The most dangerous accident of a cold neutron source is explosion of H2-air mixture inside the vacuum containment(VC).An experimental study with hydrogen-air stoichiometric concentration in VC has been done in a full-scaled set-up.The shock pressure curves of the explosion at different initial places were obtained.The results indicate that the explosion in VC is just a deflagrating

and the maximal shock pressure is about 350 kPa.The von Mises equivalent stress and the main stresses calculated are well below the stress limit of the material for VC.It can be concluded that the VC can keep its integrality even the hypothetical hydrogen explosion inside it

and the explosion will do none harm to the reactor structures around VC.