Review of spatial relations between uraninite and coffinite - implications for alteration mechanisms
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Review of spatial relations between uraninite and coffinite - implications for alteration mechanisms Lena Z. Evins1 and Keld A. Jensen2 1 SKB, Blekholmstorget 30, SE-101 24 Stockholm, Sweden ( [email protected]) 2 NRCWE, Lersø Parkallé 105, DK-2100 København Ø, Denmark ([email protected]) ABSTRACT Coffinite (USiO4•nH2O) is a common mineral in uranium ores. It is often observed to replace uraninite during alteration under reducing conditions. However, it has proven difficult to synthesize coffinite in the laboratory and quantitative thermodynamic data for coffinite are lacking. Despite these experimental difficulties, there is ample evidence in nature that many uranium deposits have encountered conditions where formation of coffinite has been favoured over uraninite during postgenetic alteration events. Coffinite is also found as a primary mineral in sandstone uranium deposits. This review elucidates the spatial relation between coffinite and uraninite as seen on different scales with different analytical methods. Some further insight into the mechanism of uraninite alteration in natural, reducing, Si-rich environments is gained and some new arguments put forward, including the question of the effect of impurities and dopants, defects, and grain size. The replacement of uraninite by coffinite is discussed in terms of solidfluid interaction. INTRODUCTION In order to evaluate the need to quantify the process of ‘coffinitization’ of old spent nuclear fuel, which potentially could occur at the time when reducing groundwater has access to the fuel in the repository;, the likelihood of this process needs to be estimated. In other words, we QHHGWRNQRZWKH¨*RIWKHIROORZLQJ VLPSOLILHG UHDFWLRQ UO2+x + H4SiO4 ĺ86L24·2H2O + 0.5xO2
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There is, however, a troublesome dearth of information regarding the thermodynamic properties of coffinite. In a recent study [1], coffinite was successfully produced in the laboratory but only as a nanocrystalline mixture between coffinite and UO2. Another experiment [2] investigated the hydrothermal alteration of pellets of nuclear-grade depleted UO2. At the end of that experiment, about 25% of the pellet surface was covered with a phase that was probably coffinite; however, it was not possible to identify this material as coffinite by XRD. Thus, laboratory experiments have not been able, so far, to produce coffinite crystals that are large and pure enough that the thermochemical properties can be measured. However, it is the process of ‘coffinitization’, i.e. alteration of a precursor that we are interested in, rather than large and pure coffinite crystals precipitated from solution in a laboratory. The aim of this review is to investigate whether coffinite, formed by the alteration of uraninite, is commonly precipitated on (and in) the reacting (dissolving) uraninite grain. The reason for this is two-fold: 1) there are remaining uncertainties regarding chemical alteration of the spent fuel matrix under reducing conditions [3] and 2) in the mineralogical literature there is an on-going
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