EnumaElish said:
Thanks, Astronuc. I now understand that there are "peaceful" vs. "militaristic" enrichment grades. The practical question is, is there a technology that can be used to enrich nuclear fuel to, say, a max of 20% (but not above)? Or have some other strict constraint (e.g. shape of the solid that comes out of the process) that would preclude making a nuclear weapon?
Also, do you think whether someone could use low-grade nuclear fuel to manufacture a "dirty" conventional weapon?
In answer to the first part, the enrichment process works by successive stages to separate U-235 from U-238 with the U in the form of UF
6, uranium hexafluoride. UF
6 is formed chemically from processed U-oxide ( U
3O
8 ) and is a gas at 56.5°C. In modern processes, the UF
6 gas is feed into successive stages of centrifuges which separate by centrifugal force the gas molecules containing heavier U-238 from those with lighter U-235. The output of one centrifuge stage (one enrichment, richer in U-235) is the input to the next. The resulting enrichment depends on the number of stages and incremental enrichment of each stage.
The key issue with regard to the Iranian program is "what enrichment do they plan to achieve?"
The UF
6 gas, once enriched, may be converted chemically to a green ceramic form of UO
2, which with additives is then mechanically pressed and then sintered into cylindrical pellets for use in nuclear reactor fuel. Alternatively, the UF
6 may be concerted to metal, e.g. by hydrogen reduction. The metal can be formed into any geometry, always of a pre-determined subcritical mass.
Of more concern would be a reprocessing program in which Pu-239 and Pu-240 would be extracted from U-targets or spent fuel, and the resulting Pu would be formed into nuclear weapons. All modern nuclear warheads us Pu spheres (pits), which are also the triggers of thermonuclear weapons.
So-called "dirty bombs" use radio-nuclides, which are a by-product of the fission process, i.e. fission products, such as Cs-137, Sr-90, etc. The objective there is to disperse radioactive material into the environment. Nuclear reactors provide a most practical means of producing radionulides, otherwise radionuclides must be produced by neutron activation of some inert/stable nuclide.