Absolute Reactivity Research Articles

Evaluation of kinetics mechanisms ability to predict natural gas reactivity using a fuel substitution method

Previous homogeneous-charge compression-ignition engine fuel substitution experiments1 were used to test the performance of five different chemical kinetics mechanisms for natural gas fuels. A two-zone model was used to simulate the engine experiments using the five mechanisms tested. Simulations of engine experiments for six distinct operating conditions using based fuel mixtures of methane/hydrogen and methane/propane were performed along with simulations of fuel substitution sweeps with fixed boundary conditions using hydrogen, ethane, ethylene, propane, and n-butane as test fuels. The simulation results were used to quantify the ability of mechanisms to predict both the absolute reactivity of the reference fuels and the reactivity of the test fuels relative to the reference fuels using two metrics: required temperature at intake valve closure (IVC) with fixed fuel concentrations and required reference fuel concentrations with fixed IVC temperature. The five mechanisms used for this study showed substantially different results, most notably related to their propane chemistry. Simulations using the LLNL mechanism resulted in the closest match to propane engine test results, while the Aramco and C5-49 mechanisms showed some disagreement, and the GRI and UBC mechanisms showed much larger discrepancies. Simulations with all five mechanisms showed good agreement with hydrogen engine test results.

Evaluation of coolant density history effect in RBMK type fuel modelling

The axial heterogeneous void distribution in a fuel channel is a relevant and important issue during nuclear reactor analysis for LWR, especially for boiling water channel-type reactors. Variation of the coolant density in fuel channel has an effect on the neutron spectrum that will in turn have an impact on the values of absolute reactivity, the void reactivity coefficient, and the fuel isotopic compositions during irradiation. This effect is referring to as the history effect in light water reactor calculations. As the void reactivity effect is positive in RBMK type reactors, the underestimation of water density heterogeneity in 3D reactor core numerical calculations could cause an uncertainty during assessment of safe operation of nuclear reactor. Thus, this issue is analysed with different cross-section libraries which were generated with WIMS8 code at different reference water densities. The libraries were applied in single fuel model of the nodal code of QUABOX-CUBBOX/HYCA. The thermohydraulic part of HYCA allowed to simulate axial water distribution along fuel assembly model and to estimate water density history effect for RBMK type fuel.

Comparison of simultaneous arterial spin labeling MRI and 15O-H2O PET measurements of regional cerebral blood flow in rest and altered perfusion states

Arterial spin labelling (ASL) is a non-invasive magnetic resonance imaging (MRI) technique that may provide fully quantitative regional cerebral blood flow (rCBF) images. However, before its application in clinical routine, ASL needs to be validated against the clinical gold standard, 15O-H2O positron emission tomography (PET). We aimed to compare the two techniques by performing simultaneous quantitative ASL-MRI and 15O-H2O-PET examinations in a hybrid PET/MRI scanner. Duplicate rCBF measurements were performed in healthy young subjects (n = 14) in rest, during hyperventilation, and after acetazolamide (post-ACZ), yielding 63 combined PET/MRI datasets in total. Average global CBF by ASL-MRI and 15O-H2O-PET was not significantly different in any state (40.0 ± 6.5 and 40.6 ± 4.1 mL/100 g/min, respectively in rest, 24.5 ± 5.1 and 23.4 ± 4.8 mL/100 g/min, respectively, during hyperventilation, and 59.1 ± 10.4 and 64.7 ± 10.0 mL/100 g/min, respectively, post-ACZ). Overall, strong correlation between the two methods was found across all states (slope = 1.01, R2 = 0.82), while the correlations within individual states and of reactivity measures were weaker, in particular in rest (R2 = 0.05, p = 0.03). Regional distribution was similar, although ASL yielded higher perfusion and absolute reactivity in highly vascularized areas. In conclusion, ASL-MRI and 15O-H2O-PET measurements of rCBF are highly correlated across different perfusion states, but with variable correlation within and between hemodynamic states, and systematic differences in regional distribution.

