Passive shutdown and decay heat removal of a small modular reactor with density locks

  • Passive Abschaltung und Nachwärmeabfuhr eines kleinen modularen Reaktors (SMR) mittels Dichtesperren

Zhang, Tian; Allelein, Hans-Josef (Thesis advisor); Macián-Juan, Rafael (Thesis advisor); Kneer, Reinhold (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020


A density lock is an open tube bundle connecting liquid inventories of different physical properties. Due to different density and other fluid properties a stratification occurs inside the density lock. Under balanced pressure, the stratification remains stable in the lock. However, once the balance is broken, fluid flows to the inventory of lower pressure automatically. Thus, the density lock works as a passive, fail-safe valve. In the design of reactors, density locks are implemented to separate the primary circuit and a cold boronated water pool. Due to height differences of density locks, in normal operation, pressure balance is established. A stable stratification in the density lock is maintained by the pump speed control. In the abnormal situation, cold boronated water flows into the primary circuit, shuts down the reactor and removes the decay heat passively to the pool. The behavior of density locks was originally explored for the PIUS and the ISIS concepts (1980s and 1990s). In this work, density locks are integrated into the safety concept of a 200 MWth iPWR concept and their behavior regarding the passive shutdown and the decay heat removal is investigated with today’s simulation methods. For this purpose, coupling between CFD code ANSYS CFX and the point kinetics PoKiMON code was developed and a model of primary circuit and RPV pool was built. Several operation and accident scenarios were analyzed to investigate the dynamic system behavior. The work proved that density locks can be implemented in such an iPWR concept. In normal operation and during slow transients the stable separation of RPV pool and primary circuit is maintained by pump speed control. In the pump-trip accident, density locks shutdown the reactor in the space of a few seconds without human action, while preventing overheating of the core. However, in the Main Steam Line Break (MSLB) accident, the response of the density lock is limited. The pump control prevents failure of the pressure balance in this slow transient and only limited amount of boron are able to enter the reactor core. The reactor shutdown passively triggered by the negative temperature feedback. After the reactor shutdown, natural circulation flow path through density locks established, passively cooling the reactor core. No boiling occurs in the system.