
Graphite != Graphene.
I don't know too much about the materials used as moderators in nuclear reactors, but I'd be prepared to bet good money the graphite they use is composed of large blocks of disordered material in order to make its neutron-absorbing properties isotropic.
Graphene, on the other hand is a highly ordered material, being composed as it is of single 'sheets' of graphite.
Rather than purifiying it from bulk graphite, as you imply, it would be manufactured using some sort of plasma vapour deposition (PVD) technique to manufacture 1-atom thick sheets. I would imagine that these would be of negligable use in a nuclear reactor as your average high-velovity neutron is going to go straight through something that is one ten millionth of a millimetre thick.
I don't think you'd happen to have too much difficulty from the authorities if found to be transporting graphene-based electronics, in much the same way that customs aren't going to be too bothered about the gold-plating on the headphone jack on your mp3 player when you go through the airport, but might be interested in a little chat over a cup of tea if you were carrying a suitcase full of gold bullion...