Heat Transfer Design for Ceramic Waste Forms From Pyro-processing

Reprocessing spent nuclear fuel to produce fast reactor fuel and fission product waste forms by pyro-processing can expand the Yucca Mountain storage capacity by a factor of five and reduce the time of concern to 300 years. The consolidation (by heating to 900 deg C) and cooling of two different sizes of fission product ceramic waste form (CWF) destined for Yucca Mountain are analyzed. The first size is a 52 cm diameter cylinder from processing EBR-II spent fuel. The second is a 185 cm cylinder from processing spent LWR fuel in a 100 MT/yr reprocessing plant. The objective is to reduce the heating cooling time cycle to increase production. The 52 cm diameter CWF heats up to 900 C in 70 hours. This length of time is due to its low thermal conductivity. Inserting a small diameter stainless steel rod in the middle of the can was investigated to see if this time could be reduced. Rather than reducing the heatup time, it prolongs it because the heat capacitance of the steel is much larger than that of the ceramic. However other materials (aluminum and carbon) yield reduced heatup time. An aluminum rod would have to be encased in a thin walled steel tube since it melts at 660 deg C. After the material has been consolidated at the high temperature, it must be cooled. The consolidated material has a higher thermal diffusivity than the unconsolidated but is encased in an annular furnace. Data indicate that the overall heat transfer coefficient is 1 W/m2K or lower resulting in cooling times of 140 hours. The aluminum center rod also helps with the cooldown. If the CWF were removed from the furnace for cooling, cooling times would be reduced further (20 hours). Thermal stresses are low enough to allow this cooling but operational problems must be overcome to handle the 900 C CWF. The 185 cm diameter (6.1 foot) CWF presents more difficulty. Dimensional analysis allows the small cylinder results to be used to estimate heatup and cooldown times. The time is proportional to the radius squared (a factor of 12.6) resulting in 886 hours for heatup plus 1140 hours for cooldown (∼ 84 days). Operations of the 100 MT/yr reprocessing plant produces this much material every 15 days. A slight increase in furnace temperature and addition of an 18.5 cm radius aluminum center rod plus other remedies can reduce the heatup and cooldown times to approximately 15 days. (authors)