How does a nuclear reactor make electricity, and heat?

Steady a chain reaction with control rods and a moderator, then follow the heat from fuel to steam

F5 of Energy Systems, Step by Step: fire a neutron, hold a chain reaction steady with control rods and a moderator, follow the heat from fuel to steam in the two U.S. plant types, and compare designs by size, outlet temperature and uses.
  • Foundations · L1
  • 15 min guided · about 5 min practice
  • After F4

The idea in one table: hold the chain reaction steady (an illustrative toy)

Control rods in Moderator left Each split leads to (k) 100 neutrons after 12 generations
0% 100% 1.25 more splits: growing 1,455
40% 100% 1.00: steady (critical) 100
40% 40% 0.64: dying out under one

From fuel to steam: the two U.S. plant types

Type Vessel pressure (MPa, megapascals) Water temperature Where the steam forms
Pressurized water (PWR) about 15 MPa about 326 °C in a steam generator, in a second loop
Boiling water (BWR) about 7 MPa about 288 °C in the vessel itself

Prairie Town's 100 MW plant (illustrative), at the 32.7% efficiency implied by EIA's 2025 tested heat rate: its core makes about 306 MW of heat. About 206 MW is rejected heat and other losses, mostly through the condenser.

Designs compared by size, outlet temperature and uses

Design Size Coolant Outlet temperature Uses
Large water-cooled 1,000 MWe or higher ordinary water about 300 °C electricity; some district heating and desalination
Small water-cooled 300 MWe or less ordinary water about 300 °C electricity; some district heating and desalination
Microreactor 1 to 20 MWt gas, liquid metal, molten salt or heat pipes depends on the coolant heat used directly, or electricity
Sodium-cooled fast varies liquid sodium, at low pressure about 500 to 550 °C electricity
High-temperature gas-cooled up to 600 MWt each helium 700 to 950 °C electricity; industrial heat at 650 to 950 °C, hydrogen production among it
Salt-cooled high-temperature (concept) varies molten fluoride salt, at low pressure 700 to 950 °C electricity or industrial heat

A design's outlet temperature can overlap a heat use's band, but overlap is not capability: heat flows only from hotter to cooler, through heat exchangers, and the chosen design decides.

With JavaScript on, you work through this step by step: fire a neutron, steady a chain reaction, open a plant, sort its parts, guess its heat and line designs up against heat uses.

Illustrative. The chain-reaction toy (its k values and generations are chosen to show the idea, not taken from a real core) and Prairie Town's 100 MW plant.

Real numbers. Pressures, temperatures, sizes and heat uses: U.S. Department of Energy, Quadrennial Technology Review 2015 and its companion assessments (2015). Efficiency: EIA's 2025 tested heat rate for nuclear units, 10,443 Btu (British thermal units, a unit of heat) per kWh, against 3,412 Btu in a kWh. All read 3 November 2026.

Need a refresher? F4 (how heat turns into electricity) and lesson 5 (why 1 MW is not always 1 MW).

What you just did, as diagrams

Fission
A neutron splits a uranium-235 nucleus into two smaller atoms, a few new neutrons and heat.
A steady chain
Control rods absorb neutrons and a moderator slows them; a reactor is held so each split leads to exactly one more (critical, k = 1).
Fuel to steam
A pressurized-water plant keeps its core water liquid at about 15 MPa and boils a second loop; a boiling-water plant boils in the vessel at about 7 MPa.
A heat source first
At the 32.7% efficiency implied by EIA's 2025 tested heat rate, Prairie Town's 100 MW plant makes about 306 MW of heat; about 206 MW is rejected heat and losses, mostly through the condenser.
Temperature limits the uses
Water-cooled designs reach about 300 °C; high-temperature designs reach 700 to 950 °C, which overlaps the 650 to 950 °C band many industrial processes need. Overlap is not capability: the design and its heat exchangers decide.
Go deeper · AnalystWhat lets a chain reaction be held steady
  • Delayed neutrons. A small share of the neutrons from fission come out seconds late, from decaying fission products. They slow the chain’s response enough for control rods to keep pace (DOE Fundamentals Handbook, 1993).
  • One water, two jobs. In U.S. plants the water around the fuel is both the coolant and the moderator (DOE, 2025). In the toy, less moderator means a smaller k: less water, less slowing.
  • No moderator at all. Sodium-cooled fast designs avoid slowing their neutrons, using coolants made of heavier atoms; water’s hydrogen is what slows neutrons in today’s plants (DOE, 2015).
  • Richer fuel. Most microreactor designs need fuel with more uranium-235 than today’s 3% to 5%; some may need less by using high-temperature moderators (DOE, 2021). One such fuel, HALEU, holds 5% to less than 20% (DOE, 2024).
  • Hotter heat, higher efficiency. EIA’s 2025 tested heat rate for U.S. nuclear units implies about 32.7%. DOE puts a high-temperature design at about 47% at 850 °C: the heat-engine idea of F4.
Go deeper · ModelerThe toy as an equation, and a plant as a model sees it

The toy. Neutrons in generation \(g+1\): \(N_{g+1} = k\,N_g\), with

\[ k = \big(k_\text{min} + (k_\text{max} - k_\text{min})\,m\big)\,(1 - a\,r). \]

Symbol Meaning In the toy
\(k\) multiplication factor: new splits per split, no unit 0.25 to 1.25
\(m\) share of the moderator left, from 0 to 1 the moderator slider
\(r\) share of the control rods pushed in, from 0 to 1 the rods slider
\(k_\text{min}\), \(k_\text{max}\) \(k\) with no moderator and with all of it, rods out 0.5 and 1.25
\(a\) share of the neutrons the rods absorb when fully in 0.5

Steady means exactly \(k = 1\): here only at \(m = 1\), \(r = 0.4\) (rods 30%, moderator 90% gives \(k = 0.999\), a slow decline). Real cores are modelled with neutron transport and kinetics, a field of its own (side topic X1).

