Clay actinides

Recently there has been interest in the sorptive behavior of natural clays toward metal ions potentially present in radioactive wastes. Initial studies of the transplutonium elements have been carried out to define their sorption behavior with such materials ( ). However, it is also important to understand the stability of the clay-actinide product with regard to radiation damage and to be able to predict what changes in behavior may occur after exposure to radiation, so that accurate transport models may be constructed. 

Torstenfelt B. 1986. Migration of the actinides, thorium, protactinium, uranium, neptunium, plutonium and americium in clay. Radiochem Acta 39 105-112. 

The most efficient matrix for retention of actinides and fission products is the uraninite mineral. However, it has been shown that other matricies such as apatite, clay minerals, zirconium silicates, and oxides (Fe, Mn) may also be important in the retention of fission products and actinides. For example, Pu was stored in apatite (Bros et al. 1996) and chlorite (Bros et al. 1993) in the core of the reactor 10. In the core of the reactors, between uraninite grains, 20-200 (j.m-sized metallic aggregates containing fissiogenic Ru, Rh, and Te associated with As, Pb, and S were found. These aggregates also exist in spent fuels of water-pressured type reactor plants, suggesting their analogy with spent fuels. 

In clay samples Zr-Th-rich coffinite was found around remnants of zircon. It is likely that it is the result of solid solution with zircon, ZrSi04 and thorite, ThSi04, which are isostruc-tural with coffinite (Finch Murakami 1999 Jensen Ewing 2001). The presence of phosphorus and sulphur in coffinite suggests that both elements substituted for Si in the coffinite structure. A previous study at the Bangombe site in Gabon has clearly shown that coffinites are most important secondary minerals for the retention of fissiogenic lanthanides and actinides (Stille et al. 2003). 

Eberly, P., Ewing, R. C., Janeczek, J. Furlano, A. 1996. Clays at the natural nuclear reactor at Bangombe, Gabon Migration of actinides. Radiochimica Acta, 74, 271-275. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

Sorption Behavior of Tri valent Actinides and Rare Earths on Clay Minerals ... 

The D values on the three clays studied indicate very little difference between the rare earths and actinides. The D values also indicate that there is a small increase in D when going from low atomic number to higher atomic number in agreement with results of ( ). The most unexpected results are that geometric or structural considerations can cause high specificity for some... 

The conclusions reached from this preliminary study is that these clays can serve as sorbative material for the trivalent actinides and that even after damage to the clay structures, the actinide will not be released as a soluble species. 

G. D., "Sorption Behavior of Trivalent Actinides and Rare Earths on Clay Minerals," ACS Symposium Series on Radioactive Waste in Geologic Storage, R. F. Gould, Ed., Miami Beach, Florida, Sept. 11-15, 1978. 

Pu(IV), which forms highly charged polymers, strongly sorbs to soils and sediments. Other actinide III and IV oxidation states also bind by ion exchange to clays. The uptake of these species by solids is in the same sequence as the order of hydrolysis Pu > Am(III) > U(VI) > Np(V). The uptake of these actinides by plants appears to be in the reverse order of hydrolysis Np(V) > U(VI) > Am(III) > Pu(IV), with plants showing little ability to assimilate the immobile hydrolyzed species. The further concentration of these species in the food chain with subsequent deposit in humans appears to be minor. Of the 4 tons of plutonium released to the environment in atmospheric testing of nuclear weapons, the total amount fixed in the world population is less than 1 g [of this amount, most (99.9%) was inhaled rather than ingested]. 

Actinide and Technetium Sorption on Iron-Silicate and Dispersed Clay Colloids... 

The differences in sorptive behavior of Th, Pu, U, and Np are evident by examining Table II. Plutonium and thorium isotopes at tracer concentrations (parts per billion, element mass/clay mass) were equilibrated for 24 hours with the < 2-pm fraction (clay) of a silt loam soil. The pH of the equilibration solutions was 6.5 and the aqueous phase contained Ca at a concentration of 5 mM. Both tetravalent actinides failed to remain 1n solution. Whether this is a direct function of sorption mechanisms or simply related to the solubility of the ions in solution is not distinguished by the results. Uranyl ion was not removed to the same extent as the tetravalent species. Neptunium(V) sorbed very poorly. It should be noted that while Np(V) is a mono-charged cation, Np02+ does not sorb like Na+. 

Diffusion of Tc and other fission products such as Cs and -Eu as well as the actinides Np, Am and natural uranium was studied in a sample of a sediment from an enclosed brackish water bay of the Baltic Sea. Under oxidizing conditions TcOq did not interact with the sediment to any large degree. Deeper laying sediments were depleted of oxygen and showed negative redox potentials. In this case the apparent diffusivity of Tc 0 =5-10 m -s was low compared to compacted clay with >a=8 -10 " m- s", indicating a reduction of TcOq [30]. 

The initiated radioactive inventory for spent reactor fuel consists of actinides, fission products and activation products. As noted previously, (Gi. 21) the shorter lived fission products, such as Sr and Cs, and transuranic elements, such as Pu, Pu, are the main contributors to the radioactivity. However, performance assessments strongly indicate that the waste form matrix and the near field engineered barriers (e.g. clay backfill, etc.), can successfully retain and prevent any migration to the far field viromnent for one thousand years and probably much longer (> lO years). After the first thousand years the long lived nuclides such as Cs, Sn, Tc and Se among the fission products and the actinides Np, Pu, Pu, and Am become the major concern. 

Experimental results, with both actinides and heavy metals, have demonstrated that the presence of water aids the above reaction scheme 15), It is believed that the water is required to hydrate the metal ion prior to complexation and also aids in deprotonation of the acidic ligand. In SFE from environmental matrices, the presence of water may also aid the breaking of bonds to the binding sites on matrices such as soil, clay and other minerals present in the matrix. This overall reaction scheme is illustrated conceptually in Figure 1. 

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