Difference between revisions of "What is nuclear energy?"

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In our classification (above) the Oklo reactors were ''solid Uranium fuelled, thermal spectrum using light water as moderator and heat transfer medium (and Real)''.
 
In our classification (above) the Oklo reactors were ''solid Uranium fuelled, thermal spectrum using light water as moderator and heat transfer medium (and Real)''.
  
''The ''Scientific American'' article ''[https://www.scientificamerican.com/article/ancient-nuclear-reactor/ The Workings of an Ancient Nuclear Reactor]'' by Alex Meshik discusses the discovery of the Oklo (and other) natural reactors, and what we have learned from them.''
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''The ''Scientific American'' article ''[https://www.scientificamerican.com/article/ancient-nuclear-reactor/ The Workings of an Ancient Nuclear Reactor]'' by Alex Meshik discusses the discovery of the Oklo (and other) natural reactors, and what we have learned from them. Wikipedia also discusses the Oklo reactors in it article:
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"[https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor Natural nuclear fission reactor]''
  
 
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== FURTHER READING ==
 
== FURTHER READING ==

Revision as of 11:45, 6 September 2020


When we burn coal, oil, gas, wood (and other biomass), hydrogen etc, their chemical molecules react with Oxygen to produce heat (or in the case of fuel cells, electricity). The molecules of fuel get broken down and their constituent atoms re-arranged into different molecules - for example Carbon and Hydrogen atoms in gas or oil break away from each other and combine with Oxygen into water and Carbon Dioxide – H
2
O and CO
2
. However the Carbon, Hydrogen and Oxygen atoms themselves are unchanged.

Nuclear energy is produced by the splitting or combining of atoms themselves. The combining of atoms – fusion – is the subject of experiment and development, but the technology is probably decades away from producing useful amounts of energy commercially.

The splitting of atoms is fission and is the basis of our current nuclear power stations.

Another process in which atoms split is the spontaneous decay of radioactive isotopes, including some of the Carbon and Potassium atoms in our bodies (and in bananas!). The heat generated by radioactive decay (of Plutonium) is used to power some spacecraft including the Voyagers and the Curiosity Mars rover.

Fission, fissile, and fertile

A diagram showing a chain transformation of uranium-235 to uranium-236 to barium-141 and krypton-92
How a neutron splits a Uranium-235 atom producing more neutrons

Uranium has several isotopes, all of which are unstable, making it (weakly) radioactive. (See Wikipedia for details.) Naturally occurring Uranium comprises mostly the Uranium-238 isotope, with less than three-quarters of a percent of Uranium-235. U-235 is "fissile": it has a certain probability of spontaneously splitting up into smaller atoms, releasing neutrons in the process. Its splitting up ("fission") can be triggered by it being hit by a neutron, releasing yet more neutrons which can split more U-235 atoms, in a chain reaction. The reaction also releases a lot of energy -- 1.5 million times as much as burning the same weight of coal.

Plutonium-239 is another fissile isotope. It doesn't occur naturally but it can be produced when neutrons hit Uranium-238 atoms. Isotopes like U-238 and Thorium-232 are known as "fertile" because they can transmute into fissile isotopes (Pu-239 and U-233, respectively) when hit by neutrons.

Fast, moderate and thermal

When a Uranium-235 atom splits, the neutrons it releases travel fast, and they are far less likely to make another U-235 atom split than slower-moving neutrons do. In a mass of concentrated U-235 (such as in an atom bomb) there can be enough neutrons making atoms split and releasing more neutrons etc for a chain reaction to occur, but with less concentrated Uranium (containing less of the U-235 isotope mixed with more of the non-fissile U-238) nothing will happen. (This is why nuclear reactors can't explode like a bomb, and ordinary nuclear reactor fuel can't be used to make a bomb.)

