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

Our plain-English summary of Numerical simulation of heat dissipation characteristics in PWR high-pressure coolant pipes during cold shutdown, by F. Tang, W. Wang, Y. Luo, et al (Scientific Reports, 2025).

The study

• Authors: F. Tang, W. Wang, Y. Luo, Y. Chen, B. Yan, S. Liu
• Journal: Scientific Reports
• Published: 2025;15(1):41685
• Read it: PubMed · Publisher · Free full text

Abstract

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.

© 2025. The Author(s).

In plain English

This engineering computer-simulation study contains no information about asbestos exposure or disease. The authors used the RELAP5 MOD3.2 program to model heat loss from a 1 m stainless steel coolant pipe in a pressurized water reactor during cold shutdown. The coolant was still and started at 353.15 K. They varied radiation, insulation thickness (5 to 15 mm), insulation material, ambient temperature and airflow. For a vertical pipe with still coolant and no wind, including radiation gave a coolant temperature drop 6.12 K larger over 86,400 s than leaving it out. Over 800,000 s, coolant under 5 mm of insulation fell 48.42 K, compared with 30.52 K under 15 mm. Asbestos was one of six insulation materials modeled, defined by a thermal conductivity of 0.046 W/(m·K). Coolant in the asbestos-insulated pipe fell 43.98 K, the largest drop, compared with 28.72 K for ultrafine glass fiber wool. Lower ambient temperature and faster airflow increased cooling. The authors concluded that, with still coolant and no wind, heat loss from the insulated pipe was mainly by radiation. They assumed perfect contact between insulation and pipe and ignored air gaps.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Engineering numerical simulation (thermal-hydraulic computer model using the RELAP5 MOD3.2 code) with parametric analyses; not a health or exposure study
• Who was studied: No people were studied. The model was a single simulated PWR high-pressure coolant pipe (outer diameter 0.325 m, wall 0.03 m, length 1 m, stainless steel UNSS30200) connecting the reactor pressure vessel and steam generator, under cold shutdown with stagnant coolant
• Exposure: Not a human exposure. The model inputs varied were radiation on or off, insulation thickness (5, 8, 10, 12, 15 mm), and six insulation materials (ultrafine glass fiber wool, glass wool, polypropylene foam, rock wool, polyvinyl chloride foam, asbestos). Asbestos was represented only by an effective thermal conductivity of 0.046 W/(m·K) and emissivity 0.77. Ambient temperature (10 to 30 degrees C) and airflow (0.1 to 0.5 m/s) were also varied
• Main results:
  • For the vertical pipe with stagnant coolant and zero wind, the radiation model gave a coolant temperature drop 6.12 K larger over 86,400 s than the no-radiation model. Coolant fell from 353.15 K to 346.94 K with radiation, and to 353.06 K without it
  • Within 800,000 s, coolant under 5 mm insulation fell 48.42 K (the largest drop among thicknesses). Under 15 mm insulation it fell 30.52 K (the smallest)
  • Among the six insulation materials, coolant in the asbestos-insulated pipe fell 43.98 K within 800,000 s, the largest drop. Coolant in the ultrafine glass fiber wool pipe fell 28.72 K, the smallest
  • At an ambient temperature of 10 degrees C, coolant fell 43.34 K, the largest drop among the ambient temperatures modeled. At 30 degrees C, coolant ended at 322.32 K
  • At 86,400 s, coolant temperature was 333.47 K at 0.1 m/s airflow and 331.82 K at 0.5 m/s airflow
• The authors' conclusion: The authors concluded that, with still coolant and no wind, heat loss from the insulated pipe was mainly by radiation, so radiation should be included in such analyses. They also concluded that insulation material and thickness should be chosen with cold-shutdown heat dissipation in mind
• Limitations the authors state:
  • The model assumed ideal contact between insulation and pipe wall and ignored air gaps from improper installation or vibration
  • Coolant flow was assumed to be static, incompressible and single-phase

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