The U-boot that you're thinking about is U-234. See the history of the boat on this website. Also, Google "U-234 submarine uranium" and you'll find a lot of info.
It's not clear exactly what U-234 was carrying. Some sources say "enriched" uranium, others say "uranium oxide", others say "uranium isotopes" but clearly the cargo was either weapons-grade uranium or a precursor uranium product that could be reasonably easily processed into weapons-grade uranium by the technology of the time.
The boat surrendered in Portsmouth New Hampshire in May of 1945. The entire contents of the shipment (including a disassembled Me-262 jet aircraft and other scarce war materials) was seized by the US Government.
Some sources also state that the uranium (or some of it) was processed at Oak Ridge Laboratories into fuel used in the bombs dropped on Japan in September, 1942. If this is true, it's ironic that atomic weapon material meant for use by Japan against the US was actually used against Japan to end the War.