Harwell, United Kingdom
Harwell, United Kingdom

Time filter

Source Type

Bartels D.M.,University of Notre Dame | Henshaw J.,Harwell Laboratory | Sims H.E.,Harwell Laboratory
Radiation Physics and Chemistry | Year: 2013

Hydrogen is added to a pressurized water reactor (PWR) to suppress radiolysis and maintain reducing conditions. The minimum hydrogen concentration needed to prevent radiolysis is referred to as the critical hydrogen concentration (CHC). The CHC was measured experimentally in the mid-1990s by Elliot and Stuart in a reactor loop at Atomic Energy of Canada (AECL), and was found to be approximately 0.5. scc/kg for typical PWR conditions. This value is well below industry-normal PWR operating levels near 40. scc/kg. Radiation chemistry models have also predicted a low CHC, even below the AECL experimental result. In the last few years some of the radiation chemical kinetic rate constants have been re-measured and G-values have been reassessed by Elliot and Bartels. These new data have been used in this work to revise the models and compare them with AECL experimental data. It is quite clear that the scavenging yields tabulated for high-LET radiolysis by Elliot and Bartels are not appropriate to use in the present context, where track-escape yields are needed to describe the homogeneous recombination kinetics in the mixed radiation field. In the absence of such data for high temperature PWR conditions, we have used the neutron G-values as fitting parameters. Even with this expedient, the model predicts at least a factor of two smaller CHC than was observed. We demonstrate that to recover the reported CHC result, the chemistry of ammonia impurity must be included. © 2012 Elsevier Ltd.

Sarsfield M.J.,National Nuclear Laboratory | Sims H.E.,Harwell Laboratory | Taylor R.J.,National Nuclear Laboratory
Solvent Extraction and Ion Exchange | Year: 2011

Formo- and aceto-hydroxamic acids are very effective reagents for stripping tetravalent actinide ions from a tri-butyl phosphate phase into nitric acid. A model describing the partitioning of actinide (IV) ions has been derived accounting for reactions in the aqueous and solvent phases, including complex formation with nitrate and hydroxamate ions. Predicted distribution ratios for Np(IV) are compared with experimental data in the presence of aceto-hydroxamate ions. Additionally, a value of 0.473 (±0.004) kgmol-1 for the Np(IV) ion interaction coefficient with nitrate ions (εNp4+-NO3-) was determined. © Taylor & Francis Group, LLC.

Marquis E.A.,University of Oxford | Hyde J.M.,University of Oxford | Hyde J.M.,Harwell Laboratory
Materials Science and Engineering R: Reports | Year: 2010

In many materials, mechanistic understanding of material microstructure/property relationships requires knowledge of alloy structures at the atomic scale. This remains one of the main challenges of materials science. Historically, because of insufficient spatial resolution of available microstructural techniques, theories relating the role of alloying elements to materials properties were inferred from phenomenological studies. More recently, with the advent of techniques such as atom-probe tomography the spatial resolution limits have been dramatically improved. For instance, since the speculation by Cottrell and Bilby in 1949, it was generally believed that solute segregation at dislocations leads to strain hardening, but the direct proof came only recently when Blavette et al. (Nature, 1999) directly imaged a solute atmosphere at an edge dislocation, using atom-probe tomography. The recent progress in atom-probe tomography (both in dataset size and materials that can be analysed) enables atomic-scale studies of the structures of alloys and, more specifically, the analysis of solute behaviour which is a crucial issue in all areas of materials science. Clustering, ordering, site occupancy and solute/defect interactions are topics that are relevant to all classes of materials and directly affect the mechanical, electrical, magnetic, transport, etc., properties of materials. The real space information from atom-probe tomography provides a direct comparison with atomic simulations. Furthermore experimental data from the early stages of phase transformation can now be used to help validate the energetics used in atomistic modelling. This review will highlight: (1) the current limits of spatial and chemical resolution of atom-probe tomography and the resulting limits of data interpretation, (2) the data analysis tools that have been developed so far and the possible future paths for development and (3) examples where atom-probe tomography has provided mechanistic insight on the behaviour of solutes, in particular clustering, ordering and interactions with defects, and their effect on material properties. © 2010 Elsevier B.V. All rights reserved.

Ni N.,University of Oxford | Lozano-Perez S.,University of Oxford | Jenkins M.L.,University of Oxford | English C.,Harwell Laboratory | And 3 more authors.
Scripta Materialia | Year: 2010

Much work has been carried out over the past 40 years on the oxidation of zirconium alloys used for nuclear fuel cladding, but there is no consensus as to the critical factors that control kinetics, even though this is vital for the design of materials for higher burn-up regimes. One unanswered question is the role of porosity in controlling oxidation. Here we show that the nature of the nanoscale porosity can be correlated to different stages of the oxidation process. © 2009 Acta Materialia Inc.

Hyde J.M.,Harwell Laboratory | Hyde J.M.,University of Oxford | Marquis E.A.,University of Oxford | Wilford K.B.,Rolls-Royce | Williams T.J.,Rolls-Royce
Ultramicroscopy | Year: 2011

Variants of the maximum separation method have become the de-facto methodologies for the characterisation of nanometre scale clusters in atom probe tomography (APT) data obtained from dilute solid solutions. All variants rely on a number of parameters and it is well known that the precise values for these parameters strongly influence estimates of cluster size and number density. Quantitative analyses require an improved understanding of the inter-relationship between user-defined parameters, experimental parameters such as detection efficiency and the resultant parameterisation of the microstructure. A series of simulations has been performed to generate clusters with a range of compositions (50-100%) and diameters (1.5-2.5 nm) in a dilute solid solution. The data were degraded to simulate the effects of the finite detection efficiencies and positioning uncertainties associated with the ECOPoSAP and LEAP-3000X HR. An extensive analysis of each resultant dataset, using a range of values for the maximum separation parameters was then performed. Optimum values for each material condition were identified and it is shown that it is possible to characterise cluster size, number density and matrix chemistry. However, accurate estimates of cluster compositions are more difficult and absolute measurements must be treated with caution. Furthermore, it is shown that DMAX must increase with decreasing detection efficiency and consequently clusters of a specific size will appear slightly larger in atom probes with a lower detection efficiency. © 2010 Elsevier B.V.

