Residual heat removal using a passive cooling system

Abstract

This study investigates the thermal-hydraulic performance of a passive Reactor Cavity Cooling System (RCCS) designed for a 250 MW molten salt reactor. The analysis focuses on the feasibility of natural convection-driven cooling as a passive safety mechanism. Simulations were performed using MATLAB and validated with COMSOL to evaluate the system under varying pipe configurations, diameters, and outlet temperatures. Key parameters such as Reynolds number, pressure drop, Nusselt number, and heat transfer characteristics were analysed to identify trends and thermal balance conditions. Results demonstrate that larger pipe diameters and higher outlet temperatures enhance heat removal performance through increased convective and buoyancy-driven flow. Intersection points between radiative and convective heat transfer curves were identified as critical for thermal equilibrium, providing insights into optimal RCCS configurations. This work provides insights into the design of efficient passive cooling systems, which can contribute to enhancing reactor safety during loss of-power scenarios. Future studies are recommended to incorporate transient simulations, two-phase flow analysis, and experimental validations for further
refinement.

Popular Abstract

Nuclear power is a critical energy source, but ensuring reactor safety is paramount, especially during emergencies when power is lost. The Fukushima nuclear disaster in 2011 highlighted the devastating consequences of prolonged power outages in reactors, as cooling systems failed and led to a meltdown. This research investigates how a Reactor Cavity Cooling System (RCCS) can passively remove heat from a molten salt reactor without relying on electricity, providing a safer solution in such scenarios. By using natural processes like convection and radiation,RCCS aims to keep reactors cool and safe even during emergencies.

The study focused on designing and testing RCCS for a 250 MW molten salt reactor. The system works by placing cooling pipes around the reactor vessel to absorb heat radiated from the core. Water flows naturally through these pipes, driven by temperature differences that create buoyancy forces, eliminating the need for pumps or other power-dependent devices.Simulations were performed using two different computer models to evaluate the effects of pipe sizes, number of pipes, and outlet temperatures on cooling performance. The analysis was based on steady-state conditions, while limiting the water outlet temperature (T₂) to below 100°C to prevent boiling and ensure stable natural circulation.The findings revealed that larger pipes significantly enhance cooling by improving water flow and heat transfer efficiency. Higher outlet temperatures strengthen buoyancy forces, which drive natural circulation. Heat is first transferred by radiation from the reactor to the pipes and then transported away by natural convection within the pipes. In the model, steady-state solutions were identified where radiative and convective heat transfer curves intersect, corresponding to a system in thermal balance. An analysis of pressure drop confirmed that buoyant forces and frictional resistance were well balanced, allowing the system to maintain natural circulation without external intervention. At higher outlet temperatures, the pressure drop increased slightly due to higher flow rates, but the system remained stable. The Reynolds number, which indicates flow behaviour, was consistently higher for larger pipes and higher temperatures, reinforcing their role in improving performance.

Results from both models were in close agreement further strengthening confidence in the RCCS design. Velocity and temperature profiles from both tools aligned well, confirming that the system could reliably remove residual heat under various configurations. The results also suggested that the RCCS could operate efficiently across a range of pipe diameters and configurations, offering flexibility for different reactor designs.
This research highlights the potential of RCCS to revolutionise reactor safety by eliminating the need for mechanical cooling systems. By offering a reliable and cost-effective way to handle emergencies, RCCS represents a step forward in making nuclear energy safer. Imagine a reactor that stays cool even when the lights go out—this is the promise of RCCS.

• author: Mani Maran, Narendran and Deenadayalan, Kaarthig
• supervisor: Jens Klingmann; Himani Garg
• organization: Department of Energy Sciences
• course: MVKM05 20241
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Reactor cavity cooling, Nuclear cooling, Buoyancy-Driven Flow, Nuclear Reactor Safety, Passive Reactor Cooling
• report number: LUTMDN/TMHP-25/5610 -SE
• ISSN: 0282-1990
• language: English
• id: 9181396
• date added to LUP: 2025-01-23 11:29:18
• date last changed: 2025-01-23 11:29:18