Search

International audience; We present molecular dynamics simulations of aqueous iron(II) systems in the presence of polyacrylic acid (PAA) under the extreme conditions that take place in the secondary coolant circuit of a nuclear power plant. The aim of this work is to understand how the oligomer can prevent iron(II) deposits, and to provide molecular interpretation. We show how, to this end, not only the complexant ability is necessary, but also the chain length compared to iron(II) concentration. When the chain is long enough, a hyper-complexation phenomenon occurs that can explain the specific capacity of the polymer to prevent iron(II) precipitation.

Reactor physics, thermal hydraulics or material sciences are mainly studied to understand phenomena occurring in nuclear power plants or to improve the performance of existing or future reactors: improvement of core performances, research of new materials (fuels, core, reactor pressure vessels, internal structures, …) and so on. Nevertheless, in the same way as these physical sciences, chemical sciences (solution chemistry, thermodynamics, kinetics, ...) can also contribute to these objectives.Firstly, to improve availability, safety and lifetime of existing reactors, the best chemical conditioning has to be used in the different nuclear reactor circuits (primary, secondary, tertiary and auxiliary circuits). In that way, solution pH (B-Li coordinated chemistry), redox (value and chemical choice), water treatment, on-line chemical automates have, for example, to be optimized. Secondly, chemistry allows understanding phenomena occurring in the different reactors circuits. In the primary circuit, the understanding of the contamination by corrosion products (Fe, Ni, Cr, …), activation products (58 Co, 60 Co, …), fission products and actinides is a crucial issue for reactor operation and design. The main processes involved in the contamination transfer are dissolution/precipitation, erosion/deposition, convection, purification, neutron activation, radioactive decrease. Consequently, the chemistry knowledge has a role to play in the same way as other sciences (nuclear physics, material science, thermohydraulics, …). In the secondary circuit, formation of concentrated media, which can lead to the tube fouling and sometimes to the tube support plates blockage of steam generators, is of major concern. These phenomena have for consequences a loss of thermal performance and efficiency of SGs. Chemistry, in addition to thermal hydraulics, can help to explain them and consequently to their mitigation by, for example, optimization of the secondary side chemical conditioning (pH level, amine choice, …). In the tertiary circuit, scale, corrosion, deposits and microbial growth affect the plant performance and can be partially controlled by a “good” chemistry.To understand the chemistry role or impact, an important R&D work is necessary. Indeed, only few data exist in the literature for the physico-chemical conditions met in a nuclear reactor. So these data have to be obtained either experimentally (solubility measurements, liquid-steam equilibrium study, solid solution interfacial flux measurements and so on) or by extrapolation (which implies the development or the use of theoretical models (for example, for temperature or ionic strength effects)). Then, chemical database have to be built and simulation codes (for example, to describe cold shutdowns or SGs tube fouling) have to be developed. These R&D studies are either home-made or realized in collaboration with many companies such as EDF, AREVA NP or GDF SUEZ.The goal of this paper is to present the CEA methodology used for these R&D studies and few applications of chemistry to the understanding of some phenomena occurring in water-cooled nuclear reactors.

In the framework of environmental studies, it is important to understand the interaction of humic substances with cations (heavy metals, radionuclides) and to determine their complexation constants in order to evaluate their potential impact on their fate. For this purpose, two techniques have been used: electrospray ionization mass spectrometry, a newly used technique in speciation studies, and time-resolved laser-induced fluorescence spectrometry, a well-known technique for such studies. As a first step, for simplification purposes and to compare both techniques, simple molecules having functional groups present in humic substances have been selected, such as acetic, glycolic, and 4-hydroxyphenylacetic acids. Both techniques have been used to obtain stoichiometries and complexation constants between these simple molecules and europium (III).

Decorporation of radionuclides is of continuous interest in order to reduce doses in case of occupational or accidental human exposure. In the present study, insights into the non-covalent interactions that hold the well-known chelating agent DTPA (diethylenetriaminepentaacetic acid) with inorganic elements of interest, such as europium and strontium, and their ability to form stable complexes, are investigated with two spectroscopic techniques, i.e., electrospray ionization mass spectrometry (ESI-MS) and time-resolved laser-induced fluorescence (TRLIF). First investigations are on DTPA and europium alone and end with a complete study of the Eu–DTPA system. The pH variation allows one to readily investigate whether different species (protonated, hydrolyzed, etc.) exist in the pH range 2–9 and evaluate the stoichiometry and conditional stability constant for the Eu–DTPA complex. Additional experiments by ESI-MS are reported for Sr(II) in interaction with DTPA and EDTA.

