Use of Similarity Analysis on Experiments of Different Size to Predict Critical Cooling Rates for Large Ceramic Waste Forms

Ceramic waste forms (CWF) are produced to store fission products for the long term. They are cast into cylindrical shape at high temperature (925°C). Rapid cooling of the product is desirable for product turnaround, but cooling has the potential to crack the coalesced product into many pieces due to thermal stress. This paper investigates the rapid-cooling process with a borosilicate-glass component of the CWF used as a surrogate. The critical cooling rate of formed cylinders (the rate which separates the damage from the no-damage region) has been determined. This paper extends previous experimental data and analysis to production temperature as a step in the extrapolation of the data to production CWF’s. The glass solidifies in the range of 650°C to 625°C. The previous tests (7.8-cm diameter) were all run starting from a solid (625°C or less) to provide a basis for the higher temperature cases. Thermal stress cannot build up until solidification begins to occur. The current tests (7.8 and 9.9cm diameter) were run from the liquid temperature of 925°C. A theoretical model has been developed to analyze the data. The model includes heat transfer and the stress developed from the thermal gradients. Similarity analysis based on this model is used to produce dimensionless charts which allow data of different initial temperatures and diameters to be analyzed. The new data corroborated the previous estimate of the critical cooling rate and analytical-model projection for the minimum in-furnace cooling times for two production size CWF’s that will be stored in Yucca Mountain (70 hours for the 52-cm diameter and 35 days for the 181.5-cm sizes). To further reduce these times, an analytical prediction was made which shows that the formed cylinder can be removed from a furnace at a temperature of 320°C without any danger of cracking.