Waste solutions generated from reprocessing of nuclear spent fuel are nitric acid solutions containing high-level radioactive waste. The waste solution is melted together with glass in the melter. After cooling, consolidated radioactive waste with glass will then be disposed in deep geological layers.
Since the glass melt containing waste components is highly corrosive to the melter, inner refractories in contact with the glass melt require high corrosion resistance. The melt contacted refractory must also have high electric resistivity so that current does not leak when the glass is heated by Joule heating.
Our current melter adopts a ready-made refractory as the melt contact refractory with high corrosion resistance and high electrical resistivity. The glass melter will be renewed regularly because of deterioration or damage of the melt contact refractory. Because it is necessary to procure melt contact refractories stably for a long time, the development of new melt contact refractories are being promoted. A Cr2O3 type sintered refractory is studied to make it a product that can be manufactured in Japan.

In the glass melter of the reprocessing plant, the nitric acid solution containing high level active waste is mixed with the glass melt. Therefore, replacement or repair of the melt contact refractories are virtually impossible after the operation of the melter. The melt contact refractory for the glass melter requires more variable functions and higher performance compared with the refractory used in general industry. It is difficult economically and time-wise to evaluate the required function and performance repeatedly by mock-up tests using same size refractory of an actual melter. In addition, qualitative and quantitative viewpoints for adopting new glass melt contact refractories have not been defined. In our development, the same performance of the refractory adopted in existing melters is used as benchmark criteria for the new refractory.
In the development, manufacturing conditions of refractory (raw material, forming and baking conditions) are researched systematically and exhaustively at lab scale at first. Next, the manufacturing conditions were determined from the viewpoint of the corrosion resistance and the electrical resistivity, which are the required functions. Corrosion is the phenomena of chemical and physical reactions at the interface between the refractory and glass melt and the electrical resistivity is a macroscopic physical property unique to the composition and construction of the refractory. Therefore, it can be considered that the dependence of corrosion resistance and electrical resistivity on the test scale is small. In the next step, the size of the candidate refractory manufactured under the selected manufacturing conditions is increased. The larger refractory is placed in the test melter and actual scale evaluation proceeds in the environment of the actual glass melter. The required performances unique to melt contact refractory (such as spalling and corrosion resistance under the electrical current in the glass melt) are estimated continuously by lab scale tests. We then determine the test items conducted by the lab scale by comparing the results obtained at lab scale and the actual scale samples.

Based on the results obtained from the lab-scale tests that can acquire detailed and multifaceted analytical information, we will develop reasonable and accountable glass melt contacted refractories. We will proceed with cross-scale studies on a lab-scale and an actual scale and these studies should advance to application development in the future