However if some of the neutrons emitted by splitting U-235 atoms are slowed down before hitting other atoms they are about 1,000 times more likely to make them split and sustain a chain reaction. Slower neutrons are called "thermal" and the slowing-down process is called "moderating". Water and graphite are good at slowing down neutrons so most nuclear reactors use either water or graphite as moderators. Water can also be used to transfer heat from the reaction to provide useful energy.

Breeders and Burners

Fast neutrons can be captured by various atoms and turn them into other isotopes. This process can burn up radioactive isotopes (such as those in the spent fuel of conventional reactors) which are hard to dispose of, and by the process of "neutron activation" it can turn fertile isotopes such as U-238 into fissile ones such as Plutonium-239. The latter process is called "breeding" and is designed to occur in "fast breeder" reactors, although it also happens in conventional ("thermal spectrum") ones.

Types of Reactors

There are many sorts of fission reactors which have been tried, and a huge variety which have been proposed, and it may help to divide them by important characteristics:

  • Fuel: Uranium, Plutonium, Thorium etc
    • Uranium: natural or enriched (and by how much)
    • Fuel: solid or molten
  • Thermal spectrum: Fast or slow neutrons
    • (For Thermal reactors): Moderator: regular (light) water, heavy water, graphite etc
  • Heat transfer/coolant medium: gas or liquid
    • Heat transfer gas: Helium, CO
      2
      etc
    • Heat transfer liquid: water, metal, salt:
      • water: regular (light water) or heavy water,
      • metal: sodium, lead, mixture etc,
      • salt: fluoride, chloride, mixture (e.g. FLiBe) etc
  • Purpose/product: experimental, research, production of isotopes, electricity, heat etc

and, last but not least:

  • whether they are a paper (or academic) reactor or a real (practical) one.

Paper v. Real reactors

US Admiral Hyman Rickover, who brought nuclear reactors for the navy and civilian power stations to reality, observed that:

An academic reactor or reactor plant almost always has the following basic characteristics:

  1. It is simple.
  2. It is small.
  3. It is cheap.
  4. It is light.
  5. It can be built very quickly.
  6. It is very flexible in purpose.
  7. Very little development will be required. It will use off-the-shelf components.
  8. The reactor is in the study phase. It is not being built now.

On the other hand a practical reactor can be distinguished by the following characteristics:

  1. It is being built now.
  2. It is behind schedule.
  3. It requires an immense amount of development on apparently trivial items.
  4. It is very expensive.
  5. It takes a long time to build because of its engineering development problems.
  6. It is large.
  7. It is heavy.
  8. It is complicated.

Real Reactors

Probably the simplest reactors, and certainly the earliest — by almost 2 billion years — were those at Oklo, in Gabon in West Africa.

The geology of the Oklo reactors:
(1) reactor zones
(2) Sandstone
(3) Uranium ore layer
(4) Granite

These comprised veins of rock rich in Uranium ore, into which water permeated. The water acted as a moderator, slowing neutrons released by spontaneous fission and creating a chain reaction. The reaction released heat which boiled the water off until the reaction stopped, after which the rocks cooled and water returned to start the reaction again. It is estimated that the reactors ran for hundreds of thousands of years, until the U-235 in the rocks had been burned up too much to sustain further activity.

The same thing could not happen now. Uranium-235 has a half life of about 700 million years compared to 4.5 billion years (about the same as the age of the Earth) for U-238, so whilst natural Uranium now contains only about 0.7% U-235, at the time of the Oklo reactors the concentration was around 3%, which is similar to that used in present-day light-water reactors, and is sufficient to sustain reactions.

The Oklo reactors probably produced less than 100kW of heat, compared to several GW in modern man-made reactors (about one-third of which gets converted to electricity).

In our classification (above) the Oklo reactors were solid Uranium fuelled, thermal spectrum using light water as moderator and heat transfer medium (and Real).