Updating the SAPRC Maximum Incremental Reactivity (MIR) scale for the United States from 1988 to 2010

ABSTRACTOzone reactivity scales play an important role in selecting which chemical compounds are used in products ranging from gasoline to pesticides to hairspray in California, across the United States and around the world. The California Statewide Air Pollution Research Center (SAPRC) box model that calculates ozone reactivity uses a representative urban atmosphere to predict how much additional ozone forms for each kilogram of compound emission. This representative urban atmosphere has remained constant since 1988, even though more than 25 years of emissions controls have greatly reduced ambient ozone concentrations across the United States during this time period. Here we explore the effects of updating the representative urban atmosphere used for ozone reactivity calculations from 1988 to 2010 conditions by updating the meteorology, emission rates, concentration of initial conditions, concentration of background species, and composition of volatile organic compound (VOC) profiles. Box model scenarios are explored for 39 cities across the United States to calculate the Maximum Incremental Reactivity (MIR) scale for 1,233 individual compounds and compound-mixtures. Median MIR values across the cities decreased by approximately 20.3% when model conditions were updated. The decrease is primarily due to changes in atmospheric composition ultimately attributable to emissions control programs between 1998 and 2010. Further effects were caused by changes in meteorological variables stemming from shifting seasons for peak ozone events (summer versus early fall). Lumped model species with the highest MIR values in 1988 experienced the greatest decrease in MIR values when conditions were updated to 2010. Despite the reduction in the absolute reactivity in the updated 2010 atmosphere, the relative ranking of the VOCs according to their reactivity did not change strongly compared to the original 1988 atmosphere. These findings indicate that past decisions about ozone control programs remain valid today, and the ozone reactivity scale continues to provide relevant guidance for future policy decisions even as new products are developed.Implications: Updating the representative urban atmosphere used for the Maximum Incremental Reactivity (MIR) scale from 1988 to 2010 conditions caused the reactivity of 1223 individual compounds and combined mixtures to decrease by an average of 20.3% but the relative ranking of the VOCs was not strongly affected. This means that previous guidance about preferred chemical formulations to reduce ozone formation in cities across the United States remain valid today, and the MIR scale continues to provide relevant guidance for future policy decisions even as new products are developed.

Integrated environmental management of pyrrhotite tailings at Raglan Mine: Part 1 challenges of desulphurization process and reactivity prediction

The aim of this project is to assess the efficiency of pyrrhotite rich tailings desulfurization and its capacity to make mine operations more sustainable and respectful to the environment. The Raglan mine ore concentrator produces acid-generating tailings, which contain pyrrhotite, that need to be properly managed. Partial desulphurization of the total tailings produced at the end-circuit was investigated to assess the option of using desulphurized material as a component of a cover to control acid mine drainage (AMD) generation. The weathering and contaminant generation potential was investigated through kinetic testing. The objective of Part 1 of this study is to present results in terms of: i) physical, chemical, and mineralogical characterization of the Raglan Mine tailings; ii) optimization of the pyrrhotite flotation process; and iii) kinetic testing on the desulphurized tailings to predict their acid generating potential, as well as their contaminant (particularly Ni) release rates. Desulphurization succeeded in reducing the sulphur content in the tailings to less than 0.4 wt% (mainly as pyrrhotite-lizardite intergrowths). Contaminant releases complied all with Quebec environmental regulations. The tailings were proven to be non-acid generating based on quantitative evaluation of mineralogy by scanning electron microscopy (QEMSCAN) results and the calculation of absolute reactivity.

Electron-hole pair effects in methane dissociative chemisorption on Ni(111).

