The Basics - Level 2

The fission process Nucleus of U-235 or Pu-329 captures a neutron - U-236, Pu-240 nucleus formed. U-236, Pu-240 very unstable, rapidly split into two (fission). Neutrons and a large burst of energy are emitted. –Complete fissioning of 1 gram of U-235 releases 23,000 kilowatt-hours of heat.

Critical mass Each nucleus undergoing fission must produce a neutron that splits another nucleus. Critical mass - the minimum mass of fissile material that can sustain a nuclear fission chain reaction. Sphere is optimum shape.

Nuclear explosions If critical mass is exceeded more neutrons are produced with each successive generation of fission. Nuclear explosions occur at super- critical masses, when the number of neutrons increases rapidly and uncontrollably.

Nuclear explosions Basic weapons contain fissile material less than critical mass. On detonation, density is increased to super-critical - a process referred to as "assembly". Within half a millionth of a second: –Temperatures - hundreds of millions degrees centigrade, and pressures - millions of atmospheres, build up.

Fusion Isotopes of hydrogen - deuterium and tritium. Extremely high temperatures required for reaction to occur. Require a fission bomb to provide energy to initiate reaction. Used mainly to ‘boost’ fission bombs - increase fission rate by providing more high energy neutrons.

Detonation techniques Gun technique –Only used with HEU. –Mass of sub-critical HEU fired at another - sum of two masses supercritical. –Simple technique. –Long assembly time. –Hiroshima bomb.

Detonation techniques Implosion technique –1/10 the assembly time of the gun technique. –HEU or plutonium can be used. –Fissile core surrounded by conventional high explosives.

Detonation techniques Implosion technique –Explosives detonate and uniformly compress the core and increase its density, making it super-critical. –Neutrons also fired into fissile material to encourage fission chain reaction.

Main components of nuclear weapons High quality, high purity conventional high explosives and reliable detonators. Electronic circuits. A tamper and neutron reflector. A core of fissile material. A neutron source.