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Varela-Castro G.,Polytechnic University of Catalonia | Cabrera J.M.,Polytechnic University of Catalonia | Cabrera J.M.,Center Tecnologic | Prado J.M.,Polytechnic University of Catalonia | Prado J.M.,Center Tecnologic
Advanced Materials Research | Year: 2012

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanisms available for the control of microstructure. This mechanism occurs during hot forming operations over a wide range of metals and alloys and it is known to be as a powerful tool which can be used to the control of the microstructure and properties of alloys. Therefore is important to know, particularly in low stacking fault energy (SFE) materials, the precise time for which DRX is available to act. At constant strain rate such time is defined by a critical strain, ε c Unfortunately this critical value is not directly measurable on the flow curve; as a result different methods have been developed to derive it. Focused on steels, in the present work the state of art on the critical strain for the initiation of DRX is summarized and a review of the different methods and expressions for determining ε c is included. The collected data is suitable to feeding constitutive models. © (2012) Trans Tech Publications, Switzerland.

Martinez-Llado X.,Center Tecnologic | Valderrama C.,Polytechnic University of Catalonia | Rovira M.,Center Tecnologic | Rovira M.,Polytechnic University of Catalonia | And 5 more authors.
Geoderma | Year: 2011

The sorption of Sb(V) onto natural calcareous soils was studied in batch and column experiments as a function of physicochemical properties of the soil, namely: organic matter, the active fraction of Fe and Al and the pH of the soils. Batch experiments were performed in order to determine the sorption capacity and the kinetic rate of the sorption. Freundlich isotherm described properly the equilibrium experimental data and the kinetic results show that the sorption was very slow in all the soils studied (equilibrium after 7 days) when compared with literature data for Sb(V) sorption onto soils with higher iron content. Column experiments were carried out in order to determine the mobility of Sb(V) in the soils. The Thomas model can describe partially the breakthrough curves obtained for the Sb(V) sorption on the soils. The fitting of the model to the experimental data would indicate that transport and kinetic parameters also affected the dynamic sorption behavior. The experimental breakthrough curves can be properly described by the convective-dispersive equation (CDE) by using the CXTFIT code, and the parameters obtained by the model were in concordance to the sorption capacity reported in the column experiments. This study indicates that sorption of Sb(V) onto the soils is mainly affected in batch experiments by their 'active' Fe and Al content; while in column experiments organic matter content appeared as an important parameter. Desorption experiments resulted in a metalloid recovery above 90%, which indicates a fast and effective method to recover Sb(V) from calcareous soils and has implications for its geochemical behavior (vertical mobility in soil profiles) as a result of its relatively low Sb retention. © 2010 Elsevier B.V.

Serrano-Purroy D.,Institute for Transuranium Elements | Clarens F.,Center Tecnologic | Gonzalez-Robles E.,Center Tecnologic | Glatz J.P.,Institute for Transuranium Elements | And 6 more authors.
Journal of Nuclear Materials | Year: 2012

Two weak points in Performance Assessment (PA) exercises regarding the alteration of Spent Nuclear Fuel (SNF) are the contribution of the so-called Instant Release Fraction (IRF) and the effect of High Burn-Up Structure (HBS). This manuscript focuses on the effect of HBS in matrix (long term) and instant release of a Pressurised Water Reactor (PWR) SNF irradiated in a commercial reactor with a mean Burn-Up (BU) of 60 GWd/tU. In order to study the HBS contribution, two samples from different radial positions have been prepared. One from the centre of the SNF, labelled CORE, and one from the periphery, enriched with HBS and labelled OUT. Static leaching experiments have been carried out with two synthetic leaching solutions: bicarbonate (BIC) and Bentonitic Granitic Groundwater (BGW), and in all cases under oxidising conditions. IRF values have been calculated from the determined Fraction of Inventory in Aqueous Phase (FIAP). In all studied cases, some radionuclides (RN): Rb, Sr and Cs, have shown higher release rates than uranium, especially at the beginning of the experiment, and have been considered as IRF. Redox sensitive RN like Mo and Tc have been found to dissolve slightly faster than uranium and further studies might be needed to confirm if they can also be considered part of the IRF. Most of the remaining studied RN, mainly actinides and lanthanides, have been found to dissolve congruently with the uranium matrix. Finally, Zr, Ru and Rh presented lower release rates than the matrix. Higher matrix release has been determined for CORE than for OUT samples showing that the formation of HBS might have a protective effect against the oxidative corrosion of the SNF. On the contrary, no significant differences have been observed between the two studied leaching solutions (BIC and BGW). Two different IRF contributions have been determined. One corresponding to the fraction of inventory segregated in the external open grain boundaries, directly available to water and very significant during the first days of the experiment; and a second one corresponding to a less accessible, most probably located at the internal grain boundaries, one order of magnitude lower than the first one at equal given dissolution times but of much longer period of incidence. Unlike matrix release results, higher Cs IRF release was found for OUT than for CORE sample. This effect can be attributed to thermal migration of Cs to the periphery of the fuel during irradiation. In the case of Rb no clear differences were observed between CORE and OUT showing equilibrium between the opposing thermal migration and matrix effects. Finally, Sr CORE/OUT release ratio showed similar behaviour to matrix release, thus proving no significant thermal migration during irradiation. © 2012 Elsevier B.V. All rights reserved.

