Marc Verwerft

Autoclave leaching experiments are conducted on three well-characterised, irradiated, and cladded mixed oxide fuel-rod segments with burnups ranging from 29 GWd/t_HM to 52 GWd/t_HM to investigate the instant release fraction of fission gases and long-lived fission products and to assess the long-term fuel matrix corrosion. The segments are exposed to bicarbonate solutions as reference groundwater at neutral pH and a synthetic young cementitious water at pH 13.5 under reducing atmosphere (4 vol% H₂ in Ar at 40 bar pressure), since 2018. The initial leaching results for the fission products caesium and iodine as representative elements of the instant release fraction were found to depend on the leachate composition as well as on the fuel burnup.

Graphical abstract

The hydrolysis of uranyl‐, Nd‐ and Ce‐cations, induced by thermal decomposition of urea was investigated and the impact of the urea content and the experiment temperature on the reaction kinetics was evaluated. Uranyl precipitated as ammonium diuranate (ADU) with varying stoichiometry. Nd(III) and Ce(III) showed comparable pH evolutions and lanthanide carbonate hydroxide (LnCO₃OH) products were identified, whereas Ce(IV) hydrolysed at lower pH and formed CeO₂. The precipitation behaviour was confirmed for mixtures of uranyl and the lanthanides. Depending on the urea content, a partial co‐precipitation occurred in U/Nd mixtures. The phases formed with Ce(III) and Ce(IV) were also identified in the precipitates of binary U/Ce mixtures. In ternary U/Nd/Ce compositions, a simultaneous precipitation of Nd(III) and Ce(III) was observed and a partial incorporation of the lanthanides into the ADU phase, whereas the precipitation in the presence of Ce(IV)/CeO₂ led to the formation of three individual phases.

In order to derive the burnup of spent nuclear fuel from the concentration of selected fission products (typically the Nd isotopes and ¹³⁷Cs), their irradiation-averaged fission yields need to be known with sufficient accuracy, as they evolve with the changes in the actinide vector over the irradiation history. To obtain irradiation-averaged values, radiochemists often resort to robust generic methods – i.e., based on simple mathematical relations – that weight the fission yields according to the actinides contributing to fission, without performing core physics calculations. In order to assess the performance of those generic methods, a database of about 30 000 spent nuclear fuel inventories has been constructed from neutron transport and depletion simulations, covering a representative range of fuel enrichment, burnup, assembly designs and reactor types. When testing several existing methods for effective fission yield calculation, some inaccuracies were identified, originating from improper one-group cross-section parameters that do not accurately reflect resonance and self-shielding effects, and too crude approximations in the estimation of the actinide concentration evolution. Revised effective fission and absorption cross-section parameters are then proposed here, as a first improvement to the earlier burnup determination methods. As a second step, a novel method is proposed that reduces the error on their radiation-averaged fission yield values, and hence on burnup, while retaining a straightforward calculation scheme.

The radiolysis behavior of a new diglycolamide for solvent extraction of actinides and lanthanides was studied. The observed degradation rate was lower than for the reference molecule and 22 degradation compounds were identified.

Improving the kinetics of selective An(iii) extraction from nitric acid feed solutions into an ionic liquid based solvent: combining CyMe₄BTPhen with TODGA in Aliquat-336 nitrate.

Thorium was precipitated homogeneously from a thorium nitrate solution by the thermal decomposition products of urea. The kinetics of the hydrolysis were studied at 90 and 100°C by pH measurement during the initial 5 h and the precipitation efficiencies of thorium and radium were measured over a 24 h period. Precipitation of the radium daughters was closely followed with the aim of co-precipitation of radium with thorium. The CO₂ formed during urea decomposition dissolved in the solution, forming CO₃ ²⁻ during the experiment upon reaching a sufficiently high pH level (>7). This allowed radium to co-precipitate partially, thus reducing the activity of the filtrate. After filtration or centrifugation, the precipitate is composed of nanocrystalline thorium dioxide (crystallite size ~10 nm), with weakly bound H₂O and CO₂.

The valence band electronic structures of mixed uranium oxides (UO₂, U₄O₉, U₃O₇, U₃O₈, and β-UO₃) have been studied using the resonant inelastic X-ray scattering (RIXS) technique at the U M₅ edge and computational methods.

