Abstract: A test tube is designed to experimentally evaluate the accidental explosion of a H2-He heat exchanger in a nuclear reactor when hydrogen leaks during work. The test tube is identical to the exchanger in size and geometry. In the experiments, H2-air mixture is filled into the tube at the different initial pressures and equivalence ratios. A spark igniter is located at the top of the designed tube. The transient pressure and data acquisition systems were used to record the pressure histories at the specified stations. Results show that deflagration can occur at the tops of the vacuum tube and the coolant box and characteristics of shock wave can be seen from pressure curves at neighboring test points. The shock waves pass through the slot between the coolant box and the tube side-wall and propagate into the divergent section to develop deflagration. The induced shock wave reflects on the end wall of the tube. The backward propagating shock interacts with the forward traveling flame following the shock wave initially, and the complex shock wave field can be generated. Compared with the stand atmospheric pressure, in the case of low initial pressure and rich fuel mixtures, combustion rather than deflagration occurs at the top of the tube, the pressure of the high-temperature gas rises lowly, and the pressure history does not appear at the shock wave character. The fuel-riched products move into the divergent section of the tube and are re-ignited to induce strong detonation.