Experimental characterization of open loop passive heat removal
Telkkä, Joonas (2025-10-10)
Väitöskirja
10.10.2025
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Energiatekniikka
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Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-412-309-9
https://urn.fi/URN:ISBN:978-952-412-309-9
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Tiivistelmä
This study characterized the functioning of an open loop gravity-driven containment passive heat removal system of a large pressurized water reactor by means of experiments performed at the PASI test facility of LUT University, Finland. PASI is height-scaled 1:2 compared to the reference system and comprises a containment vessel, a heat exchanger with 15 tubes, a water pool, and interconnecting riser and downcomer pipelines.
The objective of the study was to determine whether or not an open natural circulation loop is sensitive to disturbances that could make it an unreliable safety system. Particularly, the aim was to address the concerns about the small driving forces and unstable operation that might threaten the structural integrity of such a system. Specifically, the behavior of the natural circulation flow in the loop, and the system’s heat removal capability, were investigated.
The quasi-steady behavior of the system was characterized in the single-phase and twophase modes. Periodic testability of the system in a real power plant was demonstrated. Dynamic loads to the piping caused by flashing-based flow oscillations were assessed and found to be small. The conditions for the stable operation of the system were identified; they depend on the riser flooding and reaching of the CCFL criterion in the riser. In the quasi-steady conditions, the amplitude of flow oscillations is somewhat constant, but the frequency is roughly proportional to the heating power. The impact of the gravity head was examined; the pool water level affects the oscillation frequency and amplitude slightly. Finally, the system response to an added loop flow resistance was studied.
Based on the study, the open loop heat removal system was found to be very robust and not sensitive to distractions such as added loop flow resistance. Heat transfer from the containment was efficient in all tested conditions. Furthermore, the system was proven to be strongly self-regulating: the system immediately finds a new operation point after a change in, e.g., heating power or flow resistance. Most probably, due to the low heating power originating from reactor decay heat, in an accident situation, this type of safety system would operate in a quasi-steady mode accompanied by continuous, undamped two-phase flow oscillations.
The objective of the study was to determine whether or not an open natural circulation loop is sensitive to disturbances that could make it an unreliable safety system. Particularly, the aim was to address the concerns about the small driving forces and unstable operation that might threaten the structural integrity of such a system. Specifically, the behavior of the natural circulation flow in the loop, and the system’s heat removal capability, were investigated.
The quasi-steady behavior of the system was characterized in the single-phase and twophase modes. Periodic testability of the system in a real power plant was demonstrated. Dynamic loads to the piping caused by flashing-based flow oscillations were assessed and found to be small. The conditions for the stable operation of the system were identified; they depend on the riser flooding and reaching of the CCFL criterion in the riser. In the quasi-steady conditions, the amplitude of flow oscillations is somewhat constant, but the frequency is roughly proportional to the heating power. The impact of the gravity head was examined; the pool water level affects the oscillation frequency and amplitude slightly. Finally, the system response to an added loop flow resistance was studied.
Based on the study, the open loop heat removal system was found to be very robust and not sensitive to distractions such as added loop flow resistance. Heat transfer from the containment was efficient in all tested conditions. Furthermore, the system was proven to be strongly self-regulating: the system immediately finds a new operation point after a change in, e.g., heating power or flow resistance. Most probably, due to the low heating power originating from reactor decay heat, in an accident situation, this type of safety system would operate in a quasi-steady mode accompanied by continuous, undamped two-phase flow oscillations.
Kokoelmat
- Väitöskirjat [1214]