Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter

We report on the Hubble Space Telescope program to observe periodic comet 9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were to study the generation and evolution of the coma resulting from the impact and to obtain wide-band images of the visual outburst generated by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out: the first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a new, short-lived fan in the sunward direction on June 14. The principal campaign began two days before impact and was followed by contiguous orbits through impact plus several hours and then snapshots one, seven, and twelve days later. All of the observations were made using the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact. Baseline images of the comet, made prior to impact, photometrically resolved the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate the temporal and spatial evolution of the ejecta cloud and allow for a determination of its expansion velocity distribution. One day after impact the ejecta cloud had passed out of the field-of-view of the HRC.Comment: 15 pages, 14 postscript figures. Accepted for publication in Icarus special issue on Deep Impac

Isoprenoid and branched GDGT-based proxies for surface sediments from marine, fjord and lake environments in Chile

Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids from archaea [isoprenoid GDGTs] and bacteria [branched (br) GDGTs] in 33 surface sediments from marine, fjord and lake systems between 25°S and 50°S in Chile were analyzed. The regional TEXH86 calibration obtained from the marine and fjord sediments and mean annual surface temperature (T = 59.6 × TEXH86 + 33.0; r2 0.9; n = 23) is statistically identical to the global ocean calibration based on suspended particulate material in terms of slope, but not in terms of intercept. The regional surface and subsurface TEXH86 calibrations were statistically different from the existing global ocean core top calibrations. The TEX86 calibration model based on most of the relatively large lakes studied here (T = 50.7 × TEX86 - 11.8; r2 0.9; n = 5) is statistically identical to the global lake calibration. The relatively high TEX86 values from smaller lakes suggested an additional source for isoprenoid GDGTs, likely terrestrial or aquatic methanogenic archaea. Application of the soil-calibrated MBT'/CBT (methylation and cyclization of br GDGTs, respectively) temperature proxy to the marine and fjord sediments resulted in an overestimation of continental mean annual air temperature (MAAT), suggesting in situ production of certain br GDGTs in the water column or surface sediment. For the lakes, MBT'/CBT-based surface air temperature estimates were 3–6 °C below MAAT. However, temperature estimates from the lake-specific MBT/CBT global calibration were in good agreement with mean annual surface temperature for all the lakes. The results highlight the need for testing local vs. global calibrations of GDGT-based proxies before their application for palaeoenvironmental reconstruction

Ruthenium release modelling in air and steam atmospheres under severe accident conditions using the MAAP4 code

In a nuclear power plant (NPP), a severe accident is a low probability sequence that can lead to core fusion and fission product (FP) release to the environment (source term). For instance during a loss-of-coolant accident, water vaporization and core uncovery can occur due to decay heat. These phenomena enhance core degradation and, subsequently, molten materials can relocate to the lower head of the vessel. Heat exchange between the debris and the vessel may cause its rupture and air ingress. After lower head failure, steam and air entering in the vessel can lead to degradation and oxidation of materials that are still intact in the core. Indeed, Zircaloy-4 cladding oxidation is very exothermic and fuel interaction with the cladding material can decrease its melting temperature by several hundred of Kelvin. FP release can thus be increased, noticeably that of ruthenium under oxidizing conditions. Ruthenium is of particular interest because of its high radio-toxicity due to 103Ru and 106Ru isotopes and its ability to form highly volatile compounds, even at room temperature, such as gaseous ruthenium tetra-oxide (RuO4). It is consequently of great need to understand phenomena governing steam and air oxidation of the fuel and ruthenium release as prerequisites for the source term issues. A review of existing data on these phenomena shows relatively good understanding. In terms of oxygen affinity, the fuel is oxidized before ruthenium, from UO2 to UO2+x. Its oxidation is a rate-controlling surface exchange reaction with the atmosphere, so that the stoichiometric deviation and oxygen partial pressure increase. High temperatures combined with the presence of oxygen in the atmosphere lead to fuel expansion and formation of cracks. In these conditions, intra- and inter-granular diffusions of ruthenium in the fuel matrix are so enhanced that it is possible to consider an instantaneous volatilisation of ruthenium oxides at the fuel surface. Based on these considerations, a completely new model has been implemented in the EDF local version of the MAAP4.07 severe accident code (Modular Accident Analysis Program), owned by EPRI (Electric Power Research Institute). The fuel oxidation modelling takes into account many kinds of atmospheres (steam and/or air and/or hydrogen), the stoichiometric evolution and the oxygen partial pressure of the fuel matrix. The release of ruthenium oxides is calculated considering their particular reaction constants. The model was assessed by the simulation of different CEA-VERCORS experiments in air, steam and mixed atmospheres. These experiments are specifically designed to study FP release from fuel under different atmospheres and temperatures. This paper deals with the main results obtained with MAAP4.07 when simulating these tests