Yaroshchuk A.,Polytechnic University of Catalonia | Martinez-Llado X.,Center Tecnologic | Llenas L.,Center Tecnologic | Rovira M.,Center Tecnologic | And 2 more authors.
Journal of Membrane Science | Year: 2011

The solution-diffusion-film model previously used for the description of rejection of single salts by NF membranes is extended to the case of electrolyte mixtures consisting of a single dominant salt and (any number of) trace ions. An especially simple analytical solution is obtained in the approximation of constant (composition-independent) membrane permeabilities with respect to the dominant salt and trace ions. A quasi-analytical protocol is developed for the correction of observable trace rejections for concentration polarization within the scope of film model. It is demonstrated that the trans-membrane transfer of trace ions is essentially influenced by the electric fields arising due to the differences in the membrane permeabilities with respect to the cations and anions of dominant salt. Accordingly, information on these permeabilities can be extracted from experimental data on the rejection of trace ions as a function of trans-membrane volume flow. The membrane permeability to the trace ions themselves can also be estimated. This is clearly advantageous over just a single value of membrane permeability to the salt obtainable from the interpretation of traditional measurements with single salts within the scope of the same model. This novel theoretical approach is used for the interpretation of experimental data on the rejection of various dominant single salts and trace ions by a commercial NF membrane in a cross-flow setup with a rectangular spacer-filled feed channel. In this way, quantitative information could be obtained on the membrane permeabilities with respect to several single ions (Na +, Cl -, Ca 2+, SO 4 2-) being parts of dominant salts as well as traces. This information can be useful for the optimization of nanofiltration of mixed electrolyte solutions in various applications. © 2010 Elsevier B.V.

Meca S.,Center Tecnologic | Martinez-Torrents A.,Center Tecnologic | Martinez-Torrents A.,Polytechnic University of Catalonia | Marti V.,Center Tecnologic | And 5 more authors.
Dalton Transactions | Year: 2011

The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR code. Two different complexes were found at a H2O2/U(vi) ratio lower than 2. A graphical method was used in order to obtain the formation constants of such complexes and the STAR program was used to refine the formation constants values because of its capacity to treat multiwavelength absorbance data and refining equilibrium constants. The values obtained for the two complexes identified were: UO2 2+ + H2O2 + 4OH- UO2(O2)(OH)2 2- + 2H2O: logβ° 1,1,4 = 28.1 ± 0.1(1)UO 2 2+ + 2H2O2 + 6OH- UO2(O2)2(OH)2 4- + 4H 2O: logβ° 1,2,6 = 36.8 ± 0.2(2) At hydrogen peroxide concentrations higher than 10-5 mol dm -3, and in the absence of carbonate, the UO2(O 2)2(OH)2 4- complex is predominant in solution, indicating the significant peroxide affinity of peroxide ions for uranium and the strong complexes of uranium(vi) with peroxide. © 2011 The Royal Society of Chemistry.

Gutierrez D.,Center Tecnologic | Lara A.,Center Tecnologic | Casellas D.,Center Tecnologic | Prado J.M.,Center Tecnologic | Prado J.M.,Polytechnic University of Catalonia
Advanced Materials Research | Year: 2010

The Forming Limit Diagrams (FLD) are widely used in the formability analysis of sheet metal to determine the maximum strain, which gives the Forming Limit Curve (FLC). It is well known that these curves depend on the strain path during forming and hence on the test method used to calculate them. In this paper, different stretching tests such as the Nakajima and the Marciniak tests were performed, with different sample geometries to obtain points in different areas of the FLD. An optical analysis system was used, which allows following the strain path during the test. The increasing use of advanced high-strength steels (AHSS) has created an interest in determining the mechanical properties of these materials. In this work, FLCs for a TRIP steel were determined using Nakajima and Marciniak tests, which revealed different strain paths depending on the type of test. Determination of the FLCs was carried out following the mathematical calculations indicated in the ISO 12004 standard and was also compared with an alternative mathematical method, which showed different FLCs. Finally, the tests were verified by comparing the strain paths of the Nakajima and Marciniak tests with a well-known mild steel. © (2010) Trans Tech Publications.

