Observation of a high burnup rim-type structure in an advanced plutonium–uranium carbide fuel

The observation is reported of a ‘rim-type' structure with small subgrains in an advanced plutonium–uranium carbide (U_0.8Pu_0.2)C fuel pin, which had been irradiated in the Dounreay Fast Reactor to a burnup of 8.3% FIMA.     

Thermochemical modelling of electrotransport of uranium and plutonium in an electrorefiner

Electrotransport behaviour of U and Pu in a molten salt electrorefining cell has been numerically simulated with an improved thermochemical model. Depending upon saturated or unsaturated states of the liquid Cd electrodes with respect to U or Pu or with both U and Pu, 16 conditions of electrorefiner cell operation have been categorised and electrotransport simulated for all the realistic conditions. Algebraic equations for determining the compositions of the salt phase and the two electrodes under each condition of electrotransport are derived. Fractional mass transport coefficients and relative fractional mass transport coefficients are derived for each condition to illustrate the electrotransport behaviour. Comparison is made between modeling with concentration dependent and concentration independent activity coefficients for U and Pu in liquid Cd. The electrotransport to a solid cathode and anodic dissolution have also been simulated. Application of the model to reprocessing of spent metallic fuel is discussed with respect to U recovery, Pu enrichment and reconstitution of the spent fuel with desired fuel composition.     

Reactions between U–Zr alloys and Fe at 923 K

Interdiffusion experiments were carried out at 923 K with the diffusion couples consisting of U–23 at.% Zr/Fe and U–23 at.% Zr–1 at.% Ce/Fe. The reaction layer adjacent to the Fe was a single Zr-depleted UFe₂ phase. The phases in the reaction layers were estimated consistently with the calculated U–Zr–Fe ternary isotherm. The diffusion path obtained in this study was similar to that reported for the U–Pu–Zr/HT9-steel couple at 923 K, when those paths were expressed on the (U+Pu)–Zr–(Fe+Cr) composition triangle. The reaction layers grew in proportion to the square root of the annealing time. The addition of approximately 1 at.% of Ce to the U–23 at.% Zr alloy has little effect on the reaction between U–23 at.% Zr and Fe.     

Fundamental studies on extraction of actinides from spent fuels using liquefied gases – Conversion of copper into nitrate with NO2 and extraction of Nd(III) nitrate by CO2 with TBP

The nitrate conversion with liquefied NO₂ and the nitrate extraction with SC-CO₂ were demonstrated for copper powder and neodymium nitrate instead of actinide compounds. Copper contacted with NO₂ in an autoclave at 353 K changed to water-soluble compounds. By XRD analysis of the product in the atmospheric condition, the formation of copper nitrate hydrate (Cu(NO₃)₂·2.5H₂O) was confirmed. As for the extraction, neodymium nitrate loaded in an autoclave was successfully extracted with the fluid of TBP and SC-CO₂ at 15 MPa, 313 K.     

Effect of fluoride contamination on the growth of ZrO2 films

The mechanism by which fluoride ion degrades the oxide film on Zircaloy-2 has been investigated by deliberately contaminating specimens. Delaying the washing of specimens for 0, 60 and 1800 s after pickling gave sets of, respectively, well-pickled, poorly-pickled and pickle-stained specimens. These were oxidised initially in dry steam (300°C, 3.5 MPa) and were then transferred to water (300°C) for short periods (1, 2 or 7 days). The oxides produced were examined by weight gain, interferometry, impedance spectroscopy and optical, SEM and TEM microscopy. The initial oxidation rates in steam were little different for the three groups of specimens (1 or 2 days), although the interference coloured oxides showed a very different distribution of oxide thicknesses between the well-picked specimens and the other groups. Transfer to water rapidly resulted in thick, friable, porous oxides on the pickle-stained, but not the other specimens, that could not be examined by many techniques because of ready loss of oxide. The techniques that could be applied to these specimens showed that they consisted of apparently large oxide crystallites in multiple layers nearly normal to the oxide metal interface. The original surface topography was still visible in areas where this surface had not spalled, showing that the degradation occurred within the oxide. The severity of this attack was determined by the extent to which the original preparation technique had left oxyfluoride layers on the initial surfaces. It was deduced that these oxyfluoride layers developed porosity in which concentrated fluoride solutions could form during high temperature exposures in water. These solutions attacked the ZrO₂ film by hydrothermal dissolution and recrystallisation to give the large layered platelets in the degraded films. The oxyfluorides appear to be sufficiently hygroscopic that the same degradation process occurred generally in 300°C, 3.5 MPa steam, only locally in 0.1 MPa steam and not in moist air.