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

1-D numerical calculations of radiolysis gas detonation and mechanical response of a 12.5-m long BWR exhaust pipe have been performed. To reproduce one of scenarios of steam condensation with following radiolysis gas accumulation in an exhaust pipe initially filled with nitrogen at an initial pressure of 1.6 bar and temperature of 35°C, nitrogen diluted radiolysis gas mixtures were used for the numerical simulations. Nitrogen concentration in radiolysis gas composition was changed in the range of 0–80 mol.%. It permits to significantly change detonation properties of the test mixtures. Different gas dynamic effects such as a precursor shock wave and shock reflection on the maximum detonation pressure have shown in our calculations. Maximum pressure load of the piping can be achieved near the deflagration-to-detonation transition (DDT) point and at the tube end. At those positions the maximum pressure could be at least 2–2.5 times higher than the CJ detonation pressure. Dilution of the radiolysis gas mixture with nitrogen leads to reduction of the radiolysis gas detonability and to increase of run up distance to the DDT point. In this case so called “late detonation initiation” in a pre-compressed zone close to the reflection end can occur. It produces extremely high pressure load which can locally be 10 times higher than CJ-detonation pressure for steady state detonation. Mechanical response of the 12.5-m long austenitic steel pipe (Werkstoff Nr. 1.4541) with a diameter 510 mm and wall thickness of 15 mm was calculated for different detonation pressure loads. Maximum deformations of the pipe were obtained close to the DDT point and at the reflection end. It was shown that even for the worst case mixture with a “late detonation initiation” the deformation of tested pipe is very low (not more than 0.2%) and no danger exists for the integrity of the exhaust pipe under radiolysis gas detonation load. This 1D numerical code permits for the first time a continuously mechanistic analysis of the complicated processes with DDT and detonation propagation in closed pipes and it can be used for designers and for piping safety analysis.