MASLWR Overview and RELAP5 Simulation

The focus of this presentation is to demonstrate the functionality of the Multi Application Small Light Water Reactor (MASLWR) thermal hydraulic test facility and it’s natural circulation operating behavior as well as analyze the ability of a RELAP5-3D model to simulate this behavior. The MASLWR facility is introduced with a general overview. Test data collected from steady state operation of the natural circulation configuration of the MASLWR facility will be analyzed and compared to a similar RELAP5-3D simulation.\ud \ud The MASLWR Test Facility models the NuScale design of a next generation nuclear power reactor. It is used to provide test data to NuScale which is used to verify their thermal hydraulic codes modeling their own reactor.\ud \ud The primary loop of the MASLWR is fully contained in the RPV. The electrically heater core heats the water at the bottom of the RPV, this hot water rises through the hot leg located centrally in the RPV. At the top of the hot leg the hot water is directed outward and downward to the helical coil steam generator where it is cooled. Once the water is cooled and becomes more dense it then falls down the cold leg and is recirculated into the core. The flow through the primary loop is driven by natural circulation due to the density gradient. In the secondary loop cold water is pumped through the helical coil steam generator. The water boils and exits the coils as steam. Multiple instruments collect data from the steam and then it is vented to atmosphere. The RPV is configured to simulate being located inside a High Pressure Containment (HPC) [1].\ud \ud The test facility has multiple types of instruments that collect information on pressure drops, temperatures and flow rates in both the primary and secondary loops. Testing has been performed on the MASLWR test facility to analyze the steady state natural circulation in the primary loop. Steady state was achieved at multiple power levels by varying the flow in the secondary loop. At each power level, different flow rates and temperature distributions were observed. This data will serve as a standard for comparison of our RELAP5-3D simulation.\ud \ud Reactor Excursion and Leak Analysis Program 5 (RELAP5) is a thermal-hydraulic safety analysis software package specifically catered to nuclear reactor operations [2]. A RELAP model of the MASLWR test facility has already been built by Dr. Brian Woods and Jordan Bowser of Oregon State University. This model has been modified to fit the parameters of the test data collected. The flow losses built into this model are the most accurate that can be determined with presently available test data, but could be greatly improved with future testing of the MASLWR test facility. The model was run at multiple power levels to determine flow rates required to maintain steady state of the natural circulation system. The RELAP model consists of all components described in the MASLWR test facility overview with a simplified heat removal structure in place of the helical coil steam generator. Steady-state operation of the facility is defined as unchanging conditions in both the primary and secondary systems with respect to time during operation.\ud \ud Data from the MASLWR testing and the RELAP model were compared. The specific paremeters of the most interest are mass flow rate of the primary coolant, and temperature of the coolant at different locations throughout the natural circulation loop. The temperature data from the two sources turned out to be within the tolerances of the testing instruments. The mass flow rate data from the RELAP model underestimated the true data and was outside of the instrument tolerances.\ud \ud In conclusion the inaccuracy of the mass flow rates can be connected to the inaccurate flow losses built into the RELAP model. It is intended that future testing of the MASLWR test facility will be used to determine more accurate flow losses