Nuclear Power Reactors and Nuclear Energy
A nuclear power reactor uses a controlled fission chain reaction to produce heat, which makes steam that drives a turbine and generator; nuclear plants produce about 9–10 % of the world’s electricity with very low emissions of carbon dioxide during operation. In brief: a 1000 MW(e) reactor fissions about 1 tonne of uranium-235 per year, whereas a coal plant of the same size burns about 2.5 million tonnes of coal.
Nuclear energy is a major low-carbon source of electricity, but it is also a source of debate because of accidents, radioactive waste and the link with nuclear weapons. This article explains how a reactor works, describes the main types in a table, shows the nuclear fuel cycle, gives calculations of fuel consumption and efficiency, and summarizes the benefits and problems.
What are the parts of a reactor?
- Fuel: pellets of uranium dioxide (UO2), enriched to 3–5 % uranium-235, stacked in tubes of zirconium alloy (fuel rods), which are bundled in fuel assemblies.
- Moderator: slows the neutrons from fission: ordinary (light) water, heavy water (D2O) or graphite.
- Control rods: neutron absorbers (boron, cadmium, hafnium) that regulate the chain reaction or shut it down.
- Coolant: removes the heat from the core: water, heavy water, gas (CO2 or helium) or liquid metal (sodium).
- Steam generator and turbine: the heat produces steam, which drives a turbine connected to a generator.
- Containment and shielding: a reactor pressure vessel, a thick concrete and steel building, and cooling and emergency systems.
What types of reactors exist?
| Type | Moderator | Coolant | Fuel | Comment |
|---|---|---|---|---|
| Pressurized water reactor (PWR) | Light water | Light water at about 155 atm | 3–5 % enriched U | Most common type |
| Boiling water reactor (BWR) | Light water | Light water, boils in core | 3–5 % enriched U | Steam made directly in the core |
| CANDU | Heavy water | Heavy water | Natural uranium | Refueled while running |
| Gas-cooled reactor (AGR) | Graphite | Carbon dioxide | Enriched U | Used in the UK |
| RBMK | Graphite | Light water | Low enriched U | Type at Chernobyl |
| Fast reactor | None | Liquid sodium | Plutonium and U-238 | Can breed fuel |
About 440 power reactors are in operation in over 30 countries, supplying approximately 9–10 % of the world’s electricity. In France, about 65–70 % of the electricity is from nuclear power, and in the United States about 18–19 %. The thermal efficiency of a plant is about 33 %, similar to that of a coal plant, because of the thermodynamic limit of the steam cycle.
What is the nuclear fuel cycle?
- Mining and milling of uranium ore, producing “yellowcake” (U3O8).
- Conversion and enrichment to UF6 gas and enriched in 235U by centrifuges (gas diffusion in the past).
- Fuel fabrication: UO2 pellets in fuel rods.
- Use in the reactor for 3–6 years, during which the 235U content falls and fission products and plutonium-239 build up.
- Spent fuel storage in cooling pools and then in dry casks. Either direct disposal or reprocessing to recover the uranium and plutonium for new fuel (MOX), as in France.
- Waste disposal in a deep geological repository.
Worked examples
Example 1: uranium-235 fissioned per year
A reactor with a thermal power of 3000 MW (electrical power of 1000 MW at 33 % efficiency) operates for a year with a capacity factor of 90 %. The energy is 3.0 × 109 W × 3.156 × 107 s × 0.90 = 8.5 × 1016 J. At 8.2 × 1013 J per kg of 235U fissioned, the mass is 8.5 × 1016 / 8.2 × 1013 = 1040 kg, about 1 tonne. The mass converted into energy is only Δm = E/c2 = 8.5 × 1016 / 9.0 × 1016 = 0.94 kg.
