Mike Kuznetsov

Hydrogen Detonation in a Flat Semi-Confined Layer

Hydrogen accumulation at the top of the containment or reactor building may occur due to an interaction of molten corium and water followed by a severe accident of a nuclear reactor (TMI, Chernobyl, Fukushima Dai-ichi). Hydrogen accumulates usually in a containment of nuclear reactor as a stratified semi-confined layer of hydrogen-air mixture. Detonation of such mixture may lead to significant damage of the containment structure. A series of large scale experiments on hydrogen combustion and detonation in a semi-confined layer of uniform and non-uniform hydrogen-air mixtures in presence of obstructions or without them was performed at the Karlsruhe Institute of Technology (KIT). Critical conditions for deflagration-to-detonation transition and then for steady state detonation propagation were experimentally evaluated in a flat semi-confined layer. The experiments were performed in a horizontal semi-confined layer with dimensions of 9×3×0.6 m with/without obstacles opened from below. The hydrogen concentration in the mixtures with air was varied in the range of 0–34 vol.% without or with a gradient of 0–1.1 mol. %H2/cm. Effects of hydrogen concentration gradient, thickness of the layer, geometry of the obstructions, average and maximum hydrogen concentration on critical conditions for detonation onset and then propagation were investigated with respect to the safety analysis. Blast wave strength and mechanical response of the safety volume was experimentally measured as well.Copyright

Parameterization of Laminar Burning Velocity Dependence on Pressure and Temperature in Hydrogen/Air/Steam Mixtures

A new correlation describing the laminar burning velocity of hydrogen/air mixtures as a function of temperature, pressure, and mixture composition is proposed. The correlation is developed for reactive hydrogen/air mixtures diluted with steam for wide temperature, pressure, and composition ranges. The study includes the following stages: first, the laminar burning velocity was calculated with the detailed reaction mechanism of Maas and Warnatz (1988) implemented in the code INSFLA; second, the results obtained were validated against experimental data and a new heuristic approximation based on them was created. The correlation found consists of simple mathematical expressions and provides a good approximation of the laminar burning velocity in the range between 200 K and 600 K for temperature, between 0.1 bar and 10 bar for pressure, and up to 20 mol% steam dilution.

Numerical Simulation of Radiolysis Gas Detonations in a BWR Exhaust Pipe and Mechanical Response of the Piping to the Detonation Pressure Loads

Radiolysis gas (2H2+O2) can accumulate in steam piping of Boiling Water Nuclear Reactor (BWR) in case of steam condensation. A detonation of radiolysis gas was the likeliest cause of the pipe ruptures in the Hamaoka-1 and Brunsbuttel accidents (Nakagami, 2002; Schulz et al., 2002). In both cases the failed pipes were initially under the operating pressure of 70 bar. During the detonation accident the pressure rose up to 1000 bar or more. In the current paper we consider a typical BWR exhaust pipe and first evaluate the maximum pressure load in case of a radiolysis gas detonation at an initial pressure of 1.6 bar and a temperature of 35 °C. Next, the mechanical response of the exhaust pipe and its possible damage will be numerically evaluated. The typical exhaust pipe investigated in this study is shown in Fig. 1. It consists of two parts with an outer diameter of 510 and 419 mm fabricated from stainless steel DIN 1.4541. In reality, the exhaust pipe is filled with nitrogen initially. Radiolysis gas (RG) with steam can enter through an exhaust valve due to an opening procedure or due to a leak. In case of a slow long time steam condensation, the radiolysis gas can accumulate at the top of the exhaust pipe. Thus, without an additional ventilation, the “worst case” atmosphere in the exhaust pipe has an initial pressure of 1.6 bar (controlled by the 6 m height of the water level) and consists of radiolysis gas diluted with nitrogen. According to the recommendations of the Reactor Safety Commission (Germany) for radiolysis gas control in BWR plants it is demanded to determine the reaction pressure for the highest radiolysis gas concentration which could arise. Our previous data analysis (Kuznetsov et al., 2007a) was based on the postulated detonation of pure radiolysis gas, consisting of a stoichiometric hydrogen-oxygen mixture, as the “worst case” scenario. In this study three levels of pressure loads for “worst case” conditions were evaluated in these works: (1) the stationary detonation pressure of about 29 bar; (2) the local deflagration-todetonation transition (DDT) pressure of 62.5 bar; and (3) the reflected Chapman-Jouguet (CJ) pressure of 71 bar as the maximum detonation pressure that occurs at the tube end. The characteristic pressure loading time was estimated to be about 2 ms, which corresponds to the quasi-static loading regime for a tube of 510 mm outer diameter and 15 mm of wall

