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<p>Natural circulation systems (NCSs) find extensive application in nuclear, thermal and solar power plants. Current commercial nuclear power plants rely on natural circulation (NC) for decay heat removal, whereas many advanced reactors, such as Economic simplified boiling water reactor and several Small Modular Reactors employ NC as the normal cooling mode. The steady state and stability performance of NCSs are important for design. Hence, this paper briefly describes the steady state performance and the advances made in the techniques for stabilizing single-phase, two-phase and supercritical NCSs without significantly reducing the flow rate and hence the heat transport capability. The most common stabilizing technique relies on the insertion of an orifice. The present study shows that insertion of an orifice can stabilize an unstable point near the lower threshold of stability, whereas it destabilizes a stable point near the upper threshold of stability for both single-phase and two-phase NCSs. In case of supercritical systems, a stable point near the lower threshold can be destabilized whereas an unstable point near the upper threshold can be stabilized by the insertion of an orifice, which is opposite to the trend for single-phase and two-phase NC loops. In forced circulation systems, stabilizing techniques that reduce the flow rate, including the insertion of an orifice, can be adopted with a corresponding increase in the pump developed head without altering the flow rate. On the other hand, NCSs require stabilizing techniques that do not decrease the flow rate and hence the heat transport capability. Insertion of an orifice decreases the flow rate and the heat transport capability and hence is not desirable. This paper identifies techniques that either slightly reduce, do not affect, or enhance the heat transport capability while stabilizing. It also examines design requirements for maximizing the power output of natural circulation reactors (NCRs). In addition to increasing the flow rate, enhancing the surface area density of the reactor core is a key factor in improving the power rating of NCRs. The use of internally and externally cooled annular fuel pins, as well as plate-type fuel, can help achieve higher surface area density and, consequently, higher power output.</p>

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Advances in thermal hydraulics of natural circulation systems and the associated design and safety challenges

  • 期刊论文

  • 发表日期：

    2026年7月30日

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  • Vijayan Pallippattu Krishnan,
  • Swati Gangwar,
  • Dev Banitia

摘要

Natural circulation systems (NCSs) find extensive application in nuclear, thermal and solar power plants. Current commercial nuclear power plants rely on natural circulation (NC) for decay heat removal, whereas many advanced reactors, such as Economic simplified boiling water reactor and several Small Modular Reactors employ NC as the normal cooling mode. The steady state and stability performance of NCSs are important for design. Hence, this paper briefly describes the steady state performance and the advances made in the techniques for stabilizing single-phase, two-phase and supercritical NCSs without significantly reducing the flow rate and hence the heat transport capability. The most common stabilizing technique relies on the insertion of an orifice. The present study shows that insertion of an orifice can stabilize an unstable point near the lower threshold of stability, whereas it destabilizes a stable point near the upper threshold of stability for both single-phase and two-phase NCSs. In case of supercritical systems, a stable point near the lower threshold can be destabilized whereas an unstable point near the upper threshold can be stabilized by the insertion of an orifice, which is opposite to the trend for single-phase and two-phase NC loops. In forced circulation systems, stabilizing techniques that reduce the flow rate, including the insertion of an orifice, can be adopted with a corresponding increase in the pump developed head without altering the flow rate. On the other hand, NCSs require stabilizing techniques that do not decrease the flow rate and hence the heat transport capability. Insertion of an orifice decreases the flow rate and the heat transport capability and hence is not desirable. This paper identifies techniques that either slightly reduce, do not affect, or enhance the heat transport capability while stabilizing. It also examines design requirements for maximizing the power output of natural circulation reactors (NCRs). In addition to increasing the flow rate, enhancing the surface area density of the reactor core is a key factor in improving the power rating of NCRs. The use of internally and externally cooled annular fuel pins, as well as plate-type fuel, can help achieve higher surface area density and, consequently, higher power output.