The dissociative chemisorption of methane on metal surfaces has attracted much attention in recent years as a prototype of gas-surface reactions in understanding the mode specific and bond selective chemistry. In this work, we systematically investigate the influence of electron-hole pair excitations on the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111). The energy dissipation induced by surface electron-hole pair excitations is modeled as a friction force introduced in the generalized Langevin equation, in which the independent atomic friction coefficients are determined within the local-density friction approximation. Quasi-classical trajectory calculations for CH4/CH3D/CHD3 have been carried out on a recently developed twelve-dimensional potential energy surface. Comparing the dissociation probabilities obtained with and without friction, our results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity. Given similar observations in both water and methane dissociation processes, we conclude that electron-hole pair excitations would not play an important role as long as the reaction is direct and the interaction time between the molecule and metal electrons is relatively short.

Ticagrelor versus prasugrel in patients with high on-clopidogrel treatment platelet reactivity after PCI: The ISAR-ADAPT-PF study

Patients with high on-treatment platelet reactivity (HTPR) on clopidogrel are at high risk for adverse cardiovascular events after percutaneous coronary intervention (PCI). The aim of the ISAR-ADAPT-PF study was to assess the antiplatelet efficacy of ticagrelor versus prasugrel in patients with HTPR on clopidogrel. In a prospective and randomized clinical study, 70 patients with HTPR on clopidogrel loading dose (LD) within 24 h post PCI were assigned to receive either ticagrelor [180 mg LD followed by 90 mg maintenance dose (MD) twice daily] or prasugrel (60 mg LD followed by 10 mg MD once daily). The adenosine diphosphate-induced platelet aggregation assessed on the Multiplate analyzer on day 2 after randomization (primary end point) was as follows: the mean difference between the two treatment groups was 6 aggregation units (AU) × min with an upper 95% confidence interval (CI) of 41 AU × min, which was greater than the predefined noninferiority margin of 18 AU × min (P for noninferiority = 0.29). However, no significant differences in absolute platelet reactivity levels between ticagrelor- versus prasugrel-treated patients at that time point were observed (138 ± 100 AU × min vs. 132 ± 64 AU × min, P for superiority = 0.77). In conclusion, neither drug was statistically more effective for inhibition of platelet aggregation in patients with HTPR on clopidogrel post PCI, although the study could not formally demonstrate the assumed noninferiority of ticagrelor versus prasugrel.

N-Acetylserotonin and 6-Hydroxymelatonin against Oxidative Stress: Implications for the Overall Protection Exerted by Melatonin.

The protection exerted by N-acetylserotonin (NAS) and 6-hydroxymelatonin (6OHM) against oxidative stress was investigated using the density functional theory. It was found that these compounds are better peroxyl radical scavengers than melatonin itself, Trolox, caffeine, or genistein both in lipid and aqueous solutions. The related kinetic data is provided for the first time. The solvent polarity influences not only the absolute reactivity of NAS and 6OHM toward peroxyl radicals, but also their relative scavenging activity. In addition, they both fully inhibit the oxidative effects of copper-ascorbate mixtures, and (•)OH production via the Haber-Weiss reaction, albeit the effects on the later are only partial. On the basis of comparisons with other melatonin-related compounds, it is proposed that the role of NAS and 6OHM on the overall protection exerted by melatonin against oxidative stress is mainly related to their free radical scavenging activities. Moreover, they increase such protection. The role of the phenol moiety on such activity is demonstrated.

Reaction dynamics of initial O2 sticking on Pd(100).

We have determined the initial sticking probability of O2 on Pd(100) using the King and Wells method for various kinetic energies, surface temperatures, and incident angles. The data suggest two different mechanisms to sticking and dissociation. Dissociation proceeds mostly through a direct process with indirect dissociation contributing only at low kinetic energies. We suggest a dynamical precursor state to account for the indirect dissociation channel, while steering causes the high absolute reactivity. A comparison of our results to those previously obtained for Pd(111) and Pd(110) highlights how similar results for different surfaces are interpreted to suggest widely varying dynamics.

