Chemical and ceramic methods toward safe storage of actinides using monazite. 1998 annual progress report
- Rockwell International Corp., Thousand, Oaks, CA (US)
- Oak Ridge National Lab., TN (US)
'The use of ceramic monazite, (La,Ce)PO{sub 4}, for sequestering actinides, especially plutonium, and some other radioactive waste elements (rare earths e.g.) and thus isolating them from the environment has been championed by Lynn Boatner of ORNL. It may be used alone or, as it is compatible with many other minerals in nature, can be used in composite combinations. Natural monazite, which almost invariably contains Th and U, is often formed in hydrothermal pegmatites and is extremely water resistant--examples are known where the mineral has been washed out of rocks (becoming a placer mineral as on the beach sands of India, Australia, Brazil etc.) then reincorporated into new rocks with new crystal overgrowths and then washed out again--being 2.5--3 billion years old. During this demanding water treatment it has retained Th and U. Where very low levels of water attack have been seen (in more siliceous waters), the Th is tied up as new ThSiO{sub 4} and remains immobile. Lest it be thought that rare-earths are rare or expensive, this is not so. In fact, the less common lanthanides such as gadolinium, samarium, europium, and terbium, are necessarily extracted and much used by, e.g., the electronics industry, leaving La and Ce as not-sufficiently-used by-products. The recent development of large scale use of Nd in Nd-B-Fe magnets has further exaggerated this. Large deposits of the parent mineral bastnaesite are present in the USA and in China. (Mineral monazite itself is not preferred due to its thorium content.) In the last 5 years it has become apparent show that monazite (more specifically La-monazite) is an unrecognized/becoming-interesting ceramic material. PuPO4 itself has the monazite structure; the PO{sub 4} 3-unit strongly stabilizes actinides and rare earths in their trivalent state. Monazite melts without decomposition (in a closed system) at 2,074 C and, being compatible with common ceramic oxides such as alumina, mullite, zirconia and YAG, is useful in oxidatively stable ceramic composites: for example, use is contemplated as an enabling weak interface in oxide-oxide fiber composites (including as a high temperature starch on space shuttle blankets), and possibly as machinable ceramics, friction materials and other. The ceramic behavior of pure and doped monazite has not yet been studied in any detail. The sine-qua-non of ceramic studies and production is the reliable synthesis of reproducible starting powders and precursor chemicals that consistently reproduce the desired ceramic outcome. This has always been a more neglected (underfunded) side of ceramic studies; witness how many years passed before pure reproducible powders of alumina or silicon nitride became available for ceramic studies long after it was apparent that these were useful ceramics which, however, suffered from forming variation and degradation caused by small amounts of impurities.'
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Environmental Management (EM), Office of Science and Risk Policy
- OSTI ID:
- 13668
- Report Number(s):
- EMSP-55094-98; ON: DE00013668
- Country of Publication:
- United States
- Language:
- English
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