Search

International audience; Sensitivity analysis studies the effect of a change in a given parameter to a response function of the system under investigation. In reactor physics, this usually translates into the study of how cross sections and fission spectrum modifications affect the value of the multiplication factor, the delayed neutron fraction or the void coefficient for example. Generalized Perturbation Theory provides a useful tool for the assessment of adjoint weighed functions such as keff and void coefficient sensitivities. In this work, the capability of SERPENT code to perform sensitivity calculation based on GPT is used to study the TRIGA Mark II research reactor installed at L.E.N.A. of University of Pavia. A general sensitivity analysis to the most important reactor’s cross sections has been performed in order to highlight the biggest reactivity contributions. Two numerically challenging tasks related to GPT calculation have been performed thanks to the relatively quick Monte Carlo approach allowed by this reactor: investigating the linearity of the reactivity injection caused by the flooding of the central channel, and calculating the fuel void coefficient sensitivity to the coolant density.

Several irradiation experiments have confirmed the positive effect of adding Si to the matrix of an U(Mo) dispersion fuel plate on its in-pile irradiation behavior. E-FUTURE, the first experiment of the LEONIDAS program, was performed to select an optimum Si concentration and fuel plate heat treatment parameters for further qualification. It consisted of the irradiation of 4 distinct (regarding Si content and heat treatments), full size flat fuel plates in the BR2 reactor under bounding conditions (470 W/cm 2 peak BOL power, ∼70% peak burn-up). After the irradiation, the E-FUTURE plates were examined non-destructively and found to have pillowed in the highest burn-up positions. The destructive post-irradiation examination confirmed that the fuel evolves in a stable way up to a burn-up of 60% 235 U. Even in the deformed area (pillow) the U(Mo) fuel itself shows stable behavior and remaining matrix material was present. From the calculation of the volume fractions, the positive effect of a higher Si amount added to the matrix and the higher annealing temperature can be derived.

In this paper, we present the critical experiment performed in March 2008 in the high-flux reactor (HFR) to reproduce physical conditions similar to the core light water introduction accident. We present the HFR, the context of the study and two Monte Carlo models: TRIPOLI4 – JEFF3.1 and MCNP – ENDF/BVII. All comparisons between calculations and experimental results show, within both models, that the safety rods efficiencies, in AIC alloy, are always underestimated in normal as well as in accidental configuration of the reactor.

A method of determining partial pair correlation functions (pPCFs) for multicomponent systems from EXAFS data is presented. The method is based upon the regularization procedure of solving a Fredholm integral equation of the first kind using special iteration procedure. The effectiveness of the method has been tested on the model crystalline solids ZnBr2. We present the result for ZnBr2 aqueous solution obtained from experimental data at room temperature (303 K) and pressure 250 bar. Experiments were performed using an unique equipment at ESRF (Grenoble, France). Peculiar features of the method are high resolution for closely spaced shells and high accuracy in the determination of pPCFs.

Accurate knowledge of the single-molecule (self-) translational dynamics of liquid para-${\mathrm{H}}_{2}$ is an essential requirement for the calculation of the neutron scattering properties of this important quantum liquid. We show that, by using centroid molecular dynamics (CMD) quantum simulations of the velocity autocorrelation function, calculations of the total neutron cross section (TCS) remarkably agree with experimental data at the thermal and epithermal incident neutron energies where para-${\mathrm{H}}_{2}$ dynamics is actually dominated by the self-contributions. This result shows that a proper account of the quantum nature of the fluid, as provided by CMD, is a necessary and very effective condition to obtain the correct absolute-scale cross section values without the need of introducing any empirically adjusted quantity. At subthermal incident energies, appropriate modeling of the para-${\mathrm{H}}_{2}$ intermolecular (distinct) dynamics also becomes crucial, but quantum simulations are not yet able to cope with it. Existing simple models which account for the distinct part provide an appropriate correction of self-only calculations and bring the computed results in reasonable accord with TCS experimental data available until very recently. However, if just published cross section measurements in the cold range are considered, the agreement turns out to be by far superior and very satisfactory. The possible origin of slight residual differences will be commented on and suggests further computational and experimental efforts. Nonetheless, the ability to reproduce the total cross section in the wide range between 1 and 900 meV represents an encouraging and important validation step of the CMD method and of the present simple algorithm.

