Only 0.71 percent of natural uranium was uranium-235, and it was estimated that it would take 27,000 years to produce a gram of uranium with mass spectrometers, but kilogram amounts were required.
At the time there was disagreement about whether it was uranium-235, which made up less than 1% of natural uranium, or the more abundant uranium-238 isotope, as Fermi maintained, that was primarily responsible for fission.
In a paper co-authored with the American physicist John Wheeler, Bohr theorised that fission was more likely to occur in the uranium-235 isotope, which made up only 0.7 percent of natural uranium.
The vast majority of the work came in the form of the isotope enrichment of the uranium necessary for the weapon, since uranium-235 makes up only 1 part in 140 of natural uranium.
Only 0.7% of natural uranium is the fissionable isotope 235U.
EPR™ are only supplied with MOX (Mixed OXide) fuel to avoid the use of natural uranium.
Neutronics analyses of natural uranium fueled, light water cooled, heavy water moderated and graphite reflected nuclear reactors.
The subcritical core consists of natural uranium fuel with high density polyethylene as moderator and beryllium oxide as reflector.
Uranium-235 exists as only 0.7percentt of natural uranium ore, which is made up almost entirely of the heavier, and non-fissile, isotope uranium-238.
On Wednesday, it announced that it was close to a deal with Russia to export its stockpile of enriched uranium in return for imports of natural uranium.
There are also health dangers associated with concentrated forms of natural uranium, and since little is secure in Iraq, officials wanted to remove it.
