Nuclear Power Plant: How It Works

How a nuclear power plant works

A nuclear power plant turns the thermal energy released by fission inside a reactor’s core into electricity. In pressurized water reactors, the type the U.S. Nuclear Regulatory Commission describes in detail, a first loop of water held under pressure absorbs that heat and passes it, inside a steam generator, to a second loop; electric pumps keep both loops circulating. The resulting steam pushes the turbine’s blades, and the turbine drives a generator that converts that motion into electric current. A reactor of this kind typically holds between 150 and 200 fuel assemblies, according to the same source.

Available sources describe the pressurized water design in detail; other reactor configurations exist, but they fall outside this comparison.

Practical example: picture two separate water loops, one nested inside the other. The first, attached to the core, runs in a closed circuit and passes its heat to the second loop through the steam generator, like a kettle heating the water in a pot set on top of it without the two ever mixing. It’s the second loop, the one carrying steam, that spins the turbine.

The fission process heating the core is the same physical principle behind the atomic bomb, even though the goal is the opposite: inside a plant the reaction is slowed and controlled to produce heat continuously, while inside a weapon it’s released all at once.

Waste, a problem that outlasts the plant

Fuel that has exhausted its useful charge stays radioactive for a very long time. The Nuclear Regulatory Commission explains that no permanent disposal site exists yet, so plants have to store spent fuel safely on their own. Radioactive decay, the only process that eventually renders this waste harmless, takes an enormous span of time for high-level materials: hundreds of thousands of years, the Nuclear Regulatory Commission notes.

That extremely long timescale marks a sharp difference from other energy sources, including solar panel systems, where end-of-life mostly means recycling the panels, a material that stops posing the same risks fairly quickly.

Chernobyl and Fukushima: what the data actually show

Public perception of nuclear risk has formed largely around two accidents, both rated at the top of the INES scale, level 7: Chernobyl, in Ukraine, in 1986, and Fukushima, in Japan, in 2011. Data collected by the World Health Organization tell two very different health stories.

At Chernobyl, the WHO counted 28 direct deaths among emergency workers, firefighters and plant staff, in the three months after the accident: a confirmed count from the source. The 6,000 thyroid cancer cases diagnosed by 2005, affecting people who were still minors and living in the fallout zone back in 1986, are a different matter: the WHO considers a large share of them attributable to radioactive iodine intake, but they remain diagnoses accumulated over time, a measure of how many people fell ill, not a death count. For the general exposed population, excluding the thyroid, the WHO finds no clearly demonstrated increase in solid cancers or leukemia. Among roughly 530,000 clean-up workers, the so-called liquidators, the average estimated dose was 120 mSv; only among those who received doses above 200 mSv does some evidence of a rise in leukemia incidence emerge.

At Fukushima the picture changes. Estimated lifetime effective doses for adult residents of the prefecture averaged around 10 mSv or less, roughly double that for one-year-old infants; plant workers received an average of about 12 mSv over the first 19 months after the accident, with 35% of the workforce above 10 mSv and 0.7% above 100 mSv. The WHO records no acute injuries and no acute deaths linked to radiation, a finding that covers both plant staff and the resident population. For a group of about 160 workers who received doses above 100 mSv, the WHO still estimates a higher cancer risk over time; for the general population, aside from the thyroid, estimated cancer risks remain small next to the baseline risk. On children’s thyroids the WHO flags a specific uncertainty, because doses are hard to verify through direct measurement; it also notes that very sensitive screening itself tends to pick up more cysts, nodules and cancers than would have surfaced without such intensive monitoring.

Nuclear power compared with other energy sources

Making sense of these numbers takes a benchmark. Our World in Data, in an analysis by researcher Hannah Ritchie, compares energy sources by deaths per TWh of electricity produced, a calculation that adds up direct accidents and the deaths from air pollution inhaled across the entire supply chain. In this comparison coal comes out, in the source’s own words, as “by far” the most dangerous; wind and solar turn out to be “just as safe” as nuclear.

The hydropower figure helps put the fear of nuclear power in perspective: the Banqiao Dam collapse, in 1975, killed more people on its own than any civilian nuclear accident on record, and yet hydropower’s historical death rate is still considered low. For nuclear power, some comparative studies cite higher mortality figures: to calculate its own rate, Our World in Data uses a total of 433 deaths for Chernobyl — an assumption within the estimated range for that accident — and 2,314 for Fukushima. That second figure sums two official counts from the Japanese government: 2,313 deaths certified as disaster-related, for the stress and disruption of the forced evacuation (a figure revised in September 2020), and one death from lung cancer recognized in 2018 as caused by radiation exposure — against the zero acute radiation deaths the WHO recorded.

The comparison across energy sources also connects to other questions around the energy transition, such as the availability of critical minerals needed by both nuclear power and renewables, and to the climate effects described in the greenhouse effect article, topics that fall outside the scope of the health data discussed here, which are limited to direct and estimated mortality by energy source.