Current T esearch on

Hollandite,

E. R. VANCE, B. D. BEGG, M. W. A. STEWART, S. MORICCA, K. L. SMITH, P. A.  WALLS, D. S. PERERA, R. A. DAY, M. L. CARTER, P. J. McGLINN, Y. ZHANG and B.  THOMAS, Materials & Engineering Science, ANSTO, Menai, NSW 2234

in lattice

30 Cs, Rb

In 1978, Ringwood (1) suggested ceramic  assemblages of titanate minerals could be used to  incorporate high-level waste from nuclear fuel  reprocessing. In these assemblages waste ions are  dilutely incorporated into the crystalline mineral-  analogue phases. Synroc-C

is one of the early

titanate assemblages and it has become the  archetype from which waste forms for various  applications have been derived. Table 1 shows the  phase constitution of synroc-C, containing 20 wt%  HLW, and the radionuclides which can be

incorporated in the various phases. This material  was consolidated into a dense ceramic by uniaxial  hot pressing at - 1150Â°C .

Table 1. Composition and mineralogy of synroc-C  1 Phase I wt% I Radionuclides I

Ba(A1,Ti)2Ti(jO16 I  Zirconolite, I 3 0  1 RE,An*

Alloy phases Tc, Pd, Rh, Ru

etc.

* RE, An = rare earths and actinides respectively

In the 1 9 8 0 ~ ~  the inactive Synroc production  process was scaled-up via the Synroc

Demonstration Plant to produce - 50 kg. monoliths  containing 20 wt% of simulated HLW, with  properties as good as those of gram-sized

laboratory samples.

In the early 1 9 9 0 ~ ~  we recognised that Synroc  derivatives could be applied to a variety of high-  level radioactive wastes besides the Purex-type  waste for which it was mainly originally designed.  In addition, significant effort was put into the  development of alternative consolidation  techniques such as hot isostatic pressing, sintering,

and melting to expand the application of ANSTO's  titanate wasteforms. ANSTO has subsequently  undertaken both contract and collaborative work on  a variety of waste streams as described below.

A highlight of this latter work was a synroc  derivative developed by ANSTO and the US  Department of Energy for the immobilization of  surplus

US weapons Pu. The waste form (2) was  primarily composed of a pyrochlore-structured  phase ((CaAn)?Ti207), that can incorporate 10 and  20 wt% respectively of Pu and U oxides as well as  -10 wt% of Hf and Gd oxides (neutron absorbers).

This wasteform was selected by a competitive  process over all other candidate wasteforms by the  US government in 1997.

ANSTO's contributions to this project included  assisting in the development of the baseline ceramic  and investigation of the crystal-chemical, leaching,  and radiation damage characteristics of the  individual constituent phases (eg. brannerite).  Although the immobilisation option has been

suspended in the US, ANSTO's interest in  immobilisation of Pu-bearing waste remains.

Glass-ceramics have been devised for Hanford tank  wastes and sludges (3). These have high waste  loadings, in the 50-70 wt% range, and consist of  titanate phases designed to incorporate the fission  products and actinides in an aluminosilicate glass  matrix. They are made by melting techniques to

facilitate a high throughput.

High-uranium wastes from radioisotope production  at nuclear reactors can be immobilised in several  kinds of titanate ceramics. These have waste  loadings of up to 40 wt% and excellent leaching  behaviour.

For Al-rich wastes arising from reprocessing of Al-  clad fuels, synroc-D devised by Ringwood (1)  should be useful. This wasteform was first directed  towards the Al- and Fe-rich waste streams at

Savannah River, SC, USA but after borosilicate  glass was chosen over synroc in 1982, no further  work on this wasteform was done until 2002, when  the other Al-rich wastes became a possible  application.

"Tc is a volatile element and because of its long  lifetime, it features heavily in performance

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assessments at around the lo5 yr. mark. It can be  readily incorporated in titanate ceramics as a metal  under reducing conditions. The feasibility of  incorporating Tc as Tc4+ in a number of titanate  phases is being assessed, with the emphasis on hot

isostatic pressing to assure containment.

