Difference between revisions of "What is nuclear energy?"

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[[Category:Nuclear energy]]
[[Category:Nuclear energy]]

Latest revision as of 13:48, 23 May 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 – H2O and CO2. 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.


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, CO2 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.

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

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.

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.

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

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 CO2 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 CO2 as heat transfer medium, designed to produce electricity.


The Canada Deuterium Uranium reactor is a pressurised water design using heavy water as moderator and heat transfer medium to generate electricity. The basic CANDU design uses natural Uranium.

See also:

Nuclear Power Reactor Technology, 1950-1953 (Part 1)

Why did the US abandon a lead in reactor design? Cheryl Rofer; Physics Today; 7 Aug 2015

Sometime in the late 1960s, a great shakeup occurred in nuclear reactor research. [T]he Los Alamos Scientific Laboratory at that time ... was suddenly dissolved. ... The key player was Milton Shaw, who directed the Atomic Energy Commission’s (AEC) Reactor Development and Testing Division (RDTD) at that time. Shaw refocused the US civil nuclear program toward a single goal of the liquid-metal fast breeder reactor, making a number of strategic mistakes that have had long-term safety consequences for the industry.

Reactor Types

Nuclear Reactor Wikipedia

Nuclear power IET

An introduction to nuclear power technologies
A wide range of nuclear issues, ranging from the use of nuclear power in the UK, decommissioning of nuclear power stations, the nuclear fuel cycle, a glossary of nuclear terms, and the decay rate of Uranium238.

Nuclear reactor types IET

Introduction to the various types of nuclear reactors worldwide and information on prototype designs
Many different reactor systems have been proposed and some of these have been developed to prototype and commercial scale. Six types of reactor (Magnox, AGR, PWR, BWR, CANDU and RBMK) have emerged as the designs used to produce commercial electricity around the world. A further reactor type, the so-called fast reactor, has been developed to full-scale demonstration stage. These various reactor types will now be described, together with current developments and some prototype designs.

What is a nuclear reactor? Overview of common reactor types

What is a fission reactor a fission reactor and how does it fission reactor and how does it work? and how does it work? Sense About Science

Summary of predominantly UK sold-fuel reactor types

Generation II reactor Wikipedia

From comments by Matt Fuller on an Energy Matters post:

Thorium has gained a lot of publicity in the past few years. It shows some promise, but is possibly overhyped. Fission reactors currently come in two flavours, thermal-neutron-spectrum (most of them) and fast-neutron-spectrum (only a few left in the world, all in Russia as far as I know.) Thermal spectrum reactors have fissile U235 (about 5%) mixed with non-fissile U238. As they “burn” the U235, some of the U238 is converted to fissile plutonium Pu239 and also burned. However, less fissile fuel is created then burned and eventually the fuel rods need changing. Fast-spectrum reactors can actually create more Pu239 than the U235 they use up, and so are known as “fast breeders”. A fast breeder can therefore eventually burn ALL its uranium by converting it into plutonium.
Thorium is not in fact fissile. It’s strength is that it can be converted to fissile U233 in a thermal spectrum reactor. You can in fact mix some thorium into conventional fuel rods in pressurised water reactors and it will be converted and burned. This works better in heavy-water type reactors such as Canada’s CANDU, and the Indian reactors are also heavy-water types.
In theory then, you can run a thermal-spectrum reactor with thorium and a little fissile U235 (or U233, or Pu239) to start off, and ALL the thorium will eventually be converted and burned. Nothing’s quite so simple though: whether using uranium in a fast-spectrum breeder, or thorium in a thermal-spectrum breeder, you have to make sure you don’t end up with too much fissile material in your reactor, and you have to get rid of all the fission-product “ashes” or they’ll build up and interfere with the reaction. That means you have to be continuously cycling and reprocessing the fuel, which is messy and hazardous.


Thorium has also been publicised alongside an entirely different design of reactor, the Molten salt reactor. Instead of having your fuel as solid oxide pellets sealed in zirconium tubes, you have your fuel as a molten salt. This has some advantages – gaseous fission products bubble out and can be captured, operating temperature is higher, can run at atmospheric pressure, volatile fission products are chemically bound, meltdown is no longer a failure mode. Molten salt reactors don’t have to use thorium though, and so far only experimental reactors have ever been run. The Netherlands experiment is not a molten salt reactor. Instead they are irradiating a molten salt in a conventional research reactor to see how it responds. The Indian reactor, as already mentioned, is a heavy-water reactor that can use thorium.

Types of Nuclear Reactors Institute for Energy and Environmental Research

Nuclear reactors serve three general purposes. Civilian reactors are used to generate energy for electricity and sometimes also steam for district heating; military reactors create materials that can be used in nuclear weapons; and research reactors are used to develop weapons or energy production technology, for training purposes, for nuclear physics experimentation, and for producing radio-isotopes for medicine and research. The chemical composition of the fuel, the type of coolant, and other details important to reactor operation depend on reactor design. Most designs have some flexibility as to the type of fuel that can be used. Some reactors are dual-purpose in that they are used for civilian power and military materials production. The two tables below give information about civilian and military reactors.