Converting biomass into hydrogen can be accomplished in supercritical water. State of the art experiments are thoroughly presented. Chars and tars formation may be the most significant technological problem. However, catalysis should be the solution to obtain higher yields of hydrogen and to decrease the amounts of chars and tars. As conversion yield reaches 98% with a proportion of hydrogen higher than 50% in the gaseous phase, the next step to consider in a successful transfer to the industrial scale is to evaluate the energy cost of the process. We made an evaluation of the energy efficiency of biomass gasification, basing our work on data of the literature. Our results show that the energy efficiency from thermodynamic calculation reaches 60% when considering hydrogen, carbon monoxide and methane as valuable species in the ideal case. Including energy recovery from the water at 280 bar and 740 °C, the overall energy yield reaches 90%. These calculations are overestimated because no heat losses are taken into account. However, this study shows that the key point of the process is energy recovery as the chemical reaction is endothermic and needs high temperature and a rather large ratio of water/biomass.

We developed a new apparatus dedicated to Supercritical Water Oxidation in order to overcome the two well-known problems of this process, which are corrosion and salt plugging. This reactor has both a double shell in titanium, which prevents corrosion, and a stirrer creating a turbulent flow, which prevents the sedimentation of inorganic compounds and enables better heat transfers. We first tested our reactor with easily oxidized effluents, i.e. pure organic compounds like PEG and dodecane. Then we oxidized two different kinds of effluent: one forming a corrosive phosphoric acid, and another one containing an inorganic salt in significant amount. We get conversion yields over 99.8% with organic and salt concentrations up to 6 and 4%, respectively, without production of chars or undesired gases like CO and CH4. © 2003 Elsevier B.V. All rights reserved.

Ion-pairing and dehydration phenomena occurring in ZnBr2 aqueous solutions from normal to critical T, P conditions were investigated by x-ray absorption spectroscopy. The respective influences of temperature, pressure, and concentration were studied. The evolution of the density of solute ions, probed by the height of the absorption edge, allowed us to get information on phase diagrams and salt precipitation. The average structural evolution deduced from extended x-ray absorption fine structure was related to the formation of complexes identified from x-ray absorption near edge structure analysis. Consequently, in noncritical conditions, an increase of temperature or concentration produces dehydration and ion-pairing, while a rise of pressure destroys the ion-pairs. In contrast, concentration and pressure have weaker effects on the local order in high P, T conditions. Moreover, ion pairing formation is found not to be specifically enhanced when the fluid is close to supercritical conditions as it also occ...

Local-order evolution around ions in aqueous solutions has been investigated between normal and hydrothermal conditions. The behavior of cations and anions in aqueous ZnBr2 solution were studied by performing x-ray absorption spectroscopy experiments at both Br and Zn edges. Extended x-ray absorption fine structure analyses are made on account of anharmonic treatment and multiple scattering contributions involving H atoms at the Br edge. The extended x-ray absorption fine structure results are coupled to x-ray absorption near-edge structure simulations in order to identify the complexes formed under normal and hydrothermal conditions. It appears that both Zn and Br ions are largely hydrated under normal conditions and that Zn–Br pairs are formed in hydrothermal conditions. This is related to an octahedral-to-tetrahedral evolution of the Zn local environment, the majority of Zn atoms being surrounded by water octahedra in normal conditions and by distorted tetrahedra involving Br and O atoms in hydrotherma...

Nuclear fuel plates based on a γU-Mo/Al mixture are proposed for research reactors. In this work their thermal behavior in the [425; 550°C] temperature range has been studied mainly by neutron and high energy X-ray diffraction. Even if complementary studies will be necessary, the kinetics of first the growth of the interaction layer between γU-Mo and Al and second of the γU-Mo destabilization have been accurately measured. This basic work should be helpful for defining manufacturing conditions for fuel plates with optimized composition.

International audience; The aim of this work is to validate experimentally the results of the Monte Carlo Recoil Energy Determination (MC-RED) nuclear physics code used to determine the deposited energy in a silicon volume taking into account the probabilistic approach of the physical phenomenon. A silicon sensor has been used to measure the deposited energy spectrum after an irradiation with a 14 MeV neutron source. Neutrons of 14 MeV were produced by the SAMES accelerator of the Valduc CEA research center. The experimental results were compared with the one obtained by the MC-RED code in the same silicon volume. To compare experiment and simulations, it requires that the complete neutron field be specified. We have to take into account the facility cell environment. Nuclear simulations were performed with the MCNP code to determine with accuracy the energy spectra of neutrons and gamma rays produced by this facility on the sensor. It is shown that the contribution of secondary neutrons is not negligible

The 'service of research on neutronics and criticality' of Valduc CEA research center, proposes several experimental facilities for neutron and gamma irradiation: two metallic reactors, a solution reactor, two accelerators.

X-ray absorption spectroscopy (XAS) experiments have been performed on 1.0 and 1.6 M ZnBr2 aqueous solutions under ambient to supercritical conditions. Both the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) show striking evolutions, which indicate major structural changes, with a strong reduction in hydration and the increasing formation of ZnBr pairs. A quantitative analysis of the evolution of the Zn environment is presented that supports the existence of a transition from an octahedral to a tetrahedral environment.