Two23 wrote:
I believe that in the 1950s Kodak made an Arial Commercial Ektar lens that had uranium in the glass. The glass turns yellow with age, and will set off a Geiger counter. Not sure how dangerous it is.
I believe that was Thorium, not Uranium. Thoria glass is quite well known as having a high refractive index, though not commonly used nowadays.
Jacob D wrote:
I read somewhere that about 15kg of 235 is needed for a bomb, but only a few grams actually undergo fission, the rest is needed for some other reason, which is beyond me
The large mass is necessary to have sufficient chance for fissions to take place. You have to remember that atoms are mostly empty space (the nucleus is much smaller than the radius of atom's electron cloud).
A neutron from one fissioned U235 atom has to hit another U235 nucleus to fission that one, and the neutron from that one has to hit another nucleus, and so on. If you have just a few grams of U235 the number of reactions will be so low that there will be no chain reaction. Critical mass means that the number of fissions is enough for the reaction to be sustained.
Jacob D wrote:
I read somewhere that about 15kg of 235 is needed for a bomb, but only a few grams actually undergo fission, the rest is needed for some other reason, which is beyond me
A minimum of 52 kg is required to achieve critical mass for U235.
molson wrote:
A minimum of 52 kg is required to achieve critical mass for U235.
It all depends on the enrichment level. Wikipedia says:
"The critical mass for lower-grade uranium depends strongly on the grade: with 20% U-235 it is over 400 kg; with 15% U-235, it is well over 600 kg."
and
"The nominal spherical critical mass for an untampered 235U nuclear weapon is 56 kg, a sphere 17.32 cm (6.8") in diameter. The required material must be 85 percent or more of 235U and is known as weapons grade uranium [...]"