Mo-rich wastes would arise from reprocessing of  U-Mo fuels and present studies at ANSTO are  focussing on sintered ceramics containing powellite  (CaMo04) and sodium zirconium phosphate (NZP)  as key immobilisation phases. Waste loadings in  these materials are typically 50 wt%.

Following detailed leach studies of Cs-bearing  barium hollandite (4), we have investigated a wide  range of barium hollandite-type solid solutions  which can be sintered in air to incorporate Cs and  which have excellent leaching behaviour. In

further applications, we have been able to melt in  air Cs-bearing hollandite-rich samples without any  Cs loss or detriment to its aqueous durability.

Cements and geopolymers which are formed by  polymerisation of aluminosilicates dissolved in an  alkaline aqueous solution have been widely  advanced for immobilisation of intermediate-level

waste. However studies have only rarely been  made on the speciation of radionuclides in these  materials and we are utilising solid state nuclear  magnetic resonance, leaching tests, and scanning  microscopy to gain speciation information on Cs  and alkalis

as well as aluminium and silicon ions.

The incorporation of Iz9I into alumina or silica  beads is well known, but we are working on  encapsulation strategies using a variety of matrices  and processing options that avoid I volatilisation.

One of the strengths of ANSTO’s wasteform  development program is its extensive range of  capabilities for wasteform characterisation. Crystal  chemistry studies are targeted towards solid  solution behaviour, valences of actinides and other  ions and wasteform design. Experimental  techniques include X-ray diffraction, electron  microscopy (including electron energy loss

spectroscopy), X-ray absorption, diffuse relectance  and X-ray photoelectron spectroscopies, and solid  state nuclear magnetic resonance. Computational  techniques to calculate defect energies and  thermodynamic quantities are also being developed.

Aqueous dissolution is directed  at dissolution rates

of different actinide valences and the effect of  redox conditions, as well as the release of inactive  matrix elements. Radiation damage work involves  structural effects of heavy-ion irradiation and

doping with short-lived actinides such as 238Pu.  In conclusion, ANSTO wasteform research arid  development has broadened considerably in recent

years in regard to both wasteforms and their  processing. A recent highlight is the agreement  with the Russian Ministry of Atomic Energy to  build

a 20T/yr demonstration plant using real high-  level waste and some of ANSTO’s waste

preconditioning technology and wasteform design  skills.

REFERENCES

1. A. E. RINGWOOD, S. E. KESSON, N. G.  WARE, W. HIBBERSON and A. MAJOR,  “Geological immobilisation of nuclear reactor  wastes”, Nature, 278,219 (1979).  2 .  E. R. VANCE, A. JOSTSONS, S. MORICCA,  M. W. A. STEWART, R. A. DAY, B. D. BEGG,

M. J. HAMBLEY, K. P. HART and B. B.

EBBINGHAUS, “Synroc Derivatives for Excess  Weapons Plutonium”, in Ceramics Transactions  (Environmental Issues and Waste Management  Technologies IV), Volume 93, Eds. J. C. Marra and

G. T. Chandler, American Ceramic Society, pp.

3. E. R. VANCE, R. A. DAY, M. L. CARTER and  A. JOSTSONS, “A Melting Route to Synroc for  Hanford HLW Immobilization”, in Scientific Basis  for Nuclear Waste Management XIX, Eds. W. M.  Murphy and D. A. Knecht, Materials Research

Society, Pittsburgh, PA, USA, p. 289 (1996).  4. M. L. CARTER, E. R. VANCE, D. R. G.  MITCHELL, J. V. HANNA, Z. ZHANG and E.  LOI, “Fabrication, characterisation, and leach  testing of hollandite (Ba,Cs)(A1,Ti)2Ti601;’, J.  Mater. Res., 17,2578, (2002).

323-9 (1999).

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