Immobilization Of Simulated ～(137)Cs, ～(90)Sr Nuclide Waste In Synroc

Partitioning/conditioning strategy is considered as one of ways to treating high level liquid waste (HLLW). According to the strategy, minor actinide waste and long-lived fission products will be separated from HLLW and immobilized in glass or Synroc. The remaining intermediate-low level waste (ILLW) is immobilized in cement. The strategy will greatly reduce the volume of high level waste (HLW), and it can also reduce the long-term potential hazard associated with HLW destined for geological disposal.⁹0Sr and ¹37Cs have been the main heat-generating radionuclides for the first 1000 years. In this research, the immobilization of ⁹0Sr and ¹37Cs separated from HLLW in synroc had been systematically studied. Sr and Cs in synroc mainly inhabit perovskite phase and hollandite phase respectively.Six different formulae with Sr waste loading 5-48wt% of perovskite-rich synroc samples are designed and fabricated. The designed phase assemblage is 85wt% perovskite, 10wt% zirconclite and 5wt% rutile. The fabricated samples all have good physical properties: density (≥4.11g/cm³)and open porosity(≤ 0.1%).By X-ray diffraction (XRD), perovskite phase is the major mine phase in Synroc, zirconclite and rutile may exist as mirror phase, which is concordant with the formulations design. The ASTM C1220-98 tests (90 ℃, 28days) show that all the samples are highly durable, with mass loss leach rate and element nominalized lower 2 order of magnitude than glass waste form.Hollandite-rich phase assemblages of Synroc for Cs immobilization are designed: 85wt% hollandite,10wt% zirconclite and 5wt% rutile. The chemical formula of hollandite phase is Ba₁-xCs₂x(Al₀.5Ti₁.5)Ti₆O₁6. According to the chemical formula, eight Synroc samples with different waste loading are fabricated and evaluated. The results show that the fabricated samples have good physical properties; Synroc phase assemblage accords with the target design. However, the ASTMC1220-98 leaching results show that if the wasteloading of Cs is too high , the leach rate of Cs will increase rapidly.Al content of hollandite phase in Synroc is important factor influencing the immobilization of Cs. Synroc samples with different Al content are designed and fabricated, and the fabricated samples are systematically studied. X-ray diffraction (XRD) for the samples show that when samples have high Al content, Al will combine with Cs to form easily-leached CsAlTiO4, which will result in Cs leach rate increasing rapidly. But by the scanning electron microscopy (SEM) analysis , when Al content in Synroc is too low, hollandite phase will be difficult to form to crystals, which also will result in Cs leach rate increasing. The research find that under strongly reducing atmosphere and through displacing Ali+ by Tii+, it will be help for immobilization of Cs in hollandite phase, in which Al/Ti ratio on the C site should maintain 1:1. So chemical formula of hollandite phase is Bai-xCs2x(AliTii)Ti60i6.Based on the above research, phase assemblage of Synroc for the immobilization of Sr/Cs is designed, chemical formula of hollandite phase and perovskite phase is Bai-xCs2x(Al1Ti1)Ti6016 and Ca1^SrxTi0:i respectively. Through studying on physical properties, phase assemblage, mic-strcture and chemical durability for the fabricated samples, the results the samples have good physical properties, phase assemblage of the samples is coincident with the original design. The samples which have no more than the 10wt% waste loading are all fine grained with crystal size less than lym , and grain boundaries are narrow and triple points are clear. Thus, there are almost no glass phase in the samples, And the leach rate of Cs and Sr (ASTM C1220-98, 90°C, 28 days) is 2. 28X10:ig ? m"2 ? d"! and 4. 75X 10"2 g . m"2 . d'1 respectively, which is lower than the value from the reference document. Thus, it greatly improved the Sr/Cs immobilization ability of Synroc.