A new index to estimate reactivity and biological effect of tannins, using tropical browse legumes as a model

The aim of this paper was to propose a new approach to evaluate the effect of tannins on rumen microbial fermentation in terms of biological effect (proportion of the response if tannins are inactivated, BE) and reactivity (capability of tannins to bind chemically with different amounts of a reagent), by recording gas production in 24h in vitro incubation of substrates with increasing levels (0.025–1g/g substrate) of polyethylene glycol (PEG). Three tropical browse legumes with different tannin contents (Acacia cornigera, Albizia lebbekoides and Leucaena leucocephala) were chosen as model substrates. The reactivity index (RI, mg PEG) and absolute reactivity index (aRI, mg PEG/ml gas) were estimated from the rate coefficient of fitting gas production per unit of added PEG to an exponential equation for each incubation time. The RI characterises each specific tannin source, and depends on the potential fermentation of the plant. The average RI values of A. cornigera, L. leucocephala and A. lebbekoides were 70.4, 36.3 and 55.7mg PEG, respectively. The aRI is a tool for species comparison, because it is referred to the total volume of gas produced. It was higher in A. lebbekoides, intermediate in A. cornigera and lowest in L. leucocephala (1.57, 0.76 and 1.14mgPEG/ml gas, respectively; P=0.012). This index was higher at 6h than at 12 and 24h (1.62, 0.96 and 0.89mg PEG/ml gas, P<0.001). The estimation of these indexes is a simple and inexpensive tool that gives a more complete view of the biological activity pattern of tannins in practical conditions, complementing that of other biological effect indexes and avoiding the biases that arise from their chemical analysis.

Methodology for modal analysis at pulsed neutron source experiments in accelerator-driven systems

The evaluation of experimental techniques for the determination of the subcriticality level of an ADS core relies on reactor point kinetics. The measured parameters depend on the detector position, and the reactivity values are subject to spatial correction factors (SCFs). In this paper the contribution of different eigenmodes to the SCF for the pulsed neutron source (PNS) technique is assessed. Moreover by pulse simulations, precise values of the correction factor are obtained.As case study, the VENUS-F SC1 subcritical core is investigated, with a pulsed neutron source in the center of the core, generated by the GENEPI-3C deuteron accelerator. Much more than 100 modes need to be taken into account to precisely obtain the spatial correction factor for the area method evaluation of this core. Especially the modes with a maximum in the center of the core contribute significantly to the SCF. No spatial correction needs to be applied close to the zeros of the first mode with a maximum in the center of the core (different from the fundamental one). In the reflector zone, the absolute reactivity level is overestimated.

The Issue of Accelerator Beam Trips for Efficient ADS Operation

The development of accelerator-driven systems (ADSs) is motivated by the potential of these machines to reduce the volume and the radiotoxicity of accumulated nuclear waste, more particularly that of minor actinides currently generated by the operation of existing pressurized water reactors. The reduction of both volume and radiotoxicity of nuclear waste is achieved by transmutation and fission of minor actinides into less-active isotopes or shorter-lived by-products.Various technical challenges exist regarding designing reliable and efficient ADSs. The key points are very much linked to the design of the spallation module, the assurance that reactivity remains below criticality under any circumstances, and the accelerator reliability.This paper addresses the latter two challenges imposed on the accelerator in order to assure safe and reliable ADS operation. It discusses the possibility of performing online absolute reactivity measurements and the limits in the number of allowable accelerator beam trips, which might impede plant integrity and/or plant efficiency.

Differential cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

The potential differences in cerebrovascular responses between the anterior and posterior circulations to changes in CO2 are unclear in humans. Using transcranial Doppler ultrasound, we compared the CO2 reactivity of the (1) BA and PCA and (2) MCA and PCA during hyperoxic rebreathing in supine position. The reactivity in the BA and PCA was similar in both absolute (1.27±0.5 and 1.27±0.6cm/s/Torr; P=0.992) and relative (3.98±1.3 and 3.66±1.5%/Torr CO2; P=0.581) measures, suggesting that the PCA is an adequate surrogate measure of reactivity for the BA. The MCA reactivity was greater than the PCA in absolute (2.09±0.7 and 1.22±0.5cm/s/Torr CO2; P<0.001), but not relative measures (3.25±1.0 and 3.56±1.6%/Torr CO2; P=0.629). Our findings (a) confirm regional differences in the absolute reactivity in the human brain and (b) suggest that in cerebrovascular studies investigating functions mediated by posterior brain structures (e.g., control of breathing), the posterior vasculature should also be insonated.

