Precise REflux Condensation Investigation SEtup (PRECISE)

The project is dedicated to study reflux condensation phenomenon which occurs inside steam generator tubes of a pressurized water reactor (PWR) during severe accident conditions. Reflux condensation is capable of removing residual decay heat from the reactor coolant system (RCS) which suppresses or, at least, delays the core uncovery in a hypothetical loss of coolant (LOCA) accident. A large number of studies concerning reflux condensation were done assuming no core damage conditions, while proposed work will additionally include a severe accident scenario situations, where the reflux condensation process is heavily influenced by the presence of noncondensable gases (NC), such as hydrogen produced from oxidation of the fuel cladding. Good understanding of the phenomena is crucial for assessing the heat removal capabilities under a broad range of thermal-hydraulic conditions. The planned experimental setup is envisioned to use novel sensors to study the phenomenon under well controlled conditions and perform high resolution measurements of the thermal-hydraulic parameters and their influence on the heat removal capabilities in a single steam generator tube. Obtaining high-quality data is of interest to model development for the reflux condensation in computational fluid dynamics (CFD) codes and 1D system codes used in nuclear research and industry. Therefore, experimental results will be confronted with results of simulations and used to improve existing models.

The main goals of the project can be summarized as follows:

• provide high quality and resolution (temporal and spatial) data for reflux condensation (heat flux, temperature distribution, liquid film, NC gases distribution)
• study influence of NC gases on the heat transfer
• novel sensor development:
  • gradient heat flux sensor – first in the world application for studying reflux condensation
  • movable conductivity –void fraction probe to detect the condensate film
• provide data on NC-rich gaseous layer formation closed to condensate surface and relate its thickness to steam flow velocity
• based on gathered data, validate codes (Relap5/Mod3.2, CFD condensation model); possibly formulate an improved correlation for reflux condensation heat transfer
• reduce uncertainties in predicting nuclear accident progression by improving models and correlations in codes