Hi Lawrence, 'Undangerous' (which isn't really a word) looks similar to the word 'dangerous', which is an adjective. The opposite of 'dangerous' is 'safe'. 'Safety' and 'Security' are nouns. The noun form of 'dangerous' is 'danger'. If you want to express that something is not in danger, you could say that it is 'safe and secure' I think this is what you were looking for; if not, give us a little more information about what you mean by "undangerous" and a more detailed explanation of what you're looking for.