The effects of head‐up and head‐down tilt on cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, gravity‐dependent shifts in blood volume distribution during head‐up and head‐down tilt (HUT, HDT) have profound effects on venous return, cardiac output and MAP. Further, cerebral blood flow (CBF) is highly sensitive to the influence of changes in arterial CO2 (PaCO2) on arteriolar diameter. We tested the effects of steady‐state tilt on cerebral blood velocity (CBV) and CO2 reactivity in the middle and posterior cerebral arteries (MCA, PCA) using hyperoxic rebreathing and transcranial Doppler ultrasound in various positions on a tilt table. Following initial testing in supine position, subjects were positioned randomly and tested in each of four positions: 90° HDT, 45° HDT, 45° HUT and 90° HUT. There were no differences in steady‐state MCA or PCA CBV across positions. Absolute and relative CO2 reactivity slopes were calculated for the MCA and PCA using linear regression. Absolute reactivity was greater in the MCA than the PCA in all 5 positions, but there was no interaction between reactivity and tilt. Our data demonstrate that CBF is maintained through cerebral autoregulation in the face of superimposed steady‐state orthostatic stress and dynamic changes in PaCO2.

ChemInform Abstract: Time-Resolved Spectroscopy of Carbene-Pyridene Ylides: Distinguishing Carbenes from Diazirine Excited States

The photochemistry of diazirines and diazo compounds is not as simple as nitrogen extrusion and carbene formation. The C-H bonds adjacent to the diazo and diazirine moieties can migrate in the excited state and produce stable products without the benefit of a relaxed carbene intermediate. Additionally, cyclobutyl substituted systems exhibit carbon migration. It is unfortunate that the products of photochemical rearrangement of precursor excited states are identical to the products of thermal rearrangement of carbenes. This has prevented accurate measurement of the yield and absolute reactivity of alkylcarbenes. That pyridine reacts selectively with carbenes and not with the excited states of their nitrogenous precursors has allowed the separation of these two pathways and an appreciation of their relative importance with structural variation.

Design study of a fast spectrum zero-power reactor dedicated to source driven sub-critical experiments

In the framework of the European P&T program (IFP6-EUROTRANS), the Generation of Uninterrupted Intense NEutrons pulses at the lead VEnus REactor (GUINEVERE) project consists of an Accelerator Driven System (ADS) that is composed by a fast lead simulated-cooled reactor operated in sub-critical conditions, coupled with an updated version of the GENEPI neutron generator previously used for the MUSE experiments. The GUINEVERE facility aims at developing and improving different techniques for the reactivity monitoring of sub-critical ADS’s. As such, the GUINEVERE project will comprise a series of major experiments that will be performed in the near future. The GUINEVERE facility will be located at the VENUS light water moderated research reactor at the SCK-CEN site of Mol (Belgium), which needs to be modified in order to accommodate a completely different and new type of core. A series of constraints were taken into account in the technical design of the GUINEVERE core, in order to properly conjugate the technical feasibility of this facility and the necessity to comply with the envisioned experimental program and its associated scientific outcome. The complete design study of the GUINEVERE core is the subject of this paper. The final design of the fuel assemblies, safety and control rods is provided. Also, the critical core configuration, to be used as reference for absolute reactivity measurements, is presented along with its associated reactor physics parameters, calculated by means of Monte Carlo methodologies. Finally, for licensing purposes, the GUINEVERE facility must satisfy the required nuclear safety criteria of the Belgian safety authorities, and in this paper, an overview of the safety analysis that has been performed with regard to the core physics, thermal assessment and shielding issues is also provided.

