Effect of first wall flaws on reactor performance

The influence of primary blanket coolant leaks through flaws in the first wall on Tokamak fusion plasma performance is investigated. A one-dimensional, three region radial diffusion model of impurity transport in the plasma is developed. The model includes plasma removal and recycling in the boundary layer and is used to correlate plasma performance with coolant leakage rates. In turn, coolant leak rates are estimated via molecular or viscous flow, as appropriate through both elastically loaded cracks and cracks opened by creep. Fatigue and environmentally induced subcritical crack growth and unstable crack growth flaw sizes are estimated for austenitic and ferritic stainless steels and compared to the coolant leak rate flaw sizes. The materials and plasma analyses are combined to yield estimates of the critical leak rate for three coolant candidates: helium, water, and lithium. The results indicate that the maximum-leak-rate (MLR) flaw sizes for helium, water and lithium are 5 to 8 mm, 6 to 8 mm and 1.5 to 3 × 10⁴ mm, respectively, for elastically loaded cracks and 0.1 to 0.3 mm, 0.2 to 0.3 mm, and 700 to 1500 mm, respectively, for cracks opened by creep. The threshold flaw sizes for fatigue and corrosion fatigue subcritical crack growth of Type 316 stainless steel and HT-9 range from 0.2 to 2 mm and the flaw sizes for unstable crack growth of irradiated 316 stainless steel and HT-9 are 4 mm and 50 mm, respectively. These results suggest that the MLR sizes for helium and water are among the smallest flaws that may affect reactor performance and that the threshold for subcritical crack growth in fatigue is a critical material property. Also, creep processes are shown to have a significant effect on the MLR flaw size.