Management of Commercially Generated Radioactive Waste - U.S. ...

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* firing the calcine, at temperatures of typically 9000 to 1400 0 C (temperatures
depend on the mineral being synthesized) to form the final well-crystallized
mineral phase.
The first step is not discussed here since the partitioned wastes are in nitrate solution.
Calcination can be carried out using the types of spray calciners that have already under-
gone considerable development and testing for the solidification of radioactive wastes. No
new technology is involved to adapt these devices to synthetic minerals and the expected
difficulties are those of remote handling and metering of the solutions and of calciner
operation. Firing the calcine to form the final crystalline product in general will require
temperatures that can be reached in base metal furnaces or gas-fired kilns.
Mineral synthesis by direct solid-state reaction is done as the name implies. The
radioactive waste and the other components needed to construct the mineral phase are mixed
as solids. The solid must be intimately mixed, ground, and compacted before reaction.
Reaction temperatures are higher and reaction times are longer because the components are
crystalline solids and transport can only take place by diffusion. The main difficulty
expected here is the maintenance of equipment at the high firing temperatures. There may
be more problems with furnace burn-out and breakage or fluxing of refractories. Rare earth
and actinide oxides, for example, tend to be very refractory and will require high reaction
temperatures if this method is employed.
Hydrothermal synthesis is the technique of reacting materials using high pressure, high
temperature water as both a solvent and as a catalyst. It has the tremendous advantage of
causing reaction between poorly reactive substances at modest temperatures (2000 to 800 0 C is
the experimental range) but it has the important difficulty of requiring reaction at high
pressure (hundreds of thousands of atmospheres). To this must be added the difficulties
associated with assembling and disassembling the pressure vessel by remote handling. Hydro-
thermal synthesis is not suited to large scale processing. About the only commercial
process that uses hydrothermal synthesis on a large scale is the growth of quartz crystals
for the electronics industry. This is a batch process and inherent limitations of pressure
vessels require that the batches be fairly small. Commercial quartz-growth vessels are
2 to 3 m high and 0.3 to 0.5 m in diameter.