Sureda R.,Polytechnic University of Catalonia | Martinez-Llado X.,Center Tecnologic | Rovira M.,Polytechnic University of Catalonia | Rovira M.,Center Tecnologic | And 4 more authors.
Journal of Hazardous Materials | Year: 2010

Strontium-90 is considered the most important radioactive isotope in the environment and one of the most frequently occurring radionuclides in groundwaters at nuclear facilities. The uranyl peroxide studtite (UO2O2·4H2O) has been observed to be formed in spent nuclear fuel leaching experiments and seems to have a relatively high sorption capacity for some radionuclides. In this work, the sorption of strontium onto studtite is studied as a function of time, strontium concentration in solution and pH. The main results obtained are (a) sorption is relatively fast although slower than for cesium; (b) strontium seems to be sorbed via a monolayer coverage of the studtite surface, (c) sorption has a strong dependence on ionic strength, is negligible at acidic pH, and increases at neutral to alkaline pH (almost 100% of the strontium in solution is sorbed above pH 10). These results point to uranium secondary solid phase formation on the spent nuclear fuel as an important mechanism for strontium retention in a high-level nuclear waste repository (HLNW). © 2010 Elsevier B.V.

Gimenez J.,Polytechnic University of Catalonia | Pablo J.D.,Polytechnic University of Catalonia | Pablo J.D.,Center Tecnologic | Martinez M.,Polytechnic University of Catalonia | And 3 more authors.
Journal of Colloid and Interface Science | Year: 2010

Natural hematite was used for the removal of arsenic(III) and arsenic(V) from aqueous solution. The experimental breakthrough curves were obtained in fixed-bed columns. The transport of arsenic in a simplified fixed-bed configuration was quantified by using the CXTFIT code, which was used to estimate the transport and sorption parameters of the convective-dispersive equation (CDE) and the two-site deterministic nonequilibrium (TSM/CDE) model by fitting the models to the experimental breakthrough curves (BTC). The prediction of the breakthrough curves performed by the two-site nonequilibrium sorption model resulted in a good fit, indicating that this model can properly describe the transport and sorption processes of arsenic on natural hematite. Additionally the parameters obtained indicate that nonequilibrium sorption governs the As(III) and As(V) uptake onto hematite in a fixed-bed column. No significant differences in the transport and sorption parameters of As(III) and As(V) on natural hematite were obtained; the retardation factor values were in the same order of magnitude for both species. © 2010 Elsevier Inc.

Torrento C.,Institute Of Diagnosi Ambiental I Estudis Of Laigua Idaea | Urmeneta J.,University of Barcelona | Otero N.,University of Barcelona | Soler A.,University of Barcelona | And 2 more authors.
Chemical Geology | Year: 2011

Using chemical, isotopic and microbiologic techniques we tested in laboratory experiments the extent to which the addition of pyrite to groundwater and sediments from a nitrate-contaminated aquifer could stimulate denitrification by indigenous bacteria. In addition to this biostimulated approach, a combined biostimulated and bioaugmented treatment was also evaluated by inoculating the well-known autotrophic denitrifying bacterium Thiobacillus denitrificans. Results showed that the addition of pyrite enhanced nitrate removal and that denitrifying bacteria existing in the aquifer material were able to reduce nitrate using pyrite as the electron donor, obviating the need for the inoculation of T. denitrificans. The results of the 16S rRNA and nosZ gene-based DGGE and the quantitative PCR (qPCR) showed that the addition of pyrite led to an increase in the proportion of denitrifying bacteria and that bacterial populations closely related to the Xanthomonadaceae might probably be the autotrophic denitrifiers that used pyrite as the electron donor. Not only autotrophic but also heterotrophic denitrifying bacteria were stimulated through pyrite addition and both populations probably contributed to nitrate removal. Isotopic analyses (δ15N and δ18ONO3) were used to monitor enhanced denitrification and the N and O isotopic enrichment factors (-26.3±1.8‰ and -20.4±1.3‰, respectively) allowed to calculate the degree of natural nitrate attenuation in the aquifer. Furthermore, flow-through experiments amended with pyrite confirmed the long-term efficiency of the process under the study conditions. Further research under field conditions is needed to determine whether stimulation of denitrification by pyrite addition constitutes a feasible bioremediation strategy for nitrate-contaminated aquifers. © 2011 Elsevier B.V.

Colas E.,Amphos 21 | Grive M.,Amphos 21 | Rojo I.,Center Tecnologic | Duro L.,Amphos 21
Radiochimica Acta | Year: 2011

Thorium complexation with gluconate has been studied from solubility experiments at gluconate concentrations between 10-6 and 10 -1 mol/l, with pHc between 9 and 13 and I = 0.5 M (NaClO4). Solubility experiments indicate the formation of a 1:1 thorium: gluconate complex. The stability constant for this species, extrapolated to I = 0 by using the SIT (specific interaction theory), has been determined to be log10 K° = - 11.5±0.6 for the reaction: Th4+ +GH4 - +2H2O = Th(OH) 2(GH2)-+4H+ In the presence of ThO2 xH2O, sorption of the organic ligand onto solid surface seems to limit thorium dissolution. © by Oldenbourg Wissenschaftsverlag, München.

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