Exporting secure seed using bitcoin core
Bitcoin Core uses BIP32, but does not implement BIP39 - As such, you cannot export a set of seed words, since the seed to BIP32 master key conversions is one way only, and it is not possible to obtain seed words if you only have the master key.

You should simply back up the master key itself, and import it into another instance of bitcoin core when needed. The dumpwallet command will help you with that.