Numerical simulation of heat dissipation characteristics in PWR high-pressure coolant pipes during cold shutdown

Under cold shutdown conditions, the heat dissipation performance of the high-pressure coolant pipeline critically impacts pressurized water reactor (PWR) safety by governing residual heat removal efficiency. To quantify key drivers of thermal behavior during stagnant coolant operation, a high-fidelity thermal-hydraulic model is established by using the REALP5 MOD3.2 code. Systematic parametric analyses evaluate radiation effects and insulation-environment interactions through three dimensions. The results show that under stagnant coolant and zero-wind conditions, radiation contributes 90% of total heat transfer in the high-pressure coolant pipeline in the vertical direction, yielding a larger coolant temperature drop (ΔT) of 6.12 K over 86,400 s compared to the no-radiation model. Pipeline heat dissipation exhibits an inverse relationship with insulation cotton thickness but scales positively with material thermal conductivity. Within 800,000 s, when the insulation cotton thickness(δ) is reduced from 15 to 5 mm, the coolant temperature drops by nearly 18 K. This means that for every 10 mm reduction in the thickness of insulation cotton(δ), the heat dissipation efficiency increases by 40%. The coolant temperature drop (ΔT) of the pipe with asbestos insulation (thermal conductivity λ = 0.046W /(m·K)) reaches 43.98 K, which is 53.1% higher than that of the pipe with ultra-fine glass fiber cotton insulation(ΔT = 28.72 K) (thermal conductivity λ = 0.022W /(m·K)). Ambient temperature and airflow velocity exhibit inverse and positive correlations with coolant temperature drop (ΔT), the coolant temperature is reduced by 12.51 K more than the ambient temperature of 10℃ and 30℃. For every 1℃ decrease in ambient temperature, the temperature drop increases by 0.625 K. When the airflow velocity is 0.5 m/s compared to 0.1 m/s, the coolant temperature drops by 1.65 K more. This paper pioneers a high-fidelity thermo-hydraulic model of the main coolant pipeline of PWR under the stagnation condition after cold shutdown, which advances the theoretical framework of heat dissipation such as natural convection in the external environment and provides a quantitative safety margin for the removal of waste heat from the core. Further, it guides the design of insulation cotton material and the thickness of the high-pressure coolant pipeline and delivers predictive tools for the optimization of the heat dissipation strategy after cold shutdown.