Stefan Neumeier

Durability of potential plutonium wasteforms under repository conditions

Abstract Separated stocks of UK civil plutonium are currently held as a zero value asset in storage, as there is no final decision about whether they should be treated as a resource for future use as nuclear fuel or as waste. Irrespective of future UK government strategies regarding plutonium, at least a portion of the UK civil plutonium inventory will be designated for geological disposal. In this context, we performed a high-level review of the performance of potential wasteforms for the disposal of separated civil plutonium. The key issues considered were the durability and chemical reactivity of the wasteforms in aqueous environments and the long-term radionuclide release under conditions relevant to geological disposal. The major findings of the review, relevant not only to the situation in the UK but to plutonium disposal in general, are summarized in this paper. The review showed that, in the event of a decision being taken to declare plutonium as a waste for disposal, more systematic studies would be required to constrain the wasteform performance under repository conditions in order to derive realistic source terms for a safety case.

Site-selective time resolved laser fluorescence spectroscopy of Eu and Cm doped LaPO4

Abstract Samples of LaPO4 doped with Eu3+ or Cm3+ were synthesized by a hydrothermal process which resulted in a solid solution at temperatures less than conventional processing. Time resolved laser fluorescence spectroscopy was used to probe the incorporated Eu3+ or Cm3+ in order to gain structural information on its local environment. This revealed that Eu3+ and Cm3+ incorporate on the La site as expected. The emission spectrum of Eu3+ resolves the fully degenerate 5-fold splitting of the peaks in the F2 transition due to the low symmetry of the site, confirming previous calculations. A minor site in the Eu3+ doped sample is identified as coordinated with hydroxide contamination. Direct excitation of Cm3+ doped samples show the presence of “satellite” species. Although these spectral features have been observed in Cm3+ doped LuPO4 and YPO4, this is the first time that these satellites are resolved into their individual species. These are hypothesized to be due to a disturbance in the ideal structure which creates a break in the equivalence of the four lanthanum sites within a unit cell. The 4-fold ground state splitting of all species is identical, although slightly shifted, indicating similar environments. The fluorescence lifetimes were long (1.2 ms for Cm and 3.6 ms for Eu) indicating an absence of water in the immediate coordination sphere due to the incorporation of the doping ion.

Using Eu3+ as an atomic probe to investigate the local environment in LaPO4–GdPO4 monazite end-members

In the present study, we have investigated the luminescent properties of Eu(3+) as a dopant in a series of synthetic lanthanide phosphates from the monazite group. Systematic trends in the spectroscopic properties of Eu(3+) depending on the size of the host cation and the dopant to ligand distance have been observed. Our results show that the increasing match between host and dopant radii when going from Eu(3+)-doped LaPO4 toward the smaller GdPO4 monazite decreases both the full width at half maximum of the Eu(3+) excitation peak, as well as the (7)F2/(7)F1 emission band intensity ratio. The decreasing Ln⋯O bond distance within the LnPO4 series causes a systematic bathochromic shift of the Eu(3+) excitation peak, showing a linear dependence of both the host cation size and the Ln⋯O distance. The linear relationship can be used to predict the energy band gap for Eu(3+)-doped monazites for which no Eu(3+) luminescent data is available. Finally, mechanisms for metal-metal energy transfer between host and dopant lanthanides have been explored based on recorded luminescence lifetime data. Luminescence lifetime data for Eu(3+) incorporated in the various monazite hosts clearly indicated that the energy band gap between the guest ion emission transition and the host ion absorption transition can be correlated to the degree of quenching observed in these materials with otherwise identical geometries and chemistries.

New insights into phosphate based materials for the immobilisation of actinides

Abstract This paper focuses on major phosphate-based ceramic materials relevant for the immobilisation of Pu, minor actinides, fission and activation products. Key points addressed include the recent progress regarding synthesis methods, the formation of solid solutions by structural incorporation of actinides or their non-radioactive surrogates and waste form fabrication by advanced sintering techniques. Particular attention is paid to the properties that govern the long-term stability of the waste forms under conditions relevant to geological disposal. The paper highlights the benefits gained from synergies of state-of-the-art experimental approaches and advanced atomistic modeling tools for addressing properties and stability of f-element-bearing phosphate materials. In conclusion, this article provides a perspective on the recent advancements in the understanding of phosphate based ceramics and their properties with respect to their application as nuclear waste forms.

Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing

Abstract Monazite, a common accessory rare-earth orthophosphate mineral in the continental crust widely used in U-Pb geochronology, holds promise for (U-Th)/He thermochronology and for the immobilization of Pu and minor actinides (MA) coming from spent nuclear fuel reprocessing. Previous results obtained on natural and plutonium-doped monazite have demonstrated the ability of this structure to maintain a crystalline state despite high radiation damage levels. However, the low critical temperature (180 °C), above which amorphization cannot be achieved in natural monazite under ion irradiation, does not explain this old and unsolved paradox: why do natural monazites, independent of their geological history, remain crystalline even when they did not experience any thermal event that could heal the defects? This is what the present study aims to address. Synthetic polycrystals of LaPO4-monazite were irradiated sequentially and simultaneously with α particles (He) and gold (Au) ions. Our results demonstrate experimentally for the first time in monazite, the existence of the defect recovery mechanism, called α-healing, acting in this structure due to electronic energy loss of α particles, which explains the absence of amorphization in natural monazite samples. This mechanism is critically important for monazite geo- and thermochronology and to design and predictively model the long-term behavior of ceramic matrices for nuclear waste conditioning.