Detonation Transition in Relatively Short Tubes

Experimental data and numerical simulations on flame acceleration, shock-flame interaction and deflagration-to-detonation transition mechanism for stoichiometric hydrogen-oxygen mixtures at reduced pressure in relatively short tubes have been analysed. It was shown that the detonation occurs as a result of multiple reflections of precursor shock wave and its interaction with the flame. Adiabatic compression and heating of unreacted gas a front of the flame together with flame surface increase due to the Richtmyer-Meshkov instability provide preconditioning of the DDT process. Several times higher pressure, temperature and reaction rate within a preconditioning zone leads to significant decrease of run-up-distance to DDT in relatively short tubes. Results of the work will provide detailed information on multiple shock - flame interactions leading to the DDT process for numerical code validations.

Flame Acceleration and DDT in a Torus Geometry

A series of combustion experiments in an optically transparent annular channel were carried out with hydrogenoxygen mixtures from 15 to 85 vol.% H2. The complex formed by an oblique shock wave and a subsonic deflagration, only recently observed by the authors in an annular channel is investigated. The ensemble has the notable property of propagating with the Chapman-Jouguet velocity up to 3000 m/s in the circumferential direction being of intrinsic subsonic nature. The structure is discussed in detail utilizing an algebraic approach to provide an insight of the apparently contradictory experimental observation of a deflagration which propagates with its Chapman-Jouguet velocity.

Upward Flame Propagation Experiments on Hydrogen Combustion in a 220 cub. m Vessel

During a hypothetical severe accident in a pressurized water reactor (PWR) of nuclear power plant, hydrogen would be generated during the degradation of the reactor core. The ignition and ensuing combustion of hydrogen could cause a sharp pressure increase, which could threaten the integrity of the containment. The present work describes the execution and results of the Upward Flame Propagation Experiment (UFPE) on hydrogen combustion within the EC LACOMECO project. Two experiments were performed at the HYKA A2 experimental facility (KIT), which is a cylindrical vessel of 220 m 3 free volume, 9.1 m height and 6.0 m internal diameter. A homogeneous mixture of hydrogen (11-12 vol.%), steam (20-21 vol.%) and air was established in the vessel. The initial pressure was 1.5 bar, and the average initial temperature was about 90.0 oC. The mixture was ignited at the bottom of the vessel and the ensuing axial and radial flame propagation were observed. Pressure, temperature and flame velocity were measured at different axial and radial locations. The flame propagation was recorded by a high speed video combined with a Background Oriented Schlieren (BOS) method. The experimental results were finally compared to the results of the well-known THAI HD-22 experiment, which was performed at almost similar conditions. The objective of current work was to investigate the effect of scale on flame propagation regime in order to clarify how can phenomena observed in a scaled-down experimental facility be extrapolated to an actual containment scale. Another goal was to provide detailed experimental data for numerical code validations in a large scale.

Experimental and Numerical Simulation of Radiolysis Gas Detonations and Mechanical Response of BWR Exhaust Pipes

Radiolysis gas (2H2 +O2 ) can accumulate in BWR steam piping in case of steam condensation. An ensuing detonation of the radiolysis gas is the likeliest cause of a pipe and/or valve damage. In the current work we investigate a typical BWR exhaust pipe, which connects the high pressure steam piping with the ambient atmosphere, under the following “worst case” scenario: (a) accumulation of radiolysis gas in an exhaust pipe, (b) fast valve opening to the high pressure system with steam at 70 bar, and (c) adiabatic pressurization of the radiolysis gas by the steam. Taking into account a water surface level of 6 m from the open end this leads to an equilibrium state of 20 bar pressure and 602 K temperature for the pressurized radiolysis gas. The main purpose of the current work was an experimental and numerical evaluation of the maximum pressure load and the integrity of the BWR exhaust pipe in case of a detonation of the pressurized radiolysis gas.Copyright