Borosilcate Glass for Storing High Level Radioactive Wastes

High level wastes (HLW) arise from reprocessing of nuclear power plant fuel. The wastes which are very hazardous to man, consist of fission products (mainly Cs, Sr, Mo, Zr, and rare earths), corrosion products from stainless steel and residual actinides. Borosilicate glasses for the immobilisation of these wastes were developed by the US Atomic Energy Commission in the 1950s and were scaled up to tonne-sized canisters in the late 1960s. However, Pennsylvania State University workers in the mid-1970s recognised that there were limitations of borosilicate glass, particularly with regards to its poor aqueous durability at temperatures above 100°C.
Supercalcine Ceramics for Storing High Level Radioactive Wastes

They devised ceramics based on crystalline silicates, phosphates and molybdates as an alternative. These so-called supercalcine ceramics were sintered in air at ~1100°C and had very high loadings of fission products, typically 70 wt% (simulated by stable isotopes in the experimental work), and the chemistry of the different phases was driven by the fission products as majority components. Typical phases were pollucite, CsAlSi206; powellite, CaMo04; and rare earth apatites and phosphates (e.g. monazite, REP04, where RE = trivalent rare earth). All of these had mineral analogues which were known to be very durable in hot, wet conditions, in which some could be shown to have survived for many millions of years. Hence they should be ideal candidates to be emplaced in a deep geological repository.
Titanate Ceramics for Storing High Level Radioactive Wastes

Following work at Sandia National Laboratories on phase assemblages occurring on heating sol-gel titania particles on which HLW fission products and actinides were sorbed, Ringwood and his coworkers in the late 1970s devised a multi-phase titanate ceramic in which nearly all these fission products and actinides were incorporated substitutionally in the various mineral analogue phases. These theoretically-dense materials were made by first mixing inactive precursors with liquid (simulated) HLW, followed by drying and calcining in a H2/N2 atmosphere for 1h at 750°C. The calcine was then mixed with 2 wt% of powdered Ti metal for redox control and then subjected to uniaxial graphite die hot-pressing or hot isostatic pressing at ~1200°C The precursor composition is (wt% oxide): A1203 (5.4); BaO (5.6); CaO (11.0); Ti02 (71.4); Zr02 (6.6).
Synroc-C for Storing High Level Radioactive Wastes

Since 1984, rather than using oxides, a slurry mixture of Ba and Ca hydroxides and transesterified Al, Ti and Zr alkoxides has been used as the precursor. This provides better solid-state reactivity. The principal advantage of this Synroc-C ceramic was that the waste ions were dilutely incorporated in durable titanate mineral phases which were considerably more insoluble in water than the silicates and phosphates etc in the supercalcine ceramics mentioned above. The waste loading could be varied between zero and 35 wt% using the same inert additive chemistry without substantially changing the basic zirconolite + perovskite + hollandite + rutile phase assemblage, although the percentages of the different phases varied somewhat. There were minor alumina-rich phases, plus minor metallic phases arising from the 3d and Pd metal group elements which were reduced to metals under the reducing conditions prevailing during hot-pressing.

Table 1 shows the phase constitution of Synroc-C, containing 20 wt% HLW and which radionuclides are incorporated in the various mineral-analogue phases present.

Table 1. Composition and mineralogy of Synroc-C.

Phase


Wt.%


Radionuclides in lattice

Hollandite Ba(Al,Ti)2Ti6O16


30


Cs, Rb

Zirconolite CaZrTi2O7


30


RE, An*

Perovskite CaTiO3


20


Sr, RE, An

Ti oxides


10




Alloy phases


5


Tc, Pd, Rh, Ru, etc

*RE, An = rare earths and actinides respectively.
Synroc-D for Storing High Level Radioactive Wastes

Another variant (Synroc-D) was devised to immobilise the defence wastes at the Savannah River site, SC, USA. These wastes were basically Al-Fe hydroxide sludges containing some Na and only ~0.1 wt% of fission products. In 1981, there was a selection process in the US for candidate waste forms for this waste. Synroc came second behind borosilicate glass in this assessment.
Synroc Production

There was a recognition of improved aqueous durability of the Synroc, but it lost out through lack of engineering development. As a result, the Australian Government financed in the mid-1980s the construction of the well-known Synroc Demonstration Plant at ANSTO. This plant can produce (inactive) Synroc monoliths at the 50 kg scale with a throughput of 10 kg/h.