The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

Abstract

Spent fuel from commercial nuclear reactors consists mainly of uranium oxide. However, the changes that occur during reactor operations have a profound effect on chemical and physical properties of this material. Heat build-up in the fuel pellet during reactor operations can cause redistribution of fission products. The fission products may aggregate in one of three types of precipitates; gaseous, metallic, or oxide, depending on the burn-up and in-core treatment. Radiation damage and variations in fission and neutron capture yields across the fuel pellets lead to Pu enrichment and increased porosity with increasing burn-up. A more porous surface may make the fuel more susceptible to oxidative dissolution. As the level of actinides and fission products increases, the fuel may become more resistant to oxidation. These changes may limit the usefulness of natural uraninite (U02) analogues for predicting the geological behaviour of spent fuel disposed in a high-level waste (HLW) repository. In this Chapter, an overview of spent fuel microstructure, radiolytic effects, and alteration processes is presented. Evidence for Np incorporation into U 6+ phases, the nature of Pu surface precipitates on spent fuel, and evidence for the preferential removal of 4d-metals from e-particles in corroded spent fuel is discussed. Understanding the potential mechanisms of radionuclide attenuation through sorption and/or incorporation requires techniques with both high spatial resolution and excellent elemental sensitivity.

Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

“…Isotopic patterns of types 2 and 3 are observed only in soluble phases (#1 $ 3) in the D81N sample. The study of fission product distributions in spent fuel reveals that fissiogenic Cs migrates to grain boundaries, fractures and the gap between the fuel pellets and the surrounding cladding (Shoesmith, 2000;Buck et al, 2004). Therefore, the existence of fissiogenic Cs from the D81N sample is reasonable, considering that sample D81N is from the boundary between RZ and wall rock.…”

The Geochemistry of Technetium: A Summary of the Behavior of an Artificial Element in the Natural Environment

“…Although most of the 99 Tc is sequestered in minute dissolution-resistant metal aggregates in spent fuel rods (Buck et al 2004), reprocessing the fuel rods results in liberation of technetium, which is carried along in solution with other radioisotopes in the reclamation effort (Colton 1965;Till 1984). When exposed to the atmosphere or any moderately oxidizing environment, technetium is manifested as the pertechnetate anion ( 99 TcO 4 -), which is highly soluble in aqueous solution, meaning that this anion can be found in solution well above the drinking water standard of 900 pCi/L (EPA 2002).…”

“…These small, metallic aggregates, known as "epsilon phases" (or -phases; also known as "white phases") contain ruthenium, rhodium, platinum, molybdenum, and technetium. Therefore, in order to mobilize technetium, a great deal of uranium dioxide would have to be dissolved to expose the -phases to aqueous solution (Buck et al 2004). Although there are no specific data on the rate of aqueous dissolution of -phases, dissolution experiments on radionuclide-bearing metallic waste forms indicate that release of technetium is slow (Johnson et al 2002) and, therefore, technetium in this manifestation constitutes a minor source.…”

Formation and geochemical significance of micrometallic aggregates including fissiogenic platinum group elements in the Oklo natural reactor, Gabon

“…Aggregates that contain O, Zr, Mo, Ru, Pd, U and Pu have been observed in spent fuel (Buck et al, 2004). The individual particles forming the aggregates are only a few lm across, and the particles that contain O, Zr, U and Pu are mixed within the aggregates.…”