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

Abstract

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