Graphite aerosol release to the containment in a water ingress accident of high temperature gas-cooled reactor (HTGR)

As a design-basis accident, the water ingress due to the rupture of a single steam generator tube is a major threat for high-temperature gas-cooled reactors (HTGR). The rapid rise of primary pressure could reach the set pressure of safety valve and trigger the relief to the containment. For HTGR, the graphite dust particles, which usually carries fission product condensations, are also discharged to the containment during the pressure relief. The size distribution of the aerosol is crucial for the safety analysis of the containment in this accident scenario. Particularly, the high velocity flow in the safety valve may greatly change the size distribution of the graphite aerosol, due to the possible impact and fragmentation. In the present work we study graphite particle impact and fragment during pressure relief in the water ingress accident of HTGR by combining multiphase computational fluid dynamics (CFD) and finite element method (FEM). An Eulerian-Lagrangian scheme is employed to calculate the trajectory and instantaneous impact velocity of different sized particles. Then FEM with Hertz contact model is used to evaluate the outcome of the impact. CFD results show that the impact rate increases from 22% to nearly 100% as particle size increases from 0.1 mu m to 1.9 mu m. The critical fragment velocity is larger for smaller particle because the compressive strength limit increases due to Hall-Perch effect. By combining CFD and FEM simulation, it is shown that the possibility of fragmentation reaches 80% for particles larger than 1.5 mu m. Our work suggests that this fragmentation effect needs to be considered for the initial size distribution of graphite aerosol that is discharged to the containment.