Model for Grain Growth in DOP-26 Iridium Clad Viewgraphs.

Introduction: The fuel in a General Purpose Heat Source (GPHS) used in radioisotope power systems is contained within an iridium based DOP-26 alloy (with 0.3 wt% tungsten and doped with thorium) clad [1]. Past testing has shown that to ensure that the clad remains strong and resilient, it is important that the number of grains through the clad thickness be sufficient to avoid brittle behavior. The nominal grain size for a fresh fueled clad is about 25 μm. This allows about 26 grain diameters through the clad nominal thickness of 640 μm. The grains will grow at elevated temperatures and the growth rate will increase with temperature. This report describes the growth rate and suggests an improved model for grain growth. Past Model and Data: Measurements of grain growth in DOP-26 Ir with time and temperature were made by McKamey, et al [2, 3, 4]. To correlate the data, Reference 2 used the theoretically derived equation for normal curvature-driven grain growth, which is as shown below if p is set equal to 2: