Can anyone help answer this question?

Your supervisor asks you to calculate the design stress for a brittle nickel aluminide rod she wants to use in high temperature application in which the component is always in tension. In tensile testing of the given rods, the results yield an average tensile stress of 673 MPa and Weibull modulus of 14.7. What must be the design stress if 99.999 percent of the rods can be subject to the design stress without fracture?