What U-238 is remarkably good at is killing cats. Small spherical projectiles of the stuff when launched from slingshots are absolutely deadly against tabbies. There are literally thousands of U-238 pellets around the neighbourhbood where I live, and of course, hundreds of dead cats. Kids find the balls of Uranium and play marbles with them. They call them blackies. The risk to the population is miniscule. So far people dropping dead from cancer is not noticeable around here.
Yellowcake, which is U3O8, is hardly radioactive at all. That is the product from Uranium ore processing plants. A scintillometer (geiger counter that uses a lithium iodide crystal instead of a vacuum tube) won't kick on that stuff as much as it will on a granite boulder. A granite boulder will kick from 500 to 12,000 CPS and a drum of Yellowcake will kick at maybe 135. Background is from 40 to 100 CPS. The ore itself is far more radioactive. Ore of Uranium is hot as hades and should not be handled if it ranges to 3% Uranium. Some U oxides are lots of U and little O. You can get Ore which kicks at 100,000 CPS (about 10% U.) Some ore has lots of slow neutrons and some is faster depending on the locale. Canadian ore from Sask. dates anomalously young because its neutrons are relatively slow.
The Uranium used in shells for armour piercing is called depleted Uranium. This is also the same stuff that is used in the shell in a Hydrogen bomb to cause the fusion explosion, paradoxically. Depleted uranium has had the U235, which is the hotter isotope, largely removed from the matrix. That is why its radioactivity is so low. Some naturally occuring U238 is quite hot in fact.
Plutonium, the super hot product of heavy water reactors is not only very radioactive, it is poisonous as well. It is used as a trigger for hydrogen bombs almost exclusively because it is so rich in neutrons, and it was used exclusively as the one material for the simple atomic bomb. This material was used for the first three atomic bombs built, as there was no working gas filtration or centrifugal plant to make U235 in 1943, which was never used in the atomic bomb at first. The reason is that it is much cheaper to make plutonium if you have a reactor. This raises the question of which reactor made the plutonium for Fat Man and Little Boy, or the Hiroshima and Nagasaki bombs? Stanford University reactor was not working until 1947. I know the answer to that. Few people have even asked the question.
Another problem that was anticipated back in the twenties when the equivalence of matter and energy was discovered from the experiments of Lise Meisner was that plutonium would be necessary for explosion but the material exploded faster and with more energy than chemical explosions could compress it in order to make a true fission reaction. The materials necessary to compress the bomb and make it explode became classified and controlled in 1926. So it was much more a long term and concerted effort of governments than has previously been realized. In 1934 questions on U of T engineering exams addressed the question of calculation of critical mass for a run away fission reaction. In the mid thirties Germany moved to annex the Czech Uranium deposits. When Szilard and Teller escaped Hungary to warn Roosevelt about an imminent German atomic weapon, the joint Canadian-British weapons project had been underway on the Ottawa River for some 17 years! Building a reactor and testing weapons is not always a one year crash program as I am sure you can appreciate if you have ever done it. It just amazes me that people still believe the cover story of the Manhattan project. The first three atomic weapons used in the world were built in Canada. Los Alamos came much later and was not complete until the fifies. All the triggers for their bombs came from a small reactor in Canada.
The atomic bomb or hydrogen bomb both exclusively use the non-naturally-occurring plutonium as a trigger. U235 is an enhancer. Only heavy water reactors make weapons grade plutonium. Both India and China did not have adequate nuclear weapons programs until the bought Canadian heavy water reactors. Canada is most responsible for nuclear weapons proliferation of all countries. As a matter of fact, its reactors and mines have supplied most of the materials for the USA's nuclear weapons.
The implications of the above to an ingenious terrorists can be inferred as a danger but there are still major obstacles to making practical use of the information.
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