The Scientific American article The Workings of an Ancient Nuclear Reactor by Alex Meshik discusses the discovery of the Oklo (and other) natural reactors, and what we have learned from them. Wikipedia also discusses the Oklo reactors in it article: "Natural nuclear fission reactor

Man-made reactors

The earliest artificial reactor was the Chicago Pile experimental reactor, built as part of the WW2 Manhattan Project to build an atomic bomb. It used about 50 tonnes of Uranium, with graphite as a moderator, and produced half a watt of power.

We would classify it as a solid Uranium fuelled, thermal spectrum, graphite moderated, experimental, real reactor.

Pressurised and Boiling Water reactors

Pressurised Water Reactor

After WW2 the United States developed a nuclear reactor as propulsion for submarines, allowing them to stay submerged for days or weeks at a time and to cross oceans without surfacing, unlike earlier diesel-electric designs which had limited range and duration while submerged.

The USS Nautilus was the first nuclear powered submarine, launched in 1954. It used a Pressurised Water Reactor. PWRs were used at the US' first commercial power station at Shippingport (which also later housed a Thorium-fuelled thermal breeder reactor).

Pressurised Water Reactors are widely used in the USA, France, Germany, Russia, China, South Korea and many other countries, as well as in military submarines and aircraft carriers, and icebreakers.

See also the US Nuclear Regulatory Commission's PWR page

Boiling Water Reactor

Boiling Water Reactors are similar to PWRs but have a simpler heat transfer/cooling system. They are widely used in Japan, including in the Fukushima Daiichi reactors which suffered meltdowns after being hit by the tsunami generated by the 2011 Tohoku earthquake.

See also the US Nuclear Regulatory Commission's BWR page

PWRs and BWRs are solid, low-enriched-Uranium fuelled, thermal spectrum using light water as moderator and heat transfer medium, designed to produce electricity.

Magnox and AGRs

Advanced Gas-cooled Reactor (AGR)

After the war Britain built gas-cooled graphite-moderated pile reactors using un-enriched ("natural") Uranium, at Windscale (one of which suffered a near-catastrophic fire in 1957). These led to the design of Britain's Magnox reactor, which was used in the first commercial-scale power reactor in the world at Calder Hall (at what is now called the Sellafield nuclear plant).

Magnox reactors are solid, natural Uranium fuelled, thermal spectrum using graphite as moderator and CO
2
as heat transfer medium, designed to produce plutonium as well as electricity
.

The Advanced Gas-cooled Reactor is a development of the Magnox intended to be better at producing electricity whilst dropping the function of producing plutonium.

AGRs are solid, low-enriched-Uranium fuelled, thermal spectrum using graphite as moderator and CO
2
as heat transfer medium, designed to produce electricity
.

For more on the AGR see How an AGR power station works by British Energy Group plc, 2006

CANDU

CANDU reactor

The basic Canada Deuterium Uranium design is a pressurised water reactor using solid, natural Uranium fuel, thermal spectrum using heavy water as moderator and heat transfer medium to generate electricity.

RBMK

RBMK

This Soviet-designed reactor is notorious as the type involved in the Chernobyl accident in 1986.

The original design was solid, natural Uranium fuelled, thermal spectrum using graphite moderator and water as heat transfer medium, designed to produce electricity and able to produce plutonium, but modifications to the design after Chernobyl required it to use low-enriched-Uranium.

FURTHER READING

Wikipedia has a fairly comprehensive article on nuclear reactors and associated topics. with links to more detailed articles.

What Is Nuclear? have some resources including:

The IET has several factfiles on nuclear power including:

  • Principles of nuclear power which discusses the structure of atoms, the concept of fission, chain reactions, and the essential elements of a power reactor (using the Advanced Gas-cooled Reactor as example),
  • Nuclear Reactor Types discusses and compares Magnox, AGR, PWR, BWR, CANDU, and RBMK reactors, and some future designs.

These documents date from around 2008 and, whilst they have since been "redesigned", they still refer to, for example, the EPR as a future design.