The antonyms for the phrase "breaking open" include "closing," "sealing," "shutting," and "locking." These opposites imply the opposite of an action that forcibly opens or forces something apart, such as a box or a container. The antonyms of "breaking open" suggest an action that is designed to protect something, keep it contained, or keep it out of reach. Examples of use cases for antonyms of "breaking open" would be when you lock a safe to keep its contents secure or when you seal a jar to prevent its contents from spilling out. In sum, the antonyms of "breaking open" imply actions that result in keeping something closed or shut.