Pits Formation From Volatile Outgassing on 67P/Churyumov-Gerasimenko

International audienc

B4C oxidation modelling in severe accident codes: Applications to PHEBUS and QUENCH experiments

Boron carbide is used in many nuclear power plants like BWR, VVER, some PWR, and EPR as a neutron absorber material. Consequently, it is important to assess its role in the core degradation phenomena during a severe accident (SA), as well as that of the carbon gas released from its degradation on the fission products behaviour. This paper describes the progresses achieved in the frame of the Network of Excellence SARNET concerning the B4C control rod degradation modelling in Severe Accident codes, such as ATHLET-CD, ICARE2, ASTEC MELCOR and MAAP. These new developments complete improvements made during the European Union 5th Framework COLOSS project. Starting from basic modelling derived from available tests reported in the literature, large improvements of the kinetic correlation for B4C oxidation were obtained from analytical experiments performed at FzK (Germany) and IRSN (France), mostly in the temperature range above 1400 K. The new modelling was considered in the analysis of experiments involving a B4C control rod in small fuel rod assemblies, such as Phebus FPT3 in-pile experiment, as well as out of pile experiments Quench 07 and 09, aimed at studying the course of severe accidents. Regarding the hydrogen generation, the results given by different code simulations are consistent with the experimental values. Concerning the control rod degradation, SA codes such as ICARE2 and ATHLET-CD, using suitable modelling of B4C oxidation, predicted rather well the total carbon release. The results of the MELCOR code, involving initially a B4C oxidation model designed to be used for BWR control blades, have been largely improved in the most recently released version, with a model extended for PWR B4C control rods. Codes still have some difficulties to reproduce the final degradation of fuel bundles involving B4C rods. Spreading of molten materials from the control rods onto fuel rods of the bundle is suspected, suggesting that the main effect of the B4C control rod materials on the bundle behaviour during degradation is connected with B4C-Stainless Steel (SS) eutectics formation and B4C-SS-Zry liquid mixture relocation. These phenomena are not accounted for in the SA codes. The need for further code developments of the early phase of core degradation is recognized, involving the absorber rod material behaviour. The BECARRE experiments, on-going in the framework of the International Source Term Program, are designed to provide in-depth understanding of these phenomena and help improving their modelling. © 2009 Elsevier Ltd. All rights reserved

Comparison report on the blind phase of the OECD International Standard Problem no. 45 exercise (QUENCH-06)

