Alexander Lelyakin

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.

Adaptive LMR simulation of DDT in obstructed channel

Deflagration to detonation transition (DDT) is a challenging subject in computational fluid dynamics both from a standpoint of the phenomenon nature understanding and from extremely demanding computational efforts. In recent years, as the development of computer technology and improvement of numerical schemes was achieved, some more direct methods have been found to reproduce the DDT mechanistically without additional numerical or physical models. In the current work, highly resolved DDT simulations of hydrogen-air and of hydrogen-oxygen mixtures in 2D channel with regular repeating obstacles are present. The technique of local mesh refinement (ALMR) is implemented in the simulations to minimize the computational efforts. The criteria for the ALMR are examined and optimized in simulations.Copyright

Local Mesh Refinement for Viscid Flow in Combustion Code COM3D

After the implementation of local mesh refinement (LMR) in solution of Euler equations and detonation simulation, the technique has been used to optimize the simulation of viscid flow. In this paper, LMR has been implemented in Navier-Stokes (NS) equations and LES turbulent model. In order to implement LMR in the simulation of viscid flow accurately and efficiently, hybrid interpolation and coarse flux limiter are established. At the same time, implementation of LMR maintains the temporal refinement in solution of NS equations. In practical application, 2D advection diffusion is used to show the advantages of LMR and verify the implementation.Copyright

Local Mesh Refinement in COM3D

In order to balance the resolution and the total computational effort in turbulent combustion CFD code COM3D, a block structured local mesh refinement has been introduced. In the presented work, special attention has been given to the implementation of local mesh refinement for the alternative directions based method and time-operator splitting approach. An application of the implemented technique for the detonation simulations is examined and discussed.Copyright