On the interpretation of vapor pressure measurements on oxide fuel at very-high temperatures for fast reactor safety analysis

Safety analysis of fast reactors requires knowledge of the evaporation behavior and the total vapor pressure of oxide fuel materials in the temperature region from 3000 K upwards. Dynamic vapor pressure measurements on liquid oxide fuels performed in open-evaporation experiments with laser heating techniques imply strong alterations in the composition of the incongruently evaporating fuel surface, since, during evaporation, the depletion in the preferentially evaporating components cannot be resupplied by diffusion from the bulk material. After a short transient evaporation period stationary surface-evaporation is reached with a surface composition which differs greatly from the given fuel composition and depends on the actual evaporation temperature. When this stationary forced-congruent evaporation mode is reached, the gross vapor composition is well-defined and is identical to the bulk composition of the fuel but is quite different from the actual surface composition. In consequence, the total vapor pressure developing in open surface-evaporation of a liquid oxide fuel can substantially deviate from its thermodynamic equation-of-state, in the case of (U_0.80Pu_0.20) mixed oxide by a factor of 2 to 7 depending on the $\text{O}\text{M}$-ratio. Following these thermodynamic calculations direct measurement of the equation-of-state in open-evaporation experiments is practically impossible. Theoretically fitted expressions applicable in reactor safety analysis are presented for the equations-of-state and the vapor pressure equations for open surface-evaporation and also for the heats of evaporation of liquid (U_0.80Pu_0.20)O_1.95…2.00 mixed oxides.     

Multi-dimensional simulation of hydrogen distribution and turbulent combustion in severe accidents

A joint research project was carried out in the EU Fourth Framework Programme with the goal to develop verified and commonly agreed physical and numerical models for the analysis of hydrogen distribution, turbulent combustion and mitigation, which are suited for the multi-dimensional CFD codes that exist at the institutions of the participating partners. The work programme and the activities of the partners are described. Significant progress has been made in the areas of experiments, model development, model verification and model application to nuclear power plants. Joint benchmark tests were defined and analysed. In addition to the regular partners meetings, topical workshops were conducted to harmonize the experimental and theoretical work. The paper presents major experimental results, gives examples for the achieved multi-dimensional modelling capabilities, describes implementation into and validation of codes, and presents results of plant analysis activities.     

Conservative estimates for dynamic containment loads from hydrogen combustions

For the design of severe accident resistant reactor containments estimates are needed for limiting dynamic loads from hydrogen combustion processes. This paper presents some first exploratory results.The approach for a systematic study of dynamic loads from large scale hydrogen detonations is described. The parameter range of H₂-steam-air detonations is defined and then narrowed down to the region which seems accessible in severe reactor accidents. Maximum possible hydrogen masses, unfavorable hydrogen distributions and limiting detonation cases are discussed. The effect of gas composition, geometry, and scale on detonation pressurs and impulses was investigated with one-dimensional calculations. An accident scenario which combines many conservative parameters was analyzed. The one-dimensional modeling, which contains a high degree of conservatism, resulted in 10.5 MPa peak pressure and 60 kPa s reflected impulse (at 30 ms) over approx. 1400 m² containment surface.     

Ignition and heat radiation of cryogenic hydrogen jets

In the present work release and ignition experiments with horizontal cryogenic hydrogen jets at temperatures of 35–65 K and pressures from 0.7 to 3.5 MPa were performed in the ICESAFE facility at KIT. This facility is specially designed for experiments under steady-state sonic release conditions with constant temperature and pressure in the hydrogen reservoir. In distribution experiments the temperature, velocity, turbulence and concentration distribution of hydrogen with different circular nozzle diameters and reservoir conditions was investigated for releases into stagnant ambient air. Subsequent combustion experiments of hydrogen jets included investigations on the stability of the flame and its propagation behaviour as function of the ignition position. Furthermore combustion pressures and heat radiation from the sonic jet flame during the combustion process were measured. Safety distances were evaluated and an extrapolation model to other jet conditions was proposed. The results of this work provide novel data on cryogenic sonic hydrogen jets and give information on the hazard potential arising from leaks in liquid hydrogen reservoirs.     

