Nuclear waste forms chemical durability

Trocellier, P. 2001. Chemical durability of high level nuclear waste forms. Ann. Chim. Sci. Mat., 26, 113-130. 

The early work on phosphate glasses led to the idea that crystalline phosphates might make extremely durable waste forms, particularly for actinides. The earliest suggestion was for the use of monazite (Boatner 1978, Boatner et al. 1980, McCarthy et al. 1978, 1980). The attractive qualities of monazite as a nuclear waste form are (1) a high solubility for actinides and rare earths (10 to 20 wt %) (2) evidence from natural occurrences of good chemical durability (3) an apparent resistance to radiation damage, as natural monazites are seldom found in the metamict state, despite very high alpha-decay event doses (Boatner and Sales 1988). There have been extensive studies of monazite and apatite as potential waste form phases, and a considerable amount of work on a number of synthetic phosphate phases has been completed. 

Summary. Phosphate and silicate apatite offer a number of advantages as nuclear waste forms (1) a high capacity for the incorporation of actinide elements, as well as selected fission products such as °Sr (2) a reasonable chemical durability depending on the geochemical environment and (3) a propensity for rapid annealing of radiation damage for the phosphate compositions. Considerable work remains to be done, mainly systematic studies, under relevant repository conditions, of the effects of composition on chemical durability. 

Although the work on TPD is relatively recent, there is already a substantial amount of data on its chemical durability, and leach rates are comparable to those of other phosphate waste forms. One can anticipate important advances in the use of TPD or mixed phase assemblages of TPD-monazite (Clavier et al., in press) in reprocessing spent nuclear fuel, processing HEW, and developing new nuclear waste forms for special applications. 

The main advantages of phosphate phases as nuclear waste forms are the high capacity for actinides (up to 20 wt %), a relatively high chemical durability (Donald et al. 1997, Trocellier 2001), and the ability to anneal radiation damage effects at relatively low temperatures (300 to 400°C). Of particular importance for monazite and apatite is the fact that they occur naturally thus, models that are used to extrapolate the physical and chemical behavior of the waste form over long periods can be confirmed by comparison to natural occurrences. 

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