Double Ended Guillotine Break in a Prismatic Block VHTR Lower Plenum Air Ingress Scenario

The double ended guillotine break leading to density-driven air ingress has been identified as a low probability yet high consequence event for Very High Temperature Reactor (VHTR). The lower plenum of the VHTR contains the core support structure and is composed of graphite. During an air ingress event, oxidation of the graphite structure under high temperature conditions in an oxygen containing environment could degrade the integrity of the core support structure. Following this large break, air from the reactor containment will begin to enter the lower plenum via two mechanisms: diffusion or density driven stratified flow. The large difference in time scales between the mechanisms leads to the need to perform high fidelity experimental studies to investigate the dominant the air ingress mechanism. A scaled test facility has been designed and built that allows the acquisition of velocity measurements during stratification after a pipe break. A non-intrusive optical measurement technique provides full-field velocity measurements profiles of the two species Particle Image Velocimetry (PIV). The data allow a more developed understanding of the fundamental flow features, the development of improved models, and possible mitigation strategies in such a scenario.Two brine-water experiments were conducted with different break locations. Flow fronts were analyzed and findings concluded that the flow has a constant speed through the pipe after the initial lock exchange. The time in which the flow enters the lower plenum is an important factor because it provides the window of opportunity for mitigation strategies in an actual reactor scenario. For both cases the flow of the heavier density liquid (simulating air ingress from the reactor containment) from the pipe enters the reactor vessel in under 6 seconds. The diffusion velocity and heavy flow front of the stratified flow layer were compared for the SF6/He gas case. It is seen that the diffusion plays less of a role as the transport mechanism in comparison to the density-driven stratified flow since the velocity of the diffusion is two orders of magnitude smaller than the velocity of the stratified flow mechanism. This is the reason for the need for density-driven stratified flow investigations following a LOCA. These investigations provided high-quality data for CFD validation in order for these models to depict the basic phenomena occurring in an air ingress scenario.