If you run Nurmengard into Babelfish for German-to-English you get "only mixing pool of broadcasting corporations." That's entertainingly deranged but clearly a dead end. Nurmen also seems to mean grass in Finnish. Again, interesting but not really getting us anywhere. After some more musing it struck me that nurmen could be related to normen which is standardize, normalize. Gard is, of course, guard. Aha! Given the original stated purpose of the facility, that seems to be a likely candidate for what Rowling was suggesting.