3-D Computational Fluid Dynamics Model of Reactor Vessel Annulus with Direct Vessel Injection

The emergency core cooling system design for ABB. Combustion Engineering's System 80+ reactor includes direct vessel injection which sends cold water directly into the reactor vessel annulus. DPI differs from previous C-E designs where the water is first injected into the cold legs before it enters the annulus. The change in the injection method has necessitated a more detailed evaluation of the mixing of injected water with the system water in the annulus. The rationale for using DPI is provided by the reduction in the necessary safety injection pumping capacity while retaining the same safety margins and the increased reliability during Loss of Coolant Accident events as stated by the EPRI Utility Requirements. The injection of cold water, if not well mixed, could induce a local thermal stress in the reactor vessel wall which adds to the mechanical stress of the primary system pressure. This combination of stresses could challenge the integrity of the vessel wall, particularly at the elevation corresponding to the top of the active core where the neutron fluence in significant and the injected water may not yet have fully mixed with the fluid in the annulus. The objective of this study is to determine effects of varying the axial and circumferential location of the DPI nozzles, from a reference location, on the thermal mixing occurring within the annulus. The results are also compared to those from a second model where the safety injection water enters directly into the cold legs. A three dimensional model was developed using the computational fluid dynamics software Fluent to simulate flows through the outer annulus of the reactor core. The boundary conditions chosen for the model simulate a steam line break case with concurrent loss of reactor coolant pump operation. The case assumes a maximum DPI cold temperatures and coolant flowrate