Abstract: If liquid H₂ inside the heat exchanger leaks to the vacuum containment of the cold neutron source, the shock wave from explosion of H₂/O₂ mixture may threaten the vacuum containment in a reactor. In order to know how the shock wave goes, a full-scaled experimental mockup device was made. H₂/O₂ mixture was filled into the mockup with different equivalent ratios and initial pressure. A spark igniter was located at the end of the mockup. The transient pressure and flame sensors and data acquisition system were used to record the pressure and flame histories at definite positions. The result shows that the deflagration can occur inside the vacuum containment for both low initial pressure and equivalent ratios cases. The shock wave and the flame will travel to another side of the vacuum containment by the way of the slot for both cases. What’s more, there only occurs low speed burning without shock wave if excess H₂ is in the mixture.