Hydrogen release from a high pressure gaseous hydrogen reservoir in case of a small leak

The dynamic blow-down process of a high pressure gaseous hydrogen (GH₂) reservoir in case of a small leak is a complex process involving a chain of distinct flow regimes and gas states. This paper presents models to predict the hydrogen concentration and velocity field in the vicinity of a postulated small leak. An isentropic expansion model with a real gas equation of state for normal hydrogen is used to calculate the time dependent gas state in the reservoir and at the leak. The subsequent gas expansion to 0.1 MPa is predicted with a zero-dimensional model. The gas conditions after expansion serve as input to a newly developed integral model for a round free turbulent H₂-jet into ambient air. Predictions are made for the blow-down of hydrogen reservoirs with 10, 30 and 100 MPa initial pressure. A normalized hydrogen concentration field in the free jet is presented which allows for a given leak scenario the prediction of the axial and radial range of flammable H₂-air mixtures.     

Asymptotic approximations for probability integrals

This paper considers the asymptotic evaluation of probability integrals. The usual methods require that all random variables are transformed into standard normal variables. The method described here does not use such transformations. Asymptotic approximations are derived in the original space of the random variables. In this way it is also possible to obtain simple formulas for the sensitivity of the failure probability to changes in the distribution parameters.     

Structures of HIV TAR RNA–Ligand Complexes Reveal Higher Binding Stoichiometries

Target TAR by NMR : Tripeptides containing arginines as terminal residues and non‐natural amino acids as central residues are good leads for drug design to target the HIV trans‐activation response element (TAR). The structural characterization of the RNA–ligand complex by NMR spectroscopy reveals two specific binding sites that are located at bulge residue U23 and around the pyrimidine‐stretch U40‐C41‐U42 directly adjacent to the bulge.

     

SOME SIMPLE TESTS OF THE MOVING-AVERAGE UNIT ROOT HYPOTHESIS

Abstract. This paper deals with three test statistics for a moving-average (MA) unit root. The spectral test is based on the estimate of the spectral density at frequency zero. The variance difference statistic compares the sample variance of the integrated series with the estimated variance imposing the MA unit root constraint. Furthermore, Tanaka's score type test statistic is modified to improve the power in higher order models. The asymptotic power of the tests is considered and Monte Carlo experiments are performed to investigate the small sample properties of the tests. Finally, the tests are applied to a number of economic time series to determine the degree of integration.     

GASFLOW-MPI: A new 3-D parallel all-speed CFD code for turbulent dispersion and combustion simulations Part II: First analysis of the hydrogen explosion in Fukushima Daiichi Unit 1

The core-melt in Fukushima-Daiichi Unit 1 represents a new class of severe accidents in which combustible gas from core degradation leaked from the containment into the surrounding air-filled reactor building, formed there a highly reactive gas mixture, and exploded 25 h after begin of the station black-out. Since TMI-2 hydrogen safety research and management has focussed on processes and counter-measures inside the containment but the reactor building remained unprotected against hydrogen threats. The code GASFLOW-MPI is currently under development to simulate hydrogen behaviors, including distribution and combustion, for scenarios with containment leakage.This paper describes a first analysis of the hydrogen explosion in Unit 1. It investigates gas dispersion in the reactor building, assuming a leak at the drywell head flange, shows the evolution of a stratified, inhomogeneous H₂–O₂–N₂–steam mixture in the refueling bay, simulates the combustion of the reactive gas mixture, and predicts pressure loads to walls and internal structures of the reactor building. The blast wave propagated essentially sideways, which explains why all side walls were blown out and the ceiling just collapsed onto the floor of the refueling bay. The blast wave propagation into the free environment was also simulated. The over-pressure amplitudes are sufficiently high to cause damage to adjacent buildings and to injure people. The hitherto existing presumption that the blow-out panel of Unit 2 was removed by the Unit 1 explosion can be confirmed which likely prevented a hydrogen explosion in the Unit 2.GASFLOW-MPI provides validated models for the integral simulations of leakage related core-melt scenarios. Furthermore, the code contains extensively tested submodels for catalytic recombiners, igniters and burst foils, which allow design of new hydrogen risk mitigation systems for currently unprotected spaces in reactor buildings.     

Contributions from the ACRR in-pile experiments to the understanding of key phenomena influencing unprotected loss of flow accident simulations in LMFBRs

This paper summarizes the recently terminated in-pile experiments in the Annular Core Research Reactor (ACRR) of the Sandia National Laboratories that have been performed to study the behavior of fast reactor fuel under different conditions during an unprotected loss of flow (ULOF) accident in an LMFBR. Three groups of experiments are covered. The FD + STAR tests that study the fuel disruption and initial fuel motion in voided regions, the TRAN experiments on flowing and freezing of molten fuel in structures, and the equation of state tests with fresh and irradiated fuel under prompt burst conditions. Conclusions are drawn on the contributions which the results from these tests make to the understanding and simulation of key phenomena under ULOF accident conditions in LMFBR's.