A New Synthesis of Difluorodiazirine and the Absolute Reactivity of Difluorocarbene

Difluorodiazirine was generated by the reaction of 2,4-dinitrophenoxyfluorodiazirine with lithium fluoride and 15-crown-5 in HMPA. Laser flash photolysis of difluorodiazirine afforded difluorocarbene (CF(2)). Absolute rate constants and activation parameters were determined for the additions of CF(2) to several alkenes.

Effect of steric bulk on the absolute reactivity of allene oxides

Alpha-alkyl substituted fluorenylidene allene oxides were generated photochemically from appropriately substituted fluorenol derivatives in 2,2,2-trifluoroethanol (TFE) in order to assess the affect of steric bulk on the absolute reactivity of the allene oxides. The absolute reactivities of the allene oxides were measured using nanosecond laser flash photolysis, and were found to change little in TFE upon going from alpha-methyl to alpha-butyl to alpha-t-butyl. However, all three alpha-alkyl substituted allene oxides were considerably more reactive than the alpha-hydrogen derivative, indicating that steric bulk did not result in a decrease in reactivity as is typically found in other allene oxides. The contrary reactivity of the fluorenylidene allene oxides is explained by examining in detail the reactions of these allene oxides, determined from product studies.

Absolute reactivity of arylallyl carbocations

AbstractA series of α‐vinyl arylmethyl cations were generated and studied using nanosecond laser flash photolysis. Rate constants for the decay of the substituted α‐vinyl arylmethyl cations were determined under solvolytic conditions in pure solvents and solvent mixtures of 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) and 2,2,2‐trifluoroethanol (TFE). In addition the absolute reactivity of the carbocations with added nucleophiles were obtained. The reactivities of the α‐vinyl arylmethyl cations were then compared to the reactivities of the corresponding α‐methyl, α‐phenyl, and α‐cyclopropyl arylmethyl cations. Hammett σ+ plots of each of the series of carbocations were obtained and the substituent effects on carbocation reactivity analyzed. These data show that the influence of substituent on the reactivity of the α‐vinyl carbocations was different from the substituents effects on the reactivity of the α‐methyl, α‐phenyl, and α‐cyclopropyl series. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The comparative effects of sevoflurane versus isoflurane on cerebrovascular carbon dioxide reactivity in patients with previous stroke

The use of volatile anesthetics is reportedly related to altered cerebrovascular carbon dioxide (CO2) reactivity. We examined the comparative effects of sevoflurane versus isoflurane on cerebrovascular CO2 reactivity in patients with previous stroke. Twenty-four patients with previous stroke and 20 patients without previous stroke (serving as controls) were studied. Anesthesia was maintained with either end-tidal 1.0 minimum alveolar concentration (MAC) sevoflurane or 1.0 MAC isoflurane in 33% oxygen and 67% nitrous oxide. A 2.5-MHz pulsed transcranial Doppler (TCD) probe was attached to the patient's head at the right or left temporal window for continuous measurement of mean blood flow velocity in the middle cerebral artery (Vmca). After establishing baseline values of Vmca and cardiovascular hemodynamics, we increased end-tidal CO2 by decreasing the ventilatory frequency by 2-5 breaths x min(-1). We found that values for absolute and relative CO2 reactivity in the sevoflurane groups were lower than those in the isoflurane groups (absolute CO2 reactivity in the sevoflurane groups: control, 3.3 +/- 0.4*; previous stroke, 3.4 +/- 0.4*; absolute CO2 reactivity in the isoflurane groups: control, 4.2 +/- 0.3; previous stroke, 4.5 +/- 0.4, cm x s(-1) x mmHg(-1); *P < 0.05 compared with isoflurane group). There were no significant differences in the values for absolute and relative CO2 reactivity between the controls and the previous-stroke patients within each of the sevoflurane and isoflurane groups. Our findings suggest that, in patients with previous stroke, cerebrovascular CO2 reactivity under sevoflurane anesthesia was lower than that under isoflurane anesthesia.