Example 2: comparison with coal
Coal has a heating value of about 29 MJ/kg. To produce 8.5 × 1016 J of heat, a coal plant would need 8.5 × 1016 / 2.9 × 107 = 2.9 × 109 kg, or 2.9 million tonnes of coal per year, releasing about 7–8 million tonnes of CO2 (roughly 2.6 tonnes of CO2 per tonne of coal burned). In practice, a 1000 MW(e) coal plant uses about 2.5–3 million tonnes of coal a year, in agreement with this estimate.
Example 3: electricity produced
The electrical energy of the 1000 MW(e) reactor at 90 % capacity factor is 1000 MW × 8760 h × 0.90 = 7.9 × 106 MWh, or 7.9 TWh a year, enough for about 700,000 to 1 million homes.
Example 4: natural uranium needed
A plant like that uses about 25 tonnes of enriched uranium fuel (4 % 235U) per year. With about 8 kg of natural uranium per kg of 4 % fuel, it needs about 200 tonnes of natural uranium per year, which corresponds to well over 100,000 tonnes of ore at a typical grade of 0.1–0.2 %.
What about safety and accidents?
Modern reactors have several layers of defense: negative temperature coefficients (the reaction slows when the temperature rises), automatic shutdown by control rods and emergency cooling. There have been three major accidents:
- Three Mile Island (USA, 1979): a partial core meltdown, with a small release of radioactivity and no direct deaths.
- Chernobyl (Ukraine, 1986): an explosion and fire of an RBMK reactor with a design flaw, and a large release of radioactivity. About 30 people died in the first weeks, and the long-term health effects have been estimated, with large uncertainty, at several thousand additional cancer deaths.
- Fukushima Daiichi (Japan, 2011): the tsunami caused a loss of cooling and meltdowns in three reactors. The radiation caused no direct deaths, but over 100,000 people were evacuated.
Compared with other sources of electricity, nuclear power has had a low number of deaths per unit of energy produced, similar to that of wind and solar, and much lower than that of coal, according to most comparisons.
What happens to nuclear waste?
Low and intermediate-level waste (tools, clothing, filters) makes up the large majority of the volume and is disposed of in near-surface facilities. High-level waste, mainly spent fuel and the products of reprocessing, is small in volume (a person’s lifetime electricity from nuclear power gives a waste volume similar to that of a soft drink can), but contains the long-lived radioactive isotopes. The fission products (such as 137Cs and 90Sr, with half-lives of about 30 years) decay to a low level within several hundred years, and the actinides such as plutonium-239 remain for tens of thousands of years. The international consensus is final disposal in deep geological repositories. Finland’s Onkalo repository, built in granite bedrock, is the first of this kind, and began final testing in the 2020s.
What is the outlook?
Small modular reactors, fast breeder reactors that use uranium far more efficiently, thorium fuel cycles and, in the long term, nuclear fusion are being developed. Nuclear power is considered by many governments as an important part of the strategies for reducing carbon dioxide emissions, but the cost of building new plants, the public acceptance and the disposal of waste remain major issues.
How did nuclear power develop?
The first electricity from nuclear fission was produced in 1951 in the EBR-I experimental reactor in Idaho. The Obninsk plant in the Soviet Union (5 MW, 1954) was the first to supply power to a grid, followed by Calder Hall in the UK (1956) and Shippingport in the USA (1957), the first full-scale civilian plant. Nuclear power expanded rapidly in the 1970s and 1980s, slowed after the Three Mile Island and Chernobyl accidents, and has seen renewed interest in the 21st century.
Which mistakes are common?
- Saying that nuclear plants release no waste: they produce radioactive waste, though in small volumes.
- Mixing up electrical and thermal power: a “1000 MW(e)” plant makes about 3000 MW of heat.
- Assuming that all reactors use enriched uranium: CANDU reactors use natural uranium.
- Thinking that the steam from cooling towers is radioactive: it comes from the secondary cooling circuit.
- Treating nuclear energy as a renewable resource: uranium is finite, although available in large amounts.