In the event of hypothetical accident scenarios in a pressurized water reactor (PWR), emergency strategies have to be mapped out, in order to guarantee the reliable removal of decay heat from the reactor core. One essential passive heat removal mechanism is the reflux cooling mode. It can appear during a small break loss-of-coolant accident (LOCA) or because of loss of residual heat removal (RHR) system during midloop operation at plant outage. In the scenario of a loss-of-coolant accident (LOCA) in a PWR, it is considered that the reactor will be depressurized and vaporization will take place. This should lead to “reflux condensation,” which may be a favorable event progression; the generated steam will flow to the steam generator. This steam will condense in the steam generator, and the condensate will flow back through the hot leg to the reactor, resulting in countercurrent steam/water flow.

For a given condensate flow rate, if the steam mass flow rate increases to a certain value, a portion of the condensate will exhibit a partial flow reversal by the steam in the opposite flow direction towards the steam generator. This phenomenon is known as countercurrent flow limitation (CCFL). The keys to CCFL control are an improved understanding of these conditions, development of a suitable experimental data base, and novel tools to characterize the practical conditions in order to produce a better physical CCFL model.

We invite investigators to contribute original research articles as well as review articles that will simulate the continuing efforts to understand this important phenomenon. We are interested also in articles that explore the CCFL in a simple pipe configuration in order to support the physic behind the CCFL phenomena. Potential topics include, but are not limited to:

• New experimental investigations on the countercurrent gas-liquid two-phase flow in a PWR hot leg as well as in downcomer and tie plate during blowdown
• Development of experimental database on CCFL in a simple pipe/channel construction
• Development of phenomenological and analytical model for the prediction of CCFL in a certain part of a PWR as well as in a simple channel construction
• Computational fluid dynamics (CFD) effort on CCFL and relating phenomena, such as water hammer, steam condensation, and reflooding behavior
• Latest experimental techniques require to investigate CCFL and provide data for CFD validation
• Future discussion on R&D need about CCFL issues, with a focus on fuel coolability and relating severe accidents in a nuclear reactor

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