The superlattice and domain structures exhibited by ordered titanium monoxide Ti₅O₅ are disrupted by low energy electron beam irradiation. The effect is attributed to the disordering of the oxygen and titanium sublattices. This disordering is caused by the displacement of both oxygen and titanium atoms by the incident electrons and results in a phase transformation of the monoclinic phase Ti₅O₅ into cubic B1 titanium monoxide. In order to determine the energies required for the displacement of titanium or oxygen atoms, i.e. threshold displacement energies, a systematic study of the disappearance of superstructure reflections with increasing electron energy and electron bombardment dose has been performed in situ in a transmission electron microscope (TEM). An incident electron energy threshold between 120 and 140 keV has been observed. This threshold can be ascribed to the displacements of titanium atoms with 4 as well as with 5 oxygen atoms as nearest neighbors. The displacement threshold energy of titanium atoms in Ti₅O₅ corresponding with the observed incident electron threshold energy lies between 6.0 and 7.5 eV. This surprisingly low value can be explained by the presence of either one or two vacant oxygen lattice sites in the nearest neighbors of all titanium atoms.

Titanium stabilized 1.4970 ‘15-15Ti’ stainless steel cladding is the primary choice for fuel cladding of several current fast spectrum research reactor projects. The choice of cladding material is based on past experiences and the availability of material databases from similar steel grades that have proven their reliability in past sodium-cooled fast reactors programs. However the last production in Europe of nuclear-grade 15-15Ti was more than 20 years ago and it remained to be seen if the know-how to produce such steel with the strict specifications for nuclear fuel cladding was still available. Results of a new production of nuclear-grade 15-15Ti cladding tubes at Sandvik for SCK•CEN is presented in this paper. It is shown that materials properties are within the strict specifications similar to the ones used during past sodium-cooled fast reactors programs. Special attention is given to microstructural analysis of the newly produced steel which contains a large number of stabilizing Ti(C-N) precipitates known for their beneficial effect on in-pile material properties and thermal creep. Results from metallography, SEM and TEM investigations are presented.

Among the various products originating from fission events, the noble gas elements Xe and, in a lesser extent, Kr present important fission yields. The accumulated gas inventory in its various states (e.g. atomically dissolved, precipitated in cavities or released from the fuel) has a strong impact on the performance of LWR fuel and is presently one of the limiting factors for fuel burnup extension. A more fundamental understanding of fission gas behaviour at the atomic scale would enable to improve the modelling of the various mechanisms ultimately leading to fission gas release and to refine conservative safety margins. Lots of efforts have already been undertaken using atomistic computer simulations, ab initio calculations and Empirical Potential Molecular Dynamics (EP-MD) techniques, that relate to the bulk behaviour. This article will discuss EP-MD investigations in nanosized polycrystalline UO₂, constructed from Voronoi cells in a 3-D periodic environment. This study has focused on Xe diffusion at and close to grain boundaries.

Amorphous Si layers were grown by krypton plasma sputter deposition at 310 °C. By pulsation of the substrate potential between 0 and 50 eV, the Kr concentration in the layers could be varied to a maximum of 5.5 at. %. A model which describes trapping of inert gas atoms in the sputtered layer in terms of implantation and trapping, diffusion, growth, resputtering, and gas sputtering is presented. High-resolution electron microscopy, electrode-probe (x-ray) microanalysis, positron annihilation, Raman spectroscopy, Mössbauer spectroscopy, and bending and hardness measurements were performed on the deposited layers. It turns out that the ion assisted growth leads to a strong reduction of open volume defects. The experiments point to the presence of very small Kr agglomerates. From the Mössbauer experiments a lower limit of 250 K for the Debye temperature of the Kr agglomerates is derived. Molecular-dynamic simulations from which the Debye temperatures of Kr mono-, di-, and trimers in amorphous Si can be derived are presented. The simulations indicate the presence of predominantly Kr monomers and dimers.

We will try to illustrate here that, simply from the geometry of the electron diffraction pattern of an incommensurably modulated structure, conclusive information can be obtained on the real space shape of this modulation. The method applied here is based on the 3 + 1 dimensional description of symmetry operations and can be summarized as follows: (1) reconstruct the three‐dimensional reciprocal space geometry of the modulated structure from electron diffraction information along different zone axes; (2) deduce the complete Bravais type symbol of the four‐dimensional structure from the general reflection conditions; and (3) derive the modulation function for each atom type from the superspace symmetry elements which result from the information of both modulation and basic structure. This method will be applied here in short on the Bi₂Sr₂Ca_nCu_n+1O_6+2n structure, for which system the results are in agreement with the ones recently obtained from neutron diffraction. For Tl₂Ba₂CuO₆ where no data from other diffraction techniques are available, a more complete calculation will be performed, in order to determine the shape of the displacement function for the different atom types; the results are in agreement with the observed High Resolution Electron Microscopy (HREM) images.