Effect of non-condensable gases on reflux condensation in nuclear steam generator tubes

Of all the nuclear reactors in operation today, light water reactors and pressurized water reactors in particular remain one of the most common designs. With 298 functioning units as of 2019, this type accounts for over 66% of all operating reactors [1]. The presented work attempts to expand the understanding of a pressurized water reactor (PWR) steam generator (SG) in a particular mode of operation, referred to as reflux condensation. The scenario in which SG tubes operate in a reflux condenser mode remains relevant to PWR mid-loop operation as well as a possible loss-of-coolant (LOCA) accident. In these modes, steam condensation in SG tubes provides a very efficient path for heat removal from the primary circuit of the nuclear reactor. This phenomenon is capable of removing residual decay heat from the reactor coolant system, which suppresses/delays the disastrous core uncovery. At the same time, presence of non-condensable gas species, namely hydrogen and nitrogen, is very likely. NC gases in mixture with steam has been previously shown to severely reduce the efficiency of the water condensation process. To study the interaction of steam and non-condensable gases in the geometry of PWR SG tubes, an experimental facility is designed and constructed. The Precise Reflux Condensation Investigation Setup (PRECISE) developed and located in Paul Scherrer Institut is a concentric tube condenser, in which a working-volume, vertical test tube is surrounded with a cooling water jacket. Accurate control over the volume gas inventory and operating conditions such as pressure, temperature and coolant water properties allows to study reflux condensation in detail at different concentrations of NC gases. Nitrogen and helium (in place of flammable hydrogen) are used, as well as mixtures of the two at different ratios. Besides traditional measurement methods, PRECISE facility is equipped with two novel sensor types – heat flux sensors and a movable temperature/conductivity probe. The development of these two devices allowed to observe reflux condensation with previously unavailable spatial and temporal precision. The first part of the presented thesis provides a detailed overview of the PRECISE facility and the used measurement methods as well as explains the employed data acquisition and processing techniques. The second part discusses results from the conducted experiments and provides insight in the observe impact of the thermodynamic operating conditions on the condensation and interaction of steam and NC gases. Measurements reveal that the non-condensable gas type plays a crucial role in the plug formation. Light gas (He) forms sharp and narrow interface and limits condensation efficiently while heavy gas (N2) develops wide, diffuse interface with gradual temperature drop. The third part presents the effort to analyze the performance one of the most popular 1D safety codes RELAP5/MOD3.4 when applied to the geometry and operating conditions of the PRECISE facility. The calculation setup and modelling are presented along with the obtained results, which are validated against the previously acquired experimental values