TS-131
TRANSIENT PHENOMENA CAUSED BY RAPID CONTACT OF HIGH-PRESSURE
SATURATED WATER WITH COLD WATER
Akitoshi NAKAMURA* Akira SAITO* Tomoji TAKAMASA*
Abstract
This paper describes our preliminary study of high-pressure saturated water that rapidly contacts low-pressure, low-temperature water. The purpose of the study was to clarify the transient phenomena that occur when high-pressure water suddenly contact cold water. The results revealed that flashing of high-pressure saturated water and a subsequent flashing hammer occurred under the specified experimental settings. Pressure peaked when steam generation or flashing occurred at the wall surface and the flashing steam condensed. After the peak, pressure oscillated and reached equilibrium condition in a short time. The pressure oscillation might have been caused by a balancing action between the flashing of high-pressure saturated water and condensation of the steam generated by flashing in cold water. To check the results of the experiments, further experiments and numerical analysis were conducted for the case of saturated high-pressure water discharging into an air field.
Keywords : Flashing, Condensation Transient Phenomena, Pressure peak , Flashing hammer
INTRODUCIION
High-pressure saturated water occasionally contacts low-pressure, low-temperature on cold water in a ship's heat plant. For instances, it can be arose when opening a valve which separates high-pressure saturated water and l low-pressure, low-temperature waterline, discharging high-pressure saturated water from feed water line into a feed water-tank or a distilled water tank during boiler maintenance work, and discharging high-pressure saturated water from a pressure vessel into a containment in a wall-crack accident or loss-of-coolant accident (LOCA) in an advanced marine reactor with water-filled containment. The discharged saturated water causes flashing and generates steam in cold water. Steam is then condensed by cold water. In some cases, peaked and oscillated pressure might be caused by such thermohydraulic action.
In recent years, various countries have performed research and development in advanced passive-safety reactors with water-filled containment, with the aim of improving the safety of nuclear reactors. In the case of a wall-crack accident or loss-of-coolant accident (LOCA) in an advanced reactor, high-pressure saturated water is discharged from a pressure vessel into the containment and is cooled by pool water in the containment. Water-filled containment makes it possible for the reactor core to be kept under water even If a LOCA Occurs. Decay heal, will be removed by the passive cooling system, so secondary shielding can be eliminated or greatly reduced with water-filled containment. This reduces the weight and volume of the reactor, which is especially advantageous for marine use. In a wall-crack accident or LOCA in such reactors, the discharged saturated water causes flashing and generates steam. Steam then is condensed by the pool in the containment.
The phenomenon of flashing in pool water involves overlapping phenomena of saturated water flashing and steam condensation in pool water. The phenomenon of flashing, which consists of a flow of saturated water discharged from a nozzle or orifice into gas field with a pressure lower than that of the saturated water, has been the subject of studies in the machine industry since the end of the nineteenth century [1-3]. The phenomenon of steam condensation in pool water has also been a subject of study in relation to boiling water reactors (BWRs) with pressure-suppression containment. Such containment minimizes the volume of steam by letting it condense, thus preventing large temperature and pressure increases in the system in the event of a LOCA [4-6]. An investigation of flashing phenomenon in pool water is essential for verifying the safety and reliability of new concepts for advanced reactors.
Takamasa et al. conducted experimental studies of high-pressure saturated water discharging into pool water to clarify the phenomena that occur when high-pressure saturated water is blown down from a pressure vessel into a water-filled containment, as in the case of a wall-crack accident or LOCA in an advanced reactor [7-9]. The results may be summarized as follows: Flashing oscillation (FO) occurred when high-pressure saturated water was discharged into pool water under specified experimental settings. The occurrence of flashing oscillated between a point very close to the vent hole and a point some distance away. They called the flashing very close to the vent hole "Phase A" and the flashing at some distance from the vent hole "Phase B." The pressure in the vent tube oscillated periodically and synchronized with the pressure of the pool water. Pressure in the vent and of the pool water varied according to the FO and peaked when the pressure oscillation appeared in the Phase B region. The oscillations of pressure and flashing location were caused by a balancing action among the supply of saturated water, flashing in the control volume, and steam condensation on the steam-water interface. A linear analysis was conducted using a spherical flashing bubble model. The period of flashing oscillation in the experiments could be explained by theoretical analysis.