Classification of Reactors according to Neutron Flux Spectrum NuclearPower.net

From the physics point of view, the main differences among reactor types arise from differences in their neutron energy spectra. In fact, the basic classification of nuclear reactors is based upon the average energy of the neutrons which cause the bulk of the fissions in the reactor core. From this point of view nuclear reactors are divided into two categories:
Thermal Reactors. Almost all of the current reactors which have been built to date use thermal neutrons to sustain the chain reaction. These reactors contain neutron moderator that slows neutrons from fission until their kinetic energy is more or less in thermal equilibrium with the atoms (E < 1 eV) in the system.
Fast Neutron Reactors. Fast reactors contains no neutron moderator and use less-moderating primary coolants, because they use fast neutrons (E > 1 keV), to cause fission in their fuel.

What is Called Nuclear Waste is Mostly Unused Fuel for Molten Salt and Fast Reactors Brian Wang; Next Big Future; 14 Jun 2019

About 20 fast neutron reactors (FNR) have already been operating, some since the 1950s, and some supplying electricity commercially. Over 400 reactor-years of operating experience has been accumulated. Fast reactors more deliberately use the uranium-238 as well as the fissile U-235 isotope used in most reactors. If they are designed to produce more plutonium than the uranium and plutonium they consume, they are called fast breeder reactors (FBRs). But many designs are net consumers of fissile material including plutonium.* Fast neutron reactors also can burn long-lived actinides which are recovered from used fuel out of ordinary reactors.
Uranium 235 is naturally fissile. Easily fissioned with low energy neutrons.
Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. Uranium-238 is a fissionable material but not a fissile material.
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (100 TJ/kg), hence a speed of 14,000 km/s, or higher. They are named fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators.

Fast Neutron Reactors World Nuclear Association; ~ 2015? updated Apr 2019

encyclopedic discussion of fast reactors

Generation IV Nuclear Reactors World Nuclear Association; updated May 2019

Generation IV International Forum

The Generation IV International Forum (GIF) is a co-operative international endeavour which was set up to carry out the research and development needed to establish the feasibility and performance capabilities of the next generation nuclear energy systems.

Fuel technology

This New Fuel could make nuclear power safer and cheaper Richard Martin; MIT Technology Review; 31st Mar 2016

Lightbridge has developed a metallic fuel for nuclear reactors that it claims will tackle some of the industry’s biggest challenges, but safety questions remain


MOX Battle: Mixed Oxide Nuclear Fuel Raises Safety Questions John Matson; Scientific American; 25 Mar 2011

One of the troubled Fukushima Daiichi reactors contains a blend of uranium and plutonium fuel that may soon find use in the U.S. Does it pose more risks than standard uranium fuel?

Nuclear Industry

See also Nuclear energy by state

Toshiba Westinghouse

Westinghouse Files for Bankruptcy, in Blow to Nuclear Power DIANE CARDWELL and JONATHAN SOBLE; New York Times; 29 Mar 2017

Westinghouse Electric Company, which helped drive the development of nuclear energy and the electric grid itself, filed for bankruptcy protection on Wednesday, casting a shadow over the global nuclear industry.
The filing comes as the company’s corporate parent, Toshiba of Japan, scrambles to stanch huge losses stemming from Westinghouse’s troubled nuclear construction projects in the American South.

How two cutting edge U.S. nuclear projects bankrupted Westinghouse Tom Hals and Emily Flitter; Reuters; 2 May 2017

Westinghouse miscalculated the time it would take, and the possible pitfalls involved, in rolling out its innovative AP1000 nuclear plants, according to a close examination by Reuters of the projects.
Those problems have led to an estimated $13 billion in cost overruns and left in doubt the future of the two plants, the one in Georgia and another in South Carolina.
Overwhelmed by the costs of construction, Westinghouse filed for bankruptcy on March 29, while its corporate parent, Japan's Toshiba Corp, is close to financial ruin [L3N1HI4SD]. It has said that controls at Westinghouse were "insufficient."
The miscalculations underscore the difficulties facing a global industry that aims to build about 160 reactors and is expected to generate around $740 billion in sales of equipment in services in the coming decade, according to nuclear industry trade groups.
The sector's problems extend well beyond Westinghouse. France's Areva is being restructured, in part due to delays and huge cost overruns at a nuclear plant the company is building in Finland.
Even though Westinghouse's approach of pre-fabricated plants was untested, the company offered aggressive estimates of the cost and time it would take to build its AP1000 plants in order to win future business from U.S. utility companies. It also misjudged regulatory hurdles and used a construction company that lacked experience with the rigor and demands of nuclear work, according to state and federal regulators' reports, bankruptcy filings and interviews with current and former employees.

Lifetime of nuclear power plants

Nuclear Plants Running For 80 Years Trump Renewables And Gas Conca; Forbes

U.S. Senate Wants To Decrease CO2 By Increasing Nuclear Energy Conca; Forbes

Advanced Nuclear Summit