What is nuclear energy?

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A banana contains naturally occurring radioactive potassium-40

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
O and CO
. 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 (around 0.72% uranium-235) or enriched (and by how much: most conventional reactors use material enriched 3 to 5% 235-U)
    • Fuel: solid (fuel rods in conventional reactors) or molten (in Molten Salt Reactors)
  • 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: Argon, Helium, CO
    • 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

Oklo: naturally occurring 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
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
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 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.

See also the University of Calgary's page on CANDU reactors on their Energy Education website.

In a post on Facebook by Christoffer Keyfor his friend Chris Adlam says of the CANDU:

Back in the 1950's while the US and the rest of the world were hotly pursuing atomic weapons, Canada, who had no desire for nuclear arms, saw the power of the atom as a way to produce abundant and inexpensive electricity. Atomic Energy Canada Limited (AECL) was the Federal thinktank comprised of brilliant engineers whose goal was exactly that: come up with a nuclear reactor that didn't require enrichment (we didn't have enrichment capability because we didn't have a nuclear arms program) and whose purpose was to be used for power generation.

Utilizing deuterium as a moderator, which allowed the use of a fuel with very low fissile content (natural uranium), what would become the foundation for the CANDU was in its infancy. A pressure tube design was chosen as the low fissile content fuel would need to be swapped out frequently, thus it was a requirement that the reactor could be refuelled online. After a small radiological release incident at Chalk River, it was determined that multiple levels of containment and redundant safety systems would be absolutely necessary. A family of designs was born from this philosophy with safety being the top priority.

After NPD was constructed and successfully demonstrated the CANDU concept the first commercial unit for the purpose of power generation was constructed. This was in the early 1960's at Douglas Point, now part of the massive Bruce Power site. This ~200MWe unit was a proof-of-concept design and led to the construction of the 4 units at Pickering A in a partnership between AECL, the Federal Government, the Ontario government and Ontario Hydro. Pickering was built instead of a similar capacity (4GW) coal plant.

Pickering was a massive success and by this point AECL had come up with a larger design and Ontario Hydro was keen. This led to the construction of Bruce A whose steam generators were intentionally oversized so the units could produce process steam to run operations on the grounds, such as the massive heavy water plant designed to produce deuterium both for domestic use and export. It was expected that the CANDU would be popular abroad, as we had managed to obtain partnerships and construction contracts with India, Romania, New Brunswick, Quebec...etc. CANDU was going places and we wanted to be ready.

On the heels of Bruce A came Pickering B, now based on the standardized CANDU 6 design, but with some changes on the steam and generation side to make it more similar to the A plant, thus reducing output. Then Bruce B was built, as efforts were made to cement the design for what would be the next export-ready unit, the CANDU 9. This led to the first commercial construct of that unit design: Darlington.

Darlington is probably the best known and most maligned nuclear plant in Ontario's entire nuclear fleet. Construction started while Bruce B hadn't even come online yet (similar to Bruce A and Pickering B) and was well underway when disaster struck: Half a world away a massive and unweildly reactor designed to produce weapons-grade plutonium succumbed to operator incompetence and suffered a meltdown. Because it lacked secondary containment found on every CANDU including Douglas Point, a hydrogen explosion resulted in a large radiological release.

Everything stopped.

Construction at Darlington ceased. The world scrambled to reconcile with what happened and the entire nuclear industry, even here in Canada, despite sharing absolutely nothing in common with the Soviet RBMK design at Chernobyl, went back to the drawing board. They had to prove it couldn't happen here. While this was taking place time, and debt, marched on. Interest rates were soaring, the cost of the Darlington project, despite no actual work being done, was increasing rapidly. By the time the first unit entered commercial service 10 years had passed, a far cry from the 6 years shovel to breaker for the Bruce A units. This led to a construction cost of $14.4 billion. Darlington was a white elephant and thus the B plant was never built.

Darlington was the most mature design in the CANDU fleet. It was, at the time, the epitome of CANDU engineering. Deep water inlet and outlet diffusers, better heat transfer loop design, higher power output...etc. The list goes on.

We never exported CANDU 9.

After Chernobyl the global nuclear industry never recovered. AECL managed to land a few CANDU 6 sales but the 9 went nowhere and it was abandoned. Darlington is the only operating example of the CANDU 9.

Since then, AECL managed to partner with China on the Enhanced CANDU 6, which the Chinese had interest in because as had been demonstrated in various tests in Canada, the high neutron economy and inherently flexible nature of the deuterium pressure tube design meant that the CANDU could run on a huge variety of fuel combinations, something other reactors were simply incapable of. China's intention for the units at Qinshan was for them to run on the used fuel coming out of their neighbouring American-style light water units, and they do. When AECL failed to secure the construction contract for the ACR1000's that were supposed to be built at Darlington B in the 20-teens it was sold off to SNC Lavalin. Ontario had screwed itself with insanely generous fixed-rate contracts for industrial wind and even more highly subsidized solar projects. This drove rates through the roof, leaving no consumer tolerance for a 25 billion dollar nuclear development.

As OPG continues to refurbish Darlington, now on Unit 3, and Bruce Power refurbishes the remaining 6x Bruce units while providing the 2nd lowest cost generation in the province I think it important to note that these things are not widely celebrated. Ontario has one of the lowest emissions grids in the world and that's mostly due to our massive nuclear fleet. Who knew that before reading this?

Today, as Darlington Unit 1 soldiers on after setting the world record for continuous operation at 963 days of almost zero emissions generation we should be proud of what that stands for: a Canadian design built by Canadians for Canadians for the purpose of peaceful power production. Operated by your fellow Ontarians providing valuable employment in all corners of this massive province and, along with hydro, being one of the only things keeping your rates down after the disaster that was the GEA. This is something we can, and should, all be proud of."



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.


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.