A plant in a system model. Grid models such as lesson 6, M2 and lesson 13 take a plant as a few numbers: its size in MW, its heat rate or efficiency, its capacity factor and its refuelling outages. The U.S. fleet’s capacity factor was 91.1% in 2025 (EIA, Table 6.07.B; lesson 5).

Go deeperWhat this lesson leaves out
  • Neutron physics beyond the toy: the many fission products, cross sections and the kinetics of a real core (side topic X1).
  • Radiation, shielding and health effects.
  • The fuel cycle before the fuel arrives: mining, conversion and enrichment of uranium.
  • The final disposal of spent fuel.
  • Accident analysis and safety regulation.
  • Each advanced design's details beyond its size, coolant, outlet temperature and uses.

Check yourself

Practice, about 5 minutes: five myths, then two questions.

Myth or fact? “Spent nuclear fuel is a glowing green liquid.”

  • Myth
  • Fact
Answer

Myth. It goes in and comes out solid: ceramic pellets in sealed metal rods (DOE, 2022). Being highly radioactive, it is kept under water, later in steel and concrete casks.

Myth or fact? “All the spent fuel from U.S. commercial plants since the 1950s would cover a city.”

  • Myth
  • Fact
Answer

Myth. DOE estimated in October 2022 that all 90,000 metric tons would cover one football field, less than 10 yards deep. That is the fuel alone, without a storage site's spacing and buildings.

Myth or fact? “Fuel taken out after five years has used up nearly all of its energy.”

  • Myth
  • Fact
Answer

Myth. More than 90% of its potential energy is still there (DOE, 2022). It can be recycled into new fuel; France does this, and the United States does not today.

Myth or fact? “In a pressurized-water plant, the water in the core boils.”

  • Myth
  • Fact
Answer

Myth. At about 15 MPa it stays liquid at about 326 °C; the steam forms in a second loop, in the steam generator (DOE, 2015). Boiling-water plants do boil in the vessel.

Myth or fact? “Every safety system needs pumps, power or an operator to work.”

  • Myth
  • Fact
Answer

Myth. Passive systems use natural effects, like hot coolant rising and cool coolant sinking, to keep fuel cool (DOE, 2015). U.S. plants today rely mainly on active ones; newer water-cooled designs typically add passive ones.

A core makes 600 MW of heat. At 32.7% efficiency, the figure implied by EIA's 2025 tested heat rate, about how much electricity does the plant send out?

  • 196 MW
  • 600 MW
  • 404 MW
  • 33 MW
Answer

196 MW: 600 × 0.327. The other 404 MW is rejected heat and other losses, mostly low-temperature heat through the condenser.

A solid-oxide electrolyzer runs at 650 to 850 °C, where heat can do part of the work of splitting water. Which design's outlet temperature overlaps that band?

  • Large water-cooled, about 300 °C
  • Sodium-cooled fast, about 500 to 550 °C
  • High-temperature gas-cooled, 700 to 950 °C
  • None of them
Answer

High-temperature gas-cooled: its 700 to 950 °C outlet overlaps the band (DOE). A real plant also needs a hot enough design and heat exchangers. The other two stay below 650 °C.

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
3% to 5% Uranium-235 in U.S. power-plant fuel; natural uranium holds just over 0.7%. EIA, Nuclear explained: the nuclear fuel cycle · read 3 November 2026
15 MPa · 326 °C Pressure and water temperature in a pressurized-water vessel; a boiling-water vessel runs at about 7 MPa and 288 °C. DOE, Quadrennial Technology Review 2015, TA 4.M, Figure 4.M.4 · 2015
32.7% Efficiency implied by EIA's 2025 weighted average tested heat rate for U.S. nuclear units: 3,412 Btu in a kWh ÷ 10,443 Btu per kWh. EIA, Electric Power Annual, Table 8.1 · 2025 data, read 3 November 2026
700 to 950 °C Outlet temperature of high-temperature designs; many industrial processes need heat at 650 to 950 °C. DOE, Quadrennial Technology Review 2015, TA 4.J, p. 1 · 2015
90,000 t DOE's October 2022 estimate of all U.S. commercial spent fuel since the 1950s, in metric tons: one football field, less than 10 yards deep (fuel volume only). DOE, 5 Fast Facts about Spent Nuclear Fuel · 3 October 2022
about 20% Share of U.S. electricity made by its nuclear plants each year since 1990. EIA, Nuclear explained · read 3 November 2026

Next

F6 · How does power reach your plug, and why does it sometimes fail?

In the real world

  1. Two types: every U.S. power plant of this kind is water-cooled; more than 65% are pressurized-water and roughly a third boiling-water plants (DOE, 2025).
  2. Fuel: pellets are stacked in rods about 1 cm wide; more than 200 rods make an assembly, and a couple hundred assemblies make a core (EIA; DOE).
  3. Heat beyond electricity: some water-cooled plants already supply district heat or desalinated water (DOE, 2015).

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.