Comment on "Kinetics and thermodynamics of Eu(III) adsorption onto synthetic monoclinic pyrrhotite" by Y. Zhu et al., Journal of Molecular Liquids, 218 (2016), 565-570

International audienceThe paper of Zhu et al. [1] is of great interest, since it reported adsorption data of Eu(III) onto pyrrhotite, an uninvestigated solid surface to date. We are of the opinion that the system is of relevance in the context of nuclear waste disposal in deep repositories. In the study, Eu(III) is used as an analogue for trivalent actinides and pyrrhotite might be a relevant mineral under reducing conditions. Unfortunately, for readers interested in using the uptake data for developing a surface complexation model, some crucial information concerning experimental conditions is missing and some statements in the text require some clarification. We aim here to raise some relevant questions in a constructive way, to achieve a correct understanding of this interesting data

Solubility of Plutonium in MgCl2 and CaCl2 Solutions in Contact with Metallic Iron

The solubility of Pu(IV) hydrous oxide is studied at 22±2 °C under Ar atmosphere in 0.25 and 3.5 M MgCl2 (pHc¿9) and 3.5 M CaCl2 (pHc=11¿12) solutions in contact with iron powder. In alkaline 3.5¿4.0 M CaCl2 ternary Ca-Pu-OH complexes are formed and the solubility of PuO2·xH2O(s) increases from about 10-10 M at pHc=11 to 10-8¿10-7 M at pHc=12, both in the presence and absence of Fe powder. In the presence of iron the aqueous speciation is dominated by Pu(III) at pHc10. In the absence of reducing agents the aqueous speciation is dominated by Pu(V), with significant contributions of Pu(IV) complexes at pHc>10. The effect of ternary Ca-Pu(III,IV,V)-OH complexes in alkaline CaCl2 solutions can be described by assuming complex stoichiometries and experimental data from the oxidation state analogs Cm(III)/Nd(III), Th(IV) and Np(V). The ion interaction Pitzer approach is used for model calculations.JRC.E.6-Actinides researc

Geochemically Based Safety Assessment

Performance assessment (PA) for nuclear waste disposal based on transport processes frequently neglects significant safety factors by overestimation of radionuclide mobilization and underestimation of radionuclide retention processes. To include well understood geochemical knowledge into PA, the quasi-closed system approach (QCS) was developed. The QCS approach is described and applied to a LLW disposal in a German salt mine with respect to the disposed waste forms, geo-engineered barriers, and backfill strategies. The geochemical tools and the thermodynamic database for modelling highly concentrated salt systems are discussed. Applications are demonstrated which cover the long-term geochemical environment in the disposal caverns, the optimization of buffer materials, radionuclide retention, and the overall robustness of the approach. Also the effect of a potential solution exchange between different emplacement caverns is investigated. It is shown that the QCS approach provides essential data concerning the long-term geochemistry and related radionuclide concentrations to be used in PA and safety analysis.JRC.E.5-Nuclear chemistr

Sorption of Radionuclides onto Natural Clay Rocks

Reactions of U(VI) and Eu/Cm(III) with natural clay rock (Opalinus Clay, Switzerland, and Callovo-Oxfordian Clay, France) are investigated by batch and spectroscopic experiments as well as geochemical calculations. The aim of the studies is to identify those minerals in the heterogeneous multiphase systems with sometimes high calcite content which control lanthanide and actinide sorption. The outcome of batch sorption studies with natural clay rocks (2 g/L natural clay rock, 0.1mol/L NaClO4, pH 3¿11) and thermodynamic calculations is compared with experimental results using a synthetic mixture consisting of purified Namontmorillonite and calcite (ratio 80 : 20 wt. %). Our studies show that U(VI) sorption decreases at intermediate pH (6.5¿9) where dissolved U(VI) carbonate species predominate. In the high pH region (> 9) U(VI) sorption again increases by formation of ternary hydroxo surface complexes on the clay mineral fraction, whereby calculated U(VI) sorption overestimates experimental data for the natural rock. Complete sorption to natural clay rock and the synthetic clay/calcite mixture is observed for Eu(III) at pH > 7 for the conditions studied. Model calculations again point to the predominance of clay-sorbed species even though calculated speciation underestimates the experimentally observed sorption at pH 6¿9 slightly. Preliminary time-resolved laser-fluorescence spectroscopy (TRLFS) studies using Cm(III) as fluorescent probe reveal the appearance of several Cm(III)-clay surface species in the pH range 5¿11, but give no indications for the presence of calcite-bound Cm(III). We conclude that the clay minerals in the multiphasic clay rock act as main sorbents for tri- and hexavalent actinides.JRC.E-Institute for Transuranium Elements (Karlsruhe

Solubility of tetravalent actinides in alkaline CaCl2 solutions and formation of Ca4[An(OH)8]4+ complexes: A study of Np(IV) and Pu(IV) under reducing conditions and the systematic trend in the An(IV) series

The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22 ± 2°C under reducing conditions in alkaline CaCl2 solutions. Redox conditions were adjusted either with 2 mM Na2S2O4 or additions of iron powder. In 2.0 and 4.5 M CaCl2 and pHc = 11 - 12, the neptunium and plutonium concentrations increase with a slope of +4 (log [An] vs. pHc) as expected for the formation of the complex Ca4[An(OH)8]4+ recently identified for Th(IV). At CaCl2 concentrations = 1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log*Ks,(4,1,8) for the reaction An(OH)4(am) + 4 H2O + 4 Ca2+ ¿ Ca4[An(OH)8]4+ + 4 H+ are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5 - 4.5 M CaCl2. The log*K°s,(4,1,8) values and the complex formation constants log ß°(4,1,8) follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).JRC.E.3-Materials researc

Enhancing the EC's analytical capabilities for environmental sample analysis for nuclear safeguards purposes by the establishment of a new Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS) laboratory

Nuclear activities unavoidably leave fingerprints in the environment. Such fingerprints may consist of characteristic building structures, of typical supply lines or of minute releases of process material. In particular, the release of micron-sized aerosol particles to the immediate environment is very difficult to avoid to the complete extent, especially when nuclear materials are processed in larger quantities. These micro-particles contain the isotopic signature of the handled nuclear materials. This allows nuclear safeguards authorities with specialised sampling techniques and analytical laboratories to verify the completeness of a state's declaration and to check the consistency of measured material properties (i.e. isotopic composition) with declared operations at nuclear facilities. This methodology has been in particular applied to uranium enrichment facilities. The European Commission (EC) Joint Research Centre (JRC), the Institute of Transuranium Elements (ITU) has performed uranium particle analysis for nuclear safeguards purposes on environmental samples since the late 90's with the EC Directorate General Energy (DG-ENER) as its main user. In recent years, significant efforts have been made to enhance the used analytical techniques. The enhancements have been achieved in collaboration with leading manufactures of analytical equipment that has implemented new purpose built systems for particle analysis in the field of Secondary Ion mass Spectrometry (SIMS). This paper describes the purpose and outlines the performance of a new analytical Large Geometry – Secondary Ion Mass Spectrometry (LG-SIMS) laboratory that has been established at ITU, jointly funded by the JRC and DG-ENER. The laboratory will mainly be used for analysing uranium bearing aerosol particles collected on cotton swipes from nuclear safeguards inspections but it will also be used for other safeguards related applications and nuclear forensics. This paper will give an overview of the capabilities and enhancements that can be expected from this new laboratory and it will also describe the use and importance of environmental sampling that are followed by high performance trace analysis of particles, in the context of European nuclear safeguards.JRC.E.7-Nuclear Safeguards and Forensic