During the pressurization process, steam was accidentally released through a valve.

released 是动词 release 的过去分词形式，release 由前缀 re-（表示"再次"）和词根 lease（来自拉丁语 laxare，意为"松开"）组成，意思是"释放、放出"。在句中表示蒸汽被意外排出的被动动作。