The gas centrifuge and nuclear weapons proliferation

The most difficult step in building a nuclear weapon is the production of fissile material. One can either make plutonium-239 in a nuclear reactor or enrich uranium to increase the abundance of its fissile isotope uranium-235. Historically, enrichment has been the more obscure of the two routes, but the recent spread of one technology - the gas centrifuge - from the Netherlands to Pakistan and on to Libya, Iran, and North Korea has brought enrichment to the forefront of proliferation. That development is challenging old ideas about how to ensure the peaceful use of nuclear technology and prevent the further spread of nuclear weapons.