The rate of spontaneous fission is relatively low for U-235, so one can introduce a somewhat stronger neutron source. Note that there are two subcritical masses involved.p.tryon said:Do Uranium-235 nuclei ever undergo fission spontaneously? If not how does a nuclear bomb actually work? I understand that two pieces of Uranium (which are subcritical) are driven togther by a chemical explosion and this initiates the chain reaction; however, if nuclei can not undergo fission spontaneously, then where does the first neutron come from?
For U-235, probability of SF = 7.0E-9 % Multiply that by the decay constant and the number of atoms in 1 gram to get a specific activity.Arny said:I know that you've said that U-235 spontaneously fissions, though at a very low rate. What exactly is the rate (per minute/hour/day)?
Was spontaneous fission what was relied on for the initial neutrons to operation of Fermi's Chicago Pile 1, the first nuclear reactor?
Neutron sources for modern commerical reactors include Cf-252 for primary sources used to start the first cycle, and Sb-Be secondary (photonuclear γ,n) sources which are used in subsequent startups until sufficient transuranics are produced to give sufficient SF neutrons.Bob S said:From http://en.wikipedia.org/wiki/Neutron_source
"Neutrons are produced when alpha particles impinge upon any of several low atomic weight isotopes including isotopes of lithium, beryllium, carbon and oxygen. This nuclear reaction can be used to construct a neutron source by intermixing a radioisotope that emits alpha particles such as radium or polonium with a low atomic weight isotope, usually in the form of a mixture of powders of the two materials. Typical emission rates for alpha reaction neutron sources range from 1×106 to 1×108 neutrons per second. As an example, a representative alpha-beryllium neutron source can be expected to produce approximately 30 neutrons for every one million alpha particles. The useful lifetime for these types of sources is highly variable, depending upon the half-life of the radioisotope that emits the alpha particles. The size and cost of these neutron sources are also comparable to spontaneous fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe), or americium-lithium (AmLi). The neutron initiators of early nuclear weapons used a polonium-beryllium layers separated by nickel and gold until a neutron pulse was desired."
Natural beryllium has a very low energy threshold (2 to 3 MeV) for the photonuclear γ,n reaction, so a high α,n reaction yield is not unexpected.
Bob S
A nuclear bomb chain reaction is a series of nuclear fission reactions that occur when the nuclei of atoms split, releasing large amounts of energy in the form of heat, light, and radiation. This reaction is the basis for the immense destructive power of nuclear bombs.
A nuclear bomb chain reaction works by using a small amount of fissile material, such as uranium or plutonium, to initiate a series of nuclear fission reactions. When an atom of a fissile material is hit by a neutron, it splits into two smaller atoms and releases more neutrons, which then go on to split other atoms in a continuous chain reaction.
The strength of a nuclear bomb chain reaction is affected by several factors, including the amount and type of fissile material used, the speed and number of neutrons released, and the design of the bomb itself. Small changes in these factors can greatly impact the size and intensity of the chain reaction.
Yes, a nuclear bomb chain reaction can be controlled through the use of control rods, which absorb excess neutrons and slow down the chain reaction. This is how nuclear power plants use nuclear reactions to produce energy without causing a destructive explosion.
The consequences of a nuclear bomb chain reaction can be catastrophic, including widespread destruction, radiation poisoning, and long-term environmental and health effects. The use of nuclear weapons has been banned by international law due to the devastating consequences they can have on both human life and the environment.