Note: This is an overview of nuclear energy, longer and more detailed than the one in section (S-8). It was prepared by David P. Stern as part of the Virginia Flexbook on Physics prepared under auspices of the CK-12 foundation, under the "creative commons, cite by attribution, share alike" protocol. It was meant to serve as supplemental text for high school physics. It also includes problems and questions.

1.     Matter consists of atoms and molecules.
   
2.     Atoms and molecules contain electrically charged particles--negative electrons and positive, compact nuclei. Chemistry is ruled by electrical forces between those particles.
   
3.     Electrons and positive parts of atoms may be separated and accelerated. Much of our physics is based on studies of collisions between such particles.
   
4.     Atomic nuclei contain positive protons and uncharged neutrons (collectively "nucleons"), slightly heavier. Their numbers in a nucleus are nearly equal.
   
5.     Atomic nuclei of any chemical element contain equal number of protons, but the number of neutrons varies slightly between different isotopes.
   
6.     Almost all of the mass of an atom is in the nucleus.
   
7.     As stable atoms get heavier, their ratio of neutrons / protons increases.
   
8.     Atoms with unstable nuclei (e.g. with above ratio unusually large or small) change by emitting alpha, beta and gamma rays, so-called nuclear radiation.
   
9.     Electromagnetic radiation is a family of spreading electromagnetic waves, including light, radio, microwaves and X-rays. Gamma rays also belong, but alphas and betas are fast helium nuclei and electrons.
   
10.     As one approaches and passes atomic dimensions, laws of physics are gradually modified into those of quantum physics. In particular, an electromagnetic wave is a disturbance spreading like a wave but its energy is emitted and absorbed in particle-like packets known as photons. Quantum mechanics explains the precise frequencies of light from atoms as transitions between standing waves in atoms with differing energies.

1.     The masses of nuclei can be precisely measured by "mass spectrographs". They differ from the sum of masses of their particles by the binding energy E, the energy needed to break them apart. E is found from Einstein's E = mc² , where m is the mass difference.
   
2.     In light nuclei, binding energy per nucleon usually increases with nuclear mass, because the more nuclear mass, the more energy in the nuclear force holding them together. That force however has a short range, and past iron (56 nucleons) the repulsive force of the positive charge of the protons causes the binding energy per nucleon to decrease.
   
3.     Most elements with more than 200 nucleons are unstable because of the above disruptive force, and decay by radioactivity. None exist in nature with more than 238 nucleons (uranium 238). Artificially produced nuclei of greater mass decay even faster.

1.     Very heavy nuclei may decay by fission into two unequal parts, releasing much more energy (typically 50-100 times) than radioactive decay.
   
2.     Uranium with 235 nucleons (U-235) makes up 0.7% of natural uranium. If approached by a neutron (even a slow one) it fissions. Plutonium Pu-239 produced (with an intermediate step) when U-238 captures a neutron behaves similarly, as does U-233. (Pu-240 undergoes spontaneous fission).
   
3.     The "fission fragments" are extremely unstable, because their neutron/proton ratio is much larger than that of stable nuclei with the same number of protons. They emit on the average more than 2 neutrons per fission, in 98% of the cases "promptly" What remains is still dangerously radioactive for many years. If at least one of those neutrons is re-absorbed by U-235 or plutonium, another fission may result, leading to a "chain reaction."
   
4.     In relatively pure U-235 or Pu-239, a chain reaction can cause a nuclear explosion. In power-generating reactors, the neutrons are slowed down by collisions with atoms of a moderator like pure carbon or water (especially "heavy water"), and the reaction can be controlled by cadmium rods which absorb neutrons. In a controlled nuclear reaction, heat generated by fission is typically removed by pressurized water, which turns to steam, powering turbines which rotate generators of electric power.
   
5.     Partially spent fuel must be reprocessed--to remove "unburned" fuel, also to remove plutonium generated when U-238 absorbs neutrons, as well as fission fragments which absorb neutrons and retard the fission process. Those fragments are dangerously radioactive for centuries and need to be stored away from people and ground water
   
6.     Reactors function better and can be smaller if the fraction of U-235 in the fuel is first enriched, by cascades of porous partitions or by fast centrifuges, filtering the gas UF₆.

1.     The traditional control of nuclear reactions uses neutron-absorbing control rods, thrust automatically into the core to dampen the reaction or pulled out to speed it up. Only a 2% margin (due to neutrons emitted with a delay of a second or two) is available (for a given reactor core) between a runaway reaction and a fizzle!
   
2.     Even if a reactor is shut down, cooling water must continue to circulate, because radioactive energy continues to be released for a while, enough to produce a "meltdown" of the core. That happened in March 1978 at Three Mile Island in Pennsylvania; that reactor was inside a thick concrete "containment building" which prevented any radioactivity from leaking out.
   
3.     The Soviet power reactor at Chernobyl near Kiev (now Ukraina) was moderated by a carbon core and had no containment building. A dangerous experiment in 1986 started a runaway reaction, blowing off the roof and starting a fire which spread dangerous radioactive debris across Europe.
   
4.     A breeder reactor uses neutrons of each fission very economically--one to continue the chain reaction, and at least one to be captured by uranium 238 (or thorium 232), converting them into usable fuel--plutonium 239 or uranium 233. In principle therefore a breeder reactor can extract energy from all its uranium or thorium.
   
5.     The first experimental reactor started up on 2 December 1942 in Chicago. As of 2009 nuclear energy produces most of the electricity in France, about 20% in the USA, and comparable or larger amounts in Spain, Japan, Germany, Britain and Russia.

"Nuclear Energy" continues with (S-8A-1)   The Fundamentals: Atoms and Nuclei

"From Stargazers to Starships" continues with sections on spaceflight and spacecraft, starting with The Principle of the Rocket

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: 3-15-2009