Here are some overviews of the science and technology behind nuclear, chemical and biological weapons.

• Appendix B: A Primer on Fissile Materials and Nuclear Weapon Design, by Owen R. Cote, Jr.
• Fissile Material Basics
• Nuclear Weapons (HowStuffWorks)
• Nuclear Weapons (The Atomic Archive)
• Biological & Chemical Warfare (HowStuffWorks)
• Low-Yield Nuclear Bunker Busters (UCS)
• Biological Weapons (About.com)
• Nuclear Weapons (misc. site)