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

P.1
APPENDIX P
MINERALS THAT COULD BE USED TO CONTAIN RADIONUCLIDES
This appendix presents a review of minerals that are candidates for the isolation of
radionuclides in synthetic minerals, as discussed in Section 4.3.2.3. Analyses of the
potential hazard from certain HLLW radionuclides suggest the greatest effort in solidifica-
tion into synthetic minerals would be placed on the following groups of. elements:
* Actinides and Lanthanides. The actinides Np, Pu, Am, Cm and their daughters constitute
the major hazard in nuclear wastes from about 1000 to 5000 years of storage, with the
exception of 2 26 Ra, which does not become significant until about 105 years
(Cohen 1977). The majority of the lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu,
Gd, Tb, Dy, Ho) are present as stable isotopes after a few years, and only trace
amounts of a few Sm and Eu radionuclides have long half-lives. However, the
lanthanides could be included with the actinides for several reasons: they occur
together at one stage of partitioning; lanthanides and actinides are
crystallographically and chemically very
similar and usually occur together in the same minerals; the lanthanides can act as
diluents in synthetic minerals for a-emitting actinides in order to minimize radiation
effects.
* Strontium and cesium. These elements constitute both the major heat producers and bio-
hazards in nuclear wastes for the first 600 years or so (Cohen 1977).
* Techetium and iodine. These two fission products have long-lived isotopes ( 9 Tc, tl/2
= 2.1 x 105 y; 129I, t/2 = 1.7 x 107 y) and are biohazards. They have the additional
characteristics of forming anions that can migrate in soils and rocks as fast as the
solutions in which they are dissolved (Rai and Seine 1978), i.e., without any substan-
tial hold-up due to ion exchange or adsorption.
The minerals reviewed here are either known to contain substantial amounts of these
elements or are likely to accept these elements based on compatible crystal chemistry. The
physico-chemical and crystal chemical criteria for selecting host minerals, along with the
common mineral synthesis methods, are discussed and tables of candidates are presented. A
thorough treatment of what is known about the process of metamictization and metamict min-
erals is also included.
P.1 PHYSICO-CHEMICAL PRINCIPLES
P.1.1 Stability Criteria
The physical and chemical foundations used to define whether a known mineral is classi-
fied as very stable, relatively unstable, or very unstable with respect to alteration,
weathering and diagenesis include solubility and geologic data.