tubular part 28 between the ignition device 44 and the shear disc 32 in fig3 is shown a first alternative shock wave generator 47 which is substantially similar to the embodiment of fig2 the difference being that the shear disc 32 forms a primary shear disc and that each opening 36 is provided with a secondary shear disc 48 in fig4 is shown a second alternative shock wave generator 49 which is substantially similar to the embodiment of fig2 except that the plug assembly the ignition device and the deflagrating charge have been replaced by a piston assembly 50 including a cylinder 51 in the form of second tubular part 28 and a piston 52 arranged in the cylinder 51 the piston 52 is movable relative to the cylinder 51 in the direction of the shear disc 32 so as to compress a body of gas 54 present between the piston 52 and the shear disc 32 the piston assembly 50 furthermore includes a plug 55 screwed into the cylinder 51 and provided with a central bore 56 having an internal shoulder 58 a spring assembly 60 is arranged between the piston 52 and the plug 54 the spring assembly 60 being compressed by a threaded tie rod 62 at one end thereof connected to the piston 52 and at the other end thereof extending through the bore 56 and being retained at internal shoulder 58 by an explosive nut 64.