UPTF/TEST8B/RUN111, Flow Patterns in Hot or Cold Leg, PWR Large Break LOCA

1 - Description of test facility: The Upper Plenum Test Facility (UPTF) is a geometrical full-scale simulation of the primary system of the four-loop 1300 MWe Siemens/KWU pressurized water reactor (PWR) at Grafenrheinfeld. The test vessel, upper plenum and its internals, downcomer, primary loops, pressurizer and surge line are replicas of the reference plant. The core, coolant pumps, steam generators and containment of a PWR are replaced by simulators which simulate the boundary and initial conditions during end-of-blowdown, refill and reflood phase following a loss-of-coolant accident (LOCA) with a hot or cold leg break. The break size and location can be simulated in the broken loop. The emergency core coolant (ECC) injection systems at the UPTF are designed to simulate the various ECC injection modes, such as hot leg, upper plenum, cold leg, downcomer or combined hot and cold leg injection of different ECC systems of German and US/Japan PWRs. Moreover, eight vent valves are mounted in the core barrel above the hot leg nozzle elevation for simulation of ABB and B and W PWRs. The UPTF primary system is divided into the investigation and simulation areas. The investigation areas, which are the exact replicas of a GPWR, consist of the upper plenum with internals, hot legs, cold legs and downcomer. The realistic thermal-hydraulic behavior in the investigation areas is assured by appropriate initial and boundary conditions of the area interface. The boundary conditions are realized by above mentioned simulators, the setup and the operation of which are based on small-scale data and mathematical models. The simulation areas include core simulator, steam generator simulators, pump simulators and containment simulator. The steam production and entrainment in a real core during a LOCA are simulated by steam and water injection through the core simulator. 2 - Description of test: This separate effects test was performed to investigate flow patterns in the hot or cold leg during end-of-blowdown, refill and reflood phase of a postulated large break LOCA of a PWR. Boundary Conditions: While the pump simulator in loop 1 was closed, the broken hot and cold leg were open to the containment simulator. The pump simulator of loop 3 was set to a K-factor of 18 (blocked pump) related to 0.750 m diameter in order to establish a nearly constant differential pressure of about 0.25 bar between upper plenum and downcomer. The pump simulator of loop 2 was set to a K-factor of 18, related to 0.750 m diameter to simulate the maximum value of the pressure drop across a blocked pump during reflood. Subcooled ECC was injected into the cold leg 2 or hot leg 2, and steam was introduced below the tie plate via the core simulator system. Significant Findings: The test results showed water plug formation in the cold leg immediately after start of ECC injection. The water plug filled the pipe between ECC injection port and downcomer and showed small axial oscillations around the injection port. At ECC-flow rates lower than 250 kg/s the slug to stratified flow transition occurred. Also in the hot leg flow reversal and plug formation were observed immediately after start of ECC injection. When the hot leg water plug filled the steam generator (SG) inlet plenum and penetrated into the cyclones of the steam generator simulator (simulating the tube sheet of the PWR-SG), the corresponding net steam production of the PWR-SG was calculated within the SG-Feedback System and steam was injected into the upper plenum of the SG simulator. This steam injection caused an increased pressure difference between SG simulator and upper plenum. Owing to this increased pressure difference the subcooled water plug was pushed into the upper plenum where it immediately broke through the tie plate into the core simulator region without being affected by the up-flowing steam. Afterwards, a new subcooled water plug was built in the hot leg. The transition from plug flow to stratified flow occurred in the hot leg when the ECC injection rate was lowered from 200 to 150 kg/s. 3 - E xperimental limitations or shortcomings: The system operating pressure was limited to 20 bar. So for some experiments pressure scaling was necessary