Tutorial: Neutron Capture & Fission

Neutron Capture and Radiation

Atoms are made of neutrons, protons, and electrons.  Protons are positively charged and electrons are negatively charged, while neutrons do not have a significant charge.  The opposite charges of the protons and electrons provide an attraction that keeps the electrons attached to the atom.  The neutrons and protons are located in a central “nucleus,” with the electrons moving around the nucleus. 

The possible chemical reactions and atom can have is controlled by the total number of electrons.  Since the number of electrons depends on attraction to protons in the nucleus, the chemical identity of an atom is a function of the number of protons in the nucleus.

Different atoms of an element (i.e., same number of protons or “isotopes”) can have different numbers of neutrons.  Only some isotopes of an element are stable; unstable isotopes get rid of energy by emitting or “radiating” electromagnetic waves and charged particles. These isotopes are called “radioactive.”  The radiation of charged particles changes the positive nuclear charge, creating a different element.  The radiation emitted by an isotope usually has very specific energies. About 10% of all naturally occurring elements have isotopes that are radioactive.

If a neutron is added to the nucleus of an atom, it usually becomes radioactive.  Radiation is generated in a 2-step process: as soon as the neutron is absorbed, some energy is given up as the new isotope is formed; most of the time the new isotope is unstable and has an average lifetime until it emits a nuclear charged-particle and “decays” to a different element. 

Neutron Activation Animation

Neutron Capture and the Fission Process

A handful of very heavy isotopes are so close to being unstable that they sometimes blow apart when the additional neutron is absorbed in the nucleus.  This (called the fission process) releases an enormous amount of energy.  Neutrons are also usually released when an atom fissions, leading to the possibility that a self-sustaining “chain reaction” could be created.    

Fission Process Animation

During World War II, some scientists in the U.S. studying radioactivity realized that the energy release from fission was so large that an uncontrolled chain reaction would generate a shock wave much larger than conventional explosives.  It was known that German scientists were also studying fission, and a crash research program (called the “Manhattan Project”) was started to try to stay ahead of German research.  Research showed that although a chain reaction could be engineered for several isotopes, only a couple of them could be configured to release enough energy in a time short enough to cause an explosion.  These materials do not exist in naturally in large amounts, and have to be increased in concentration by a large amount (in the case of ²³⁵U) or made by neutron absorption (in the case of ²³⁹Pu). 

Nuclear reactors surrounded by water burn ²³⁵U and ²³⁹Pu and make other isotopes that make it difficult to build a nuclear weapon (hard to separate, unstable enough to cause uncontrollable detonation).  Nuclear weapons material is best generated in reactors with either little water, or with water where the hydrogen atoms have two neutrons in the nucleus (known as “heavy” water).  On the other hand, fission energy can boil water, use steam to rotate a turbine, and generate electricity.  Several pilot plants using nuclear power to generate electricity were built in the U.S. to prove the technology.  In order to reduce the likelihood of nations creating nuclear weapons, President Eisenhower offered to give light water reactor technology to the world.  (However, other nations developed competing reactor concepts based on graphite technology with very little water, gas cooling, and cooling by molten metal.)