Molten ceramic solidification during molten state processing of HLW
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Molten ceramic solidification during molten state processing of HLW Hajime Kinoshita1, Masayoshi Uno2, Shinsuke Yamanaka2 and William E. Lee1 1 Immobilisation Science Laboratory, The University of Sheffield, Sheffield, S1 3JD, UK 2 Department of Nuclear Engineering, Osaka University, Suita, Osaka 565-0871, Japan
ABSTRACT Solidification of molten ceramics during molten state processing of High-Level Waste (HLW) was studied to investigate a possibility of a simpler and flexible process for immobilisation of HLW. Simulated HLW was heated together with different reducing agents (TiN with AlN or TiO2) either at 1673 K or 1873K for 1 to 3 hours under Ar atmosphere. Cooling rates of 5 K/min and 10 K/min did not result in any obvious differences in the products obtained under the standard condition used in the present study. The composition and quantity of the reducing agent had a significant effect on the melting and solidifying behaviour of the simulated HLW. The results suggest the possible further studies to improve the solidification process and the final product.
INTRODUCTION Molten state processing for HLW has been studied to develop a simple, rational and effective solidification route for immobilisation of nuclear wastes [1-4]. This heating process consists of 5 stages as is shown in Figure 1: (1) all the wastes are in nitric acid solution as High Level Liquid Waste (HLLW) together with corrosion products (CP) from the vessels, (2) H2O and NOx in HLLW first vaporize at 973 K in the calcination process, (3) Cs and Rb vaporize, at temperatures up to 1273 K, during heating of calcined HLLW and would be recovered by a cold trap for storage, (4) further heating with a desired amount of reducing agents, typically a mixture of TiN and AlN causes reduction and melting of waste elements, and (5) the molten wastes separate into an alloy phase and an oxide phase depending on the stability of their oxides. Both phases are obtained separately after cooling. One of the key aspects of the process is that some of the elements are reduced to the metal and excluded from the oxide phase that will be treated as a waste form. This reduction of certain elements reduces the quantity of waste. As Figure 2 shows, elements with higher standard free Reducing
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