Styman P.D.,University of Oxford | Hyde J.M.,University of Oxford | Hyde J.M.,Harwell Laboratory | Wilford K.,Rolls-Royce | Smith G.D.W.,University of Oxford
Ultramicroscopy | Year: 2013

Atom Probe Tomography (APT) is extensively used for the analysis of RPV steels. However, many different analysis methods and cluster search parameters are used, making comparisons between different datasets difficult. Suitable dmax and Nmin parameters for the maximum separation method are investigated. In a randomised distribution of solute there is a finite probability that a group of more than Nmin solute ions exists within the dmax distance. The same is true for experimental datasets from samples which have been thermally aged or irradiated, however these background clusters are not the result of ageing, they are purely statistically random co-incidences. A method is presented for identifying such "background" statistical clusters in real APT data sets, based upon their size and composition, which allows for improved sensitivity to small clusters. © 2012.

English C.A.,Harwell Laboratory | English C.A.,University of Oxford | Jenkins M.L.,University of Oxford
Philosophical Magazine | Year: 2010

A series of TEM experiments was carried out to systematically investigate the formation of vacancy dislocation loops in displacement cascades in molybdenum. Single-crystal foils of high-purity molybdenum were irradiated with Sb+ single ions and Sb2 + and Sb3 + molecular ions to low doses (≤1016 ions m-2). Three different ion energies were employed (60, l20 and 180 keV) in order to systematically vary the total cascade energy and the energy per atom in the molecular ion. Dislocation loop sizes and defect yields were found to be larger for molecular ions than for single ions of the same energy. In (011) foils, most loops had Burgers vectors b = a/2111 lying in the plane of the foil. However, in molecular ion irradiations, a small fraction of loops with b = a100 was also found. This fraction was higher for Sb3 + than for Sb 2 + ions and also increased with ion energy. In (001) foils, defect yields were much smaller because of the loss of glissile a/2111 loops to the surface, but a100 loops were still present in molecular ion irradiations. The habit planes of both a/2111 and a100 loops were consistent with nucleation on {110} planes by the Eyre-Bullough mechanism. These results are compared with recent molecular dynamics simulations of the effect of the mass of primary knock-on atoms on displacement cascades in iron. © 2010 Taylor & Francis.

Styman P.D.,University of Oxford | Hyde J.M.,University of Oxford | Hyde J.M.,Harwell Laboratory | Wilford K.,Rolls-Royce | And 2 more authors.
Progress in Nuclear Energy | Year: 2012

Copper precipitation in irradiated RPV steels is well known to have a deleterious effect on mechanical properties. In order to understand the contribution of thermal ageing to RPV embrittlement a high copper (0.44 at.%), high nickel (1.6 at.%) model RPV weld was thermally aged at 365 °C for times up to 90,000 h. Atom Probe Tomography (APT) was employed to study the precipitation of solutes, primarily copper, nickel, manganese and silicon within the matrix and at grain boundaries. As expected, a high number density of 1-4 nm radius copper rich precipitates was observed. Nickel, manganese and silicon were found at the precipitate matrix interface, and the evolution of the composition of this interface was investigated with ageing time. Segregation of solutes to grain boundaries particularly P, Mo and C was observed, along with enrichments of Ni, Mn and Si, which have not previously been reported in long term thermally aged RPV steels. Preliminary results on several large (>10 nm) Ni-Mn-Si rich features observed at a grain boundary are also presented. These features are rich in Ni (∼30%), Mn (∼15%) and Si (∼12%) and are virtually copper-free. © 2011 Elsevier Ltd. All rights reserved.

Ortner S.R.,Harwell Laboratory
Fatigue and Fracture of Engineering Materials and Structures | Year: 2011

Several models of fracture in ferritic steel consider matrix strain to be an indicator of the condition required for second-phase particle cracking (i.e. microcrack nucleation). Recent simulations predict that stress à - strain (W) is a better supported indicator. This paper examines the effect of replacing strain by W within the EOH fracture model. There is no improvement in the descriptions of fracture toughness and initiation site properties for eight different steels over a range of temperatures. The use of W, however, leads to the convergence of data from different steels, and permits estimates of the particle cracking criterion to be made from metallographic data, when suitable fractographic data are unavailable. Overall, experiment agrees with simulation in finding W a more appropriate parameter to use than strain alone. © 2011 Crown.

Bullough R.,Harwell Laboratory
Philosophical Magazine | Year: 2013

Sir Alan Cottrell has made huge seminal contributions to our basic understanding of radiation damage processes in both fissile and non-fissile materials. Much of this ground-breaking work was accomplished in the mid-1950s when Cottrell was working at Birmingham University and later at Harwell Laboratory. It is interesting to relate the earlier progress in the 1950s to our present understanding of the phenomenon. © 2013 Taylor & Francis.

Loading Harwell Laboratory collaborators
Loading Harwell Laboratory collaborators