Immobilization of High Level Nuclear Reactor Wastes in Synroc: A Current Appraisal
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IMMOBILIZATION OF HIGH LEVEL NUCLEAR REACTOR WASTES IN SYNROC: APPRAISAL
A CURRENT
V.M. OVERSBY* AND A.E. RINGWOOD Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia
ABSTRACT 0
Results are presented for leach testing at 95 C and 0 200 C of SYNROC containing 9% and 20% simulated high level radioactive waste, synthetic hollandite and pervoskite samples, and natural zirconolite and pervoskite samples. Single phase synthetic minerals show much higher leach rates than natural mineral samples and polyphase SYNROC samples. Natural zirconolite samples with low 0 radiation damage have leach rates at 200 C based on U which are identical to those measured on SYNROC samples. Natural zirconolites with very large accumulated a dose 0 and radiation damage have leach rates at 200 C which are only 5 times higher than those of low dose samples.
INTRODUCTION SYNROC is a titanate ceramic material. The formulation proposed for co'mercial reactor high level wastes consists of three minerals - zirconolite CaZrTi 2 0 7 , "hollandite" BaAl 2 Ti 6 O0 6 , and perovskite CaTiO3 . These minerals have the capacity to accept nearly all of the elements present in high level radioactive wastes (HLW) into their crystal lattices as solid solutions. The SYNROC minerals occur in nature. This allows us to study the long term stability of minerals which have existed in a variety of geochemical environments for periods of hundreds of millions of years. Information gathered from natural mineral studies can be combined with laboratory experiments on synthetic waste form minerals to aid in predicting long term performance of SYNROC as a waste form. This paper will discuss the results obtained from leach testing of SYNROC containing simulated high level waste, synthetic perovskite and hollandite samples, and natural zirconolite and perovskite samples. A more detailed account of the SYNROC leaching data may be found in reference [I]. Sample preparation and production technology are discussed in reference [2]. PREPARATION OF SYNROC AND SYNTHETIC MINERALS Two chemical techniques have been used to prepare materials used in our leach testing program. The first method (which will be referred to as the oxide route) involved mechanically mixing CaC0 3 , BaC0 3 , Zr02 , Al(OH) 3 *Present address:
Lawrence Livermore National Laboratory, Livermore,
CA
76
and Ti0 2 (anatase). The mixture was then slurried in water, nitric acid added to decompose carbonates and finally a solution of simulated waste elements was mixed in. The slurry was rapidly pre-calcined, mixed 0 mechanically and then calcined in an Ar-H 2 atmosphere at 750 C to produce the desired redox state. Samples were then hot pressed in graphite dies at 0 4000 psi and temperatures of about 1200 C. (Exact temperature depended on the nature of the sample being hot pressed. For hollandite, best results were 0 obtained at 1230 C). If samples were pressed in metal capsules, it was necessary to add about 2% Ti metal to the calcined material to ensure proper redo
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