under accidental thermal loads. The FE model was validated by comparing the analysis results with those of published tests. Then, the validated finite element model was used to investigate the critical condition, ultimate capacity, and failure mode of the reinforced concrete beams under accidental thermal loads combined with other mechanical loads. The influence of the geometry constraint was considered, and the effects of the axial force, reinforcement ratio, and temperature on the load effect values were examined. The obtained cross-sectional thermal  bending moments were compared with the ACI Code values. The results showed that the thermal moment increased with an increase in the reinforcement ratio and temperature, and the ACI Code values appeared to overestimate the thermal bending moment, particularly in the case of high temperatures. As temperature increased, the ACI Code values became more conservative. Therefore, a formula was proposed to modify the flexural capacity calculated using the ACI Code. The results based on the formula were in good agreement with the finite element results under various operating conditions.

Key words: nuclear power plant, reinforced concrete beam, accidental condition, thermal bending moment, finite element analysis