Das Internationale Standard Problem (ISP) No. 45 ist Teil des ISP Programms der OECD/NEA und zielt auf die Untersuchung des Kernverhaltens in Kernkraftwerken beim Aufheizen und verzoegertem Fluten mit Wasser. ISP-45 wurde als Versuch No. 6 am 13. Dezember 2000 in der out-of-pile Versuchsanlage QUENCH im Forschungszentrum Karlsruhe durchgefuehrt. Ein wesentliches Ziel ist die Untersuchung der Wasserstoffquellterms beim Fluten. Um den derzeitigen Stand der Kernschmelzcodes in Bezug auf Simulation der Kernaufheizung und schnellen Abkuehlung durch Wasser (quench) adaequat beurteilen zu koennen, wurden nur die notwendigsten Anfangs- und Randbedingungen fuer die Rechnungen vorgegeben (Blinde Phase). In der blinden Phase des ISP-45 nahmen 8 Code-Systeme teil (ATHLET-CD, ICARE/-CATHARE, IMPACT/SAMPSON, GENFLO, MAAP, MELCOR, SCDAPSIM, SCDAP-3D). Die Ergebnisse der Blind-Rechnungen der 21 Teilnehmer aus 15 Staaten werden mit den experimentellen Ergebnissen und den Resultaten der Nachrechnung mit SCDAP/RELAP5 mod3.2.irs verglichen. Waehrend der Aufheizphase weichen die meisten Ergebnisse nur geringfuegig voneinander, wobei offensichtliche Benutzer-Fehler ausgeklammert wurden. Dies rechtfertigt die Definition eines sogenannten 'Hauptfeldes'. Waehrend der Quench-Phase divergieren die Ergebnise jedoch signifikant, was zum Teil auf eine unzureichende Modellierung der Thermohydraulik zurueckzufuehren ist. Um dieses Defizit zu umgehen, wurden neue Modelle in MAAP und MELCOR entwickelt und eingesetzt. Im Versuch QUENCH-06, der eine Maximaltemperatur von ca. 2200 K erreichte, verhinderte eine ausreichend dicke Oxydschicht ein fruehzeitiges Versagen der Huellrohre und damit die Freisetzung von metallischer Schmelze. Daher ist die zusaetzliche Wasserstofffreisetzung gering, sie kann mit den normalen Oxidationsmodellen weitgehend beschrieben werden. Im 'Hauptfeld' erhoehte sich die Streuung der freigesetzten H_2-Masse von ca. +/-15% vor dem Fluten auf ca. +/-40% nach dem Test. Verschiedene Teilnehmer ueberschaetzten jedoch deutlich den zusaetzlichen H_2-Quellterm durch Einsatz eines sog. 'Shattering models', das kuenstlich die protektive Oxydschicht entfernt. Auch wurden Schmelzeverlagerungen und Debris-Bildung berechnet. Eine detaillierte Untersuchung zeigte auch, dass die Codes noch Probleme mit der korrekten Berechnung des Buendelzustandes vor dem Fluten haben. Ein anderer ueberraschender Aspekt ist, dass die Energiebilanz sorgfaeltig ueberprueft werden muss, bevor man an die Interpretation der Ergebnisse gehen kann. Fehlende Erfahrung der Codebenutzer sowie Schwierigkeiten bei der Simulation der QUENCH-Anlage waren weitere Ursachen fuer die beobachtete Streuung der Ergebnisse. (orig.)The International Standard Problem (ISP) No. 45 is part of the overall ISP program of the OECD/NEA and is dedicated to the behavior of heat-up and delayed reflood of fuel elements in nuclear reactors. ISP-45 is related to the out-of-pile bundle quench experiment QUENCH-06, performed at Forschungszentrum Karlsruhe (FZK), Germany, on December 13, 2000. Special attention was paid to hydrogen production. To assess the ability of severe accident codes to simulate processes during core heat-up and reflood at temperatures above 2000 K, the behavior of the bundle during the whole experiment should be caculated on the basis of experimental initial and boundary conditions, but without knowing further experimental details (blind phase). In the blind phase 21 participants from 15 nations contributed with 8 different code systems (ATHLET-CD, ICARE/CATHARE, IMPACT/SAMPSON, GENFLO, MAAP, MELCOR, SCDAPSIM, SCDAP-3D). After the end of the blind phase all measured data were made available and the participants were invited to deliver a second calculation, where this knowledge could be used (open phase). In this report, results of the blind calculations are presented, analyzed, and compared to experimental data. Additionally, post-test calculations using the in-house version SCDAP/RELAP5 mod3.2.irs are used for comparison. During heat-up most results do not deviate significantly from one another, except as a consequence of some obvious user errors, so that a definition of a mainstream is justified. During quenching the lack of adequate hydraulic modeling becomes obvious: some participants could not match the observed cool-down rates, others had to use a very fine mesh to compensate code deficiencies. To overcome this insufficiency some newly developed reflood models were used in MAAP and MELCOR. In QUENCH-06, the sufficiently thick oxide layers protected the cladding from melting and failure below 2200 K, so that no massive hydrogen release during reflood was found. This behavior could be simulated by most of the codes if no artificial shattering options were used. With respect to hydrogen production the mainstream shows a spreading of +/-15% prior to reflood initiation and a range of +/-40% after reflood. However, a group of SCDAPSIM users activated an extreme shattering option, which overestimates the produced hydrogen mass by a factor of 5. In the mainstream, most of the codes predict correctly that no bundle damage occurred, whereas others calculate slight material relocations, mainly due to overestimation of the cladding temperatures. However, detailed inspection showed that the codes still have difficulties to predict correctly the bundle conditions prior to reflood. Another suprising aspect was that the energy balance has to be checked prior to further interpretation of the results. Lacking user experience to analyze such problems and to model them adequately was a main reason for larger deviations. (orig.)Available from TIB Hannover: ZA 5141(6677) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman