Question

U-236 is an important neutron poison in reactors that is created in uraniumbased fuels from U-235 capturing a neutron. Derive an expression for the time at which the peak U-236 concentration occurs. Note that not all neutron absorption by U-235 becomes U-236; some U-235 atoms fission.

Answer 1

Nuclear Fission: The "235" in Uranium 235 refers to the total number of protons (92) and neutrons (143) contained in the nucleus of the atom. Natural uranium has only 0.7% Uranium 235 (U-235). U-235 has the property that it decays naturally throwing off small amounts of radiation, i.e. alpha particles consisting of two protons and two neutrons. Most importantly, it can also be "induced" to decay rapidly by bombarding it with a free neutron, which is called "fission". Fission means "splitting".

When a free neutron is absorbed by a U-235 atom, it temporarily becomes a U-236 atom which is unstable. U-236 immediately splits into two smaller atoms and in addition throws off radiation (alpha particles) and three more neutrons. If there are no more U-235 atoms around, the neutrons simply fly off into space. However, if the free neutrons are surrounded by a dense mass of other U-235 atoms, these new free neutrons then split more atoms and so on, causing a chain reaction. The splitting of one U-235 atom into two smaller atoms releases an enormous amount of energy - about a million electron volts for each split atom. The stored energy in a pound of "enriched" uranium is equivalent to about a million gallons of gasoline.