Rewetting processes during PWR reflood
Creators: name: Ilyas, Muhammad
Publisher: name: Imperial College London
Year: 2011

description: Rewetting of heated surfaces is important in many physical processes and has important technological applications. Understanding of this phenomenon is required in many engineering and scientific fields. It is one of the most crucial phenomena to be considered for the safety analysis of the design basis Loss‐of‐Coolant Accident (LOCA) in light water reactors (Pressurized Water and Boiling Water Reactors). To mitigate the consequences of LOCA, water is fed into the reactor core via an emergency core cooling system; in the PWR, this water is fed to the core via the lower plenum (“bottom reflooding”) and in the BWR, this water is sprayed onto the top of the core (“top reflooding”). In both the cases, a quench front is formed which moves rather slowly. Ahead of quench front, complex and chaotic processes are occurring over a very small axial region where high temperature gradient exists. The heat transfer mechanism is not well known in this region. In this work, the detailed physics of the rewetting processes has been investigated both theoretically and experimentally. The thermal hydraulic behaviour of hot vertical channels during emergency core cooling conditions would be expected to be flow direction‐dependent, it was important to consider the two cases (top reflooding and bottom reflooding) separately. It was possible for the first time, to the author’s knowledge, to apply the fast response infra‐red thermal imaging system to study the rewetting process during top and bottom reflooding of heated vertical surfaces. The important contribution of this work was the use of this new technology to sense the variation of temperature with time at multiple nearby locations at the quench front. In the top reflood experiments, a heated stainless steel pate was quenched by a falling film flow. Through an infrared‐transparent substrate embedded in the plate and coated with platinum, temperature measurements at a location near the rewetting front were achieved using infrared thermal imaging system. The temperature/time traces showed fluctuations in temperature indicating occurrence of intermittent contacts at the quench front. A high speed video camera was also employed to capture rewetting processes by the visual observation of the rewetting front from the top surface. In the visual observations, the liquid film has been seen making intermittent contacts with the hot surface. In these experiments, the effect of the flow rate and the degree of sub‐cooling of the feed liquid has been studied. The rewetting temperature and the characteristic length of the intermittent contact region have been deduced from the experimental results. Experiments were also done to measure temperature changes at the rewetting front for the case of bottom reflooding of a heated tube using a similar technique to that employed for the studies of top reflooding. The results suggested that the rewetting behaviour was different depending on whether the reflood rate was high or low. For high reflood rate, the observations are consistent with the regime above the rewetting front being of the inverted annular type and, for lower reflooding rates, the results are consistent with the rewetting front corresponding to a film dryout in annular flow. An important finding from these experiments is the identification of transient temperature fluctuations in the transition region for the high flooding rate case. These are similar to those observed in the top reflood case and it seems very likely that these fluctuations are associated with intermittent wetting of the surface in this region. An attempt has been made to model rewetting phenomena in which the mechanism of heat transfer at the quench has been proposed. The postulated mechanism is transient near‐surface cooling resulting from intermittent solid‐liquid contacts, followed by recovery of the surface temperature of the metal substrate, with explosive vaporization occurring when the homogeneous nucleation temperature is restored at the metal‐water interface. A one‐dimensional rewetting model was constructed to explain the cyclical process; this model predicted the cyclical behaviour, with the expected qualitative dependence on system parameters. Its predictions are quantitatively consistent with experimental observation, in that the unsteady model analysis brackets the experimentally observed periodicity of the quasi‐steady actual process. The one‐dimensional model of the process has been complemented by twodimensional simulations using a commercial finite element code (ANSYS). In these simulations, an intermittent contact region has been modelled by imposing a heat transfer coefficient over a certain length between dry and wet regions. A parametric study was performed to see the effect of the rewetting velocity, the wet side heat transfer coefficient, intermittent contact heat transfer coefficient, and the length of intermittent contact region.