 The STR (Submarine Thermal Reactor) pressurized water reactor designed at Argonne powered the first nuclear-powered submarine, , and became the basis of nearly all the reactors installed in warships.The other branch of reactor development at the Argonne National Laboratory, and the one closer to Zinn's heart, was the fast breeder reactor.At the time it was believed that uranium was a scarce resource, so it would be wise to make the best use of it.