Critical concern. (Letters).I am concerned about the article "Neptunium neptunium (nĕpt  `nēəm), radioactive chemical element; symbol Np; at. no. 93; at. wt. 237.0482; m.p. about 640°C;; b.p. 3,902°C; (estimated); sp. gr. 20. Nukes? Little-studied metal goes critical" (SN: 10/26/02, p. 259). It addresses the mass of fissile fis·sile adj. 1. Possible to split. 2. Physics Fissionable, especially by neutrons of all energies. 3. Geology Easily split along close parallel planes. materials needed to "make a bomb, yet it's clear that the critical masses given--10 kilograms for plutonium-239, 50 kg for uranium-235, and 60 kg for neptunium-237--are for bare spheres with no neutron moderation, reflection, or other factors contributing to going critical. Consider that the International Atomic Energy Agency International Atomic Energy Agency: see Atomic Energy Agency, International. International Atomic Energy Agency (IAEA) International organization officially founded in 1957 to promote the peaceful use of nuclear energy. controls quantities of 8 kg of [sup.239]Pu and 25 kg for [sup.235]U and that these quantities probably relate to masses needed for crude "bombs" Further, in some processing plants, a mass of about one-half a kilogram is used as a guideline for the minimum quantity of [sup.239]Pu or [sup.235]U, which can, given optimum conditions, go critical. I think the article could mislead people on what quantities need protection. ROBERT MARSHALL The following people have the name Robert Marshall:
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