Plutonium vs. Uranium

Suppose you find a nuclear bomb and decide to transport the device to a safe hiding place. Would there be any difference with regard to your safety as to whether the device is made of uranium-235 or plutonium-239?

Answer

A plutonium bomb would be much safer to handle. Weapons-grade plutonium (Pu-239) emits primarily alpha particles and low-energy gamma rays, both being easy to shield. The trace amounts of even-numbered Pu isotopes have spontaneous fission reactions that emit neutrons. However, neutron detectors would need to be within a few meters to detect these neutrons above background. So a lost plutonium bomb could be very difficult to find.

The plutonium usually is coated with beryllium or another appropriate sealant because the exposed element will react chemically with the oxygen in the air or in water and increase its temperature considerably. The person holding a plutonium bomb would probably feel that the protective casing is warm, because the alpha particles deposit their kinetic energy in the casing material.

Of course, inhaled or ingested Pu is one of the worst carcinogens known. Any explosion releasing Pu into the air creates a hazard for all life that would remain for a long time.

In contrast, weapons-grade U-235 releases gamma rays at several energies, the most intense at 186 KeV. So the detection of a device containing U- 235 is much easier than trying to find plutonium. Some films portray the differences correctly, while other films dramatize any nuclear device and its possible dangers with remarkably adolescent scare techniques.