Roasting and Dissolution Studies on Nonirradiated Thorium Dioxide/Uranium Dioxide Pellets.

摘要

Bench scale roasting and dissolution of ThO sub 2 and ThO sub 2 /UO sub 2 ractor-grade ceramic pellets were studied at the Savannah River Laboratory to define the key parameters affecting dissolution. Pellet breakup, and subsequent dissolution rates, were determined for ThO sub 2 and ThO sub 2 /UO sub 2 pellets roasted in air or in oxygen. Roasting ThO sub 2 /UO sub 2 pellets in air at temperatures from 400 to 900 exp 0 C caused the pellets to crack but not fragment. Roasting whole pellets or fine powdered materials decreased the rate of dissolution in a nitric acid solution containing a fluoride catalyst. Roasting 100% ThO sub 2 pellets did not cause cracking or affect the subsequent dissolution rate. Mixed ThO sub 2 /UO sub 2 ceramic pellets dissolved at a faster rate than the 100% ThO sub 2 pellets. The effect of MgO and CaO on dissolution rate was determined. MgO (approx. 1.0 wt %) increased the dissolution rate of ThO sub 2 pellets, an effect which was similar to that obtained by the addition of 20% UO sub 2 to the ThO sub 2 pellets. The combination of 1% MgO and 20% UO sub 2 did not result in an additional increase in dissolution rate. However, the addition of 0.25 to 0.50 wt % CaO did increase the dissolution rate of 80% ThO sub 2 /UO sub 2 ceramic pellets. High temperatures (and pressure) were ineffective in dissolving thoria-based fuels in HNO sub 3 in the absence of a fluoride catalyst. A process flowsheet outlining the required head end steps for the reprocessing of thoria-based fuels was developed. (ERA citation 07:052867)