It is said that a German sub, U-234, with a large "cargo" type left Germany on March 25, 1945 with a disasembled ME 26r, some German experts on rockets and air defence, two Japanese men, and most importantly, 50 lead cases of uranium oxide. It was bound for Japan. The sub was still at sea when Germany surrendered and received orders to return to base. The captain decided not to and eventually surrendered to an American ship, the USS Sutton. The sub was taken to New Hamshire where the cargo of uranium was discovered and the story was out.
Uranium oxide which has not been bombarded by neutrons in a reactor or sub-reactor can be packed up to 500 kilos in a stout paper bag since common-or-garden uranium oxide emits no gamma radiation, and little other corpuscular radiation.
The material aboard U-234 was transported in lead cylinders lined with gold. There is no indication of the size of each container but the weight of the cylinder would have ruled out the quantity of substance transported being very much. Radioactive isotopes are transported in lead isotope containers for biological shielding and do not need an extra gold lining. Gold has a good cross-section for preventing the fragments of fission, but fission would not occur in a small lead cylinder. The dockyard document advises US authorities that the "uranium oxide" may be handled "like crude TNT", suggesting that it had explosive properties, but that the cylinders should never be opened because the substance "would react" (presumably on contact with air) and "become dangerous." One should not necessarily conclude that "become dangerous" is synonymous with "explode".
There is a British BIOS Intelligence document extant which suggests that a catalyst or reagent was added to a normal explosive in tests at Berlin Doberitz in late 1944. Allegedly it vastly magnified the destructive effects of the conventional explosive by creating a lightning storm at ground level over several kilometres. Such a device would kill principally by suffocation after the initial blast had dissipated. Scientists in Germany had not been able to iron out certain lethal side-effects by the war's end and there is some evidence that there might have been a Luftwaffe mutiny against carrying the material in March 1945. It is interesting that the Me 262 bomber alleged by the German crew to be aboard U-234 has never been admitted by US official sources and did not appear on the USN Unloading List. It is unlikely that the German commander and crew were lying and one assumes that the jet had some special significance in connection with the material.
Perhaps this is all bullshit...
http://uboat.net/boats/u234.htm