Analysis of the NUPEC PSBT Tests with FLICA-OVAP

This paper discusses the results of a computational activity devoted to the prediction of two-phase flows in subchannels and in rod bundles. The capabilities of the FLICA-OVAP code have been tested against an extensive experimental database made available by the Japanese Nuclear Power Energy Corporation (NUPEC) in the frame of the PWR subchannel and bundle tests (PSBT) international benchmark promoted by OECD and NRC. The experimental tests herein addressed involve void fraction distributions and boiling crisis phenomena in rod bundles with uniform and nonuniform heat flux conditions. Both steady-state and transient scenarios have been addressed, including power increase, flow reduction, temperature increase, and depressurization, representative of PWR thermal-hydraulics conditions. After a brief description of the main features of FLICA-OVAP, the relevant physical models available within the code are detailed. Results obtained in the different tests included in the PSBT void distribution and DNB benchmarks are therefore reported. The relevant role of selected physical models is discussed

UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -- 14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10$^{-5}$ molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower ($<$ 10$^{-6}$ molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH$_3$OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH$_3$, OH, H$_2$CO and CH$_3$O/CH$_2$OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH$_3$O, therefore opening new gas phase chemical routes for the formation of complex molecules.Comment: 13 pages, 2 figures, 1 tabl

Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process

UV ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e. the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the subsurface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factors of a few (compared to the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.Comment: 21 pages 5 figure

X-ray photodesorption of complex organic molecules in protoplanetary disks -- I. Acetonitrile CH3CN

X-rays emitted from pre-main-sequence stars at the center of protoplanetary disks can induce nonthermal desorption from interstellar ices populating the cold regions. This X-ray photodesorption needs to be quantified for complex organic molecules (COMs), including acetonitrile CH3CN, which has been detected in several disks. We experimentally estimate the X-ray photodesorption yields of neutral species from pure CH3CN ices and from interstellar ice analogs for which CH3CN is mixed either in a CO- or H2O-dominated ice. The ices were irradiated at 15 K by soft X-rays (400-600 eV) from synchrotron light (SOLEIL synchrotron). X-ray photodesorption was probed in the gas phase via quadrupole mass spectrometry. X-ray photodesorption yields were derived from the mass signals and were extrapolated to higher X-ray energies for astrochemical models. X-ray photodesorption of the intact CH3CN is detected from pure CH3CN ices and from mixed 13CO:CH3CN ices, with a yield of about 5x10^(-4) molecules/photon at 560 eV. When mixed in H2O-dominated ices, X-ray photodesorption of the intact CH3CN at 560 eV is below its detection limit, which is 10^(-4) molecules/photon. Yields associated with the desorption of HCN, CH4 , and CH3 are also provided. The derived astrophysical yields significantly depend on the local conditions expected in protoplanetary disks. They vary from 10^(-4) to 10(-6) molecules/photon for the X-ray photodesorption of intact CH3CN from CO-dominated ices. Only upper limits varying from 5x10^(-5) to 5x10^(-7) molecules/photon could be derived for the X-ray photodesorption of intact CH3CN from H2O-dominated ices. X-ray photodesorption of intact CH3CN from interstellar ices might in part explain the abundances of CH3CN observed in protoplanetary disks. The desorption efficiency is expected to vary with the local physical conditions, hence with the disk region

Wavelength-Dependent UV Photodesorption of Pure N2N_2 and O2O_2 Ices

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: $N_2$ and $O_2$ are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure $N_2$ and $O_2$ thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic $N_2$ and $O_2$ isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: $N_2$ photodesorption mainly occurs through excitation of the $b^1\sqcap_u$ state and subsequent desorption of surface molecules. The observed vibronic structure in the $N_2$ photodesorption spectrum, together with the absence of $N_3$ formation, supports that the photodesorption mechanism of $N_2$ is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, $O_2$ photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of $N_2$ and $O_2$ integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between $10^{-3}$ and $10^{-2}$ photodesorbed molecules per incoming photon.Astronom

Spectrally-resolved UV photodesorption of CH4 in pure and layered ices

Context. Methane is among the main components of the ice mantles of insterstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium. Aims. This study aims at experimentally determining absolute photodesorption yields for the CH4 molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH4:CO ice, indirect desorption processes triggered by the excitation of the CO molecules is monitored and quantified. Methods. Tunable monochromatic VUV light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7 - 13.6 eV (177 - 91 nm) range to irradiate pure CH4 or layers of CH4 deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry and absolute photodesorption yields of intact CH4 are deduced. Results. CH4 photodesorbs for photon energies higher than ~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH4, which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH4 desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules. Conclusions. The photodesorption of CH4 from the pure ice in various interstellar environments is around 2.0 x 10^-3 molecules per incident photon. Results on CO-induced indirect desorption of CH4 provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles

Indirect Ultraviolet Photodesorption from CO:N2 Binary Ices — An Efficient Grain-Gas Process

Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.Astronom

Development of the CATHARE 3 three-field model for simulations in large diameter horizontal pipes

International audienceDuring hypothetical Loss of coolant accident in a PWR caused by a large break in the primary circuit, the reactor core is uncovered. Emergency core cooling is then activated and the accident enters the reflooding phase. Complex two phase flows and heat transfer phenomena occur during this phase and their analysis request detailed models, included in system codes such as CATHARE. A three-field model has been previously implemented in CATHARE 3 and applied to reflooding calculations in the core. This paper aims to describe the extension of this three-field model to the primary circuit, especially for hot legs, in order to improve the simulation of liquid droplet transport from the core to the steam generators. For vertical flows, such as in the core, a comparison of correlations for the entrainment and deposition terms against steam-water data series allowed to select a model for the film-droplets mass exchange. These correlations established for vertical flows are not applicable in horizontal geometries with large diameters such as the hot legs where, during the reflooding phase, the flow regime is dispersed or stratified with entrained droplets. In this case, a specific deposition term based on a model developed from air-water experiments, accounting gravity and turbulent diffusion mechanisms, is used. Dedicated correlations for the entrainment rate and the droplet diameter complete the package of closure laws. These models are assessed against experimental data obtained in horizontal pipes with various diameters and with various inlet gas and liquid superficial velocities

SAPIUM - Selection of adequate test database for IUQ

International audienc