Time-resolved laser-induced fluorescence has been used to investigate Eu complexes formed with a few main ligands encountered in natural waters: hydroxide, carbonate and humic substances. By varying pH and concentrations of ligands at fixed europium concentration and ionic strength, it was possible, together with free europium Eu3+, to identify spectrally and temporally carbonate complexes, namely Eu(CO3)+, Eu(CO3)2− and Eu(CO3)33− and humate complexes EuHA. For hydroxide complexes, no differences were found in terms of fluorescence spectra and lifetimes. A spectral interpretation is described by using deconvolution for all systems.

Peptides (33-34 amino acids long) corresponding to the helix-turn-helix (EF-hand) motif of the calcium binding site I of Paramecium tetraurelia calmodulin have been synthesized. The linear sequence was unable to acquire a native-like conformation and calcium binding. However, incorporation of a well-positioned disulfide bond bridging the two putative helical regions greatly improved the ordered structure and binding properties. Analyzed by electrospray mass spectrometry, circular dichroism and time-resolved laser-induced fluorescence, such a disulfide-stabilized peptide is shown to acquire a calcium-dependent helical conformation and exhibits native-like affinity for calcium, terbium and europium ions with 30+/-1, 3.5+/-0.6 and 0.6+/-0.1 microM dissociation constants, respectively. Comparable affinities were calculated within the biological construct comprising the entire domain I of Arabidopsis taliana calmodulin. Single sequence mutation (Glu25Asp) in the binding loop of the peptide abolishes calcium affinity, but preserves lanthanide affinity, showing that metal selectivity can be modulated by specific mutations. Such disulfide-stabilized peptides represent useful models to engineer metal specificity in new calmodulin proteins, facilitating the development of new systems to monitor metal pollution in biosensors and to increase metal binding capability of bacterial and plant cells in bioremediation techniques.

Time-resolved laser-induced fluorescence (TRLIF) has been used to investigate Eu complexes formed with inorganic and organic ligands encountered in natural waters as well as Eu complexes formed during the interaction of simulated nuclear waste glass with claywater. By comparing spectral and temporal data obtained by TRLIF on various europium solutions containing humic acids (HA) and carbonates (CO3), it has been possible to confirm the presence of mixed europium complexes such as Eu–CO3–HA. These results are quite important for speciation purposes, in particular in the framework of environmental studies.

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

A detailed study, using a panel of spectroscopic analytical methods, of the complexation between 1-hydroxyethane-1,1'-diphosphonic acid (HEDP) and uranyl ion (UO(2)(2+)) is reported. Results suggest that the metal complex is present as only 1:1 (metal/ligand) species at low concentration (10(-)(4) M). The conditional constants of this complex were determined at various pH using time-resolved laser-induced fluorescence (TRLIF) and electrospray ionization mass spectrometry (ESI-MS). Further investigations indicate the presence of a 1:2 (metal/ligand) complex at higher concentrations ( approximately 10(-)(2) M). Selectivity studies as well as structural aspects are presented.

The protonation constants (logKiH) of new N-acetate or N-propionate substituted azamacrocycles derived from 12-membered pyridinoazamacrocycles were determined by potentiometric methods. The number of coordinated water molecules, determined from the luminescence lifetime of the EuIII complexes, indicates a decreasing degree of hydration upon increasing the ring size of the pendant arms. Each ligand forms a 1:1 complex with EuIII, for which the stability constants (logKtherm) have been measured by time-resolved laser-induced europium luminescence. The replacement of acetate pendant groups by propionate increases the overall basicity of 12-membered pyridinoazamacrocycles but decreases the stability constant of the EuIII complexes from logK = 21.1 with three N-acetate pendant arms to logK = 13.7 with two N-propionate and one N-acetate pendant arms and to logK = 10.7 with three N-propionate pendant arms. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Characterisation of the molecular structure of aquatic fulvic acids (FA) has been performed using a quadrupole time-of-flight (Q-TOF) mass spectrometer equipped with an electrospray ionisation interface. Molecular masses centred around 450 Da and sinusoidal spectral distributions have been obtained for all fulvic acids. Tandem mass spectrometry (MS/MS) experiments showed losses of 18 Da (H2O) and 44 Da (CO2), and possible molecular structures were determined for the first time to our knowledge. A methodology is reported for evaluating the average elemental composition of FA from high-resolution mass spectra by processing post-acquisition data calculations using molecular size distributions and atomic compositions of ions. The results are found to be consistent with elemental analysis data. Copyright © 2001 John Wiley & Sons, Ltd.