Abstract: The invention relates to a bottom plate member (6) which is designed and made for initially tightening a chamber in which a high gas pressure is generated, for example the propellant charge chamber (4) in an ammuniton round for a recoilless weapon system or in pipes or containers within the process industry. The plate member (6) is breakable and opens for the gases to exit when the gas pressure within the chamber exceeds a certain level. The plate member has a through hole (12) in the center which gives an effective starting point for the cracking process as well as fractural impressions (10) to control the cracking process so that the plate is divided into a number of well defined parts of flaps (11) which are connected to each other. The plate member (6) further comprises a number of straight edges (16), which number corresponds to the number of flaps (11), against which edges the base of the flaps are arranged to be folded when the plate member opens due to the gas pressure.