In addition, the Council urged the Conference on Disarmament to quickly negotiate a treaty banning the production of fissile materials for explosive devices. (politics.co.uk)
It refers both to the assembling of critical masses of fissile material for the new weapons, and to the bringing together of the right concentrations of brainpower and other resources to achieve the goals of the enterprise. (newscientist.com)
To make uranium suitable for an atomic bomb, you have to somehow enrich it up to weapons grade, so that it is almost pure U 235, the element's most fissile isotope. (scientificamerican.com)