Follow the heat: from the reactor to the generator

The reactor supplies heat; the generator supplies electricity. Commercial nuclear electricity generation relies on controlled fission. Heat supports a steam cycle, steam drives a turbine, and the turbine drives the generator. Nuclear reaction, heat transfer, mechanical rotation and electrical generation are distinct stages.

Our introduction to nuclear energy explains the energy source. Fossil-fuel thermal stations can also use turbines and generators; the major difference is the process supplying their heat.

Fuel and the controlled chain reaction

Common light-water reactors use fuel assemblies containing ceramic uranium-dioxide pellets enclosed in sealed metal tubes. Obtaining and preparing the fuel precedes power production at the station. Other reactor types can have different fuel arrangements.

During fission, a suitable nucleus splits, releasing energy and additional neutrons. Some neutrons participate in further fissions, sustaining a chain reaction. Control rods absorb neutrons to help control the reaction. In light-water reactors, water also slows neutrons as a moderator, as well as carrying heat. Moderation and neutron absorption are different functions.

Where the steam forms depends on the reactor

A pressurized-water reactor (PWR) keeps its primary coolant liquid under high pressure. A steam generator transfers heat from that water to a separate circuit, where steam forms for the turbine. The primary coolant and turbine-circuit water do not mix in normal operation.

In a boiling-water reactor (BWR), steam forms inside the reactor vessel and flows to the turbine. Consequently, the route taken by condensed water cannot be described identically for all reactor designs. The diagram below represents a PWR example.

The turbine turns the shaft; the generator converts the motion

A steam turbine converts steam energy into mechanical work. Its shaft drives a generator, which produces electricity through electromagnetic induction. Suitable transformers and connections deliver power to the grid. The reactor does not operate like a battery supplying electricity directly.

Exhaust steam gives up heat in the condenser and becomes water again. In a PWR, pumps return this water to the steam generator. A separate cooling system rejects heat that was not converted into electricity. This heat-rejection requirement belongs within site selection and environmental assessment.

Thermal power and net electrical output are different quantities

A reactor's thermal rating is not the station's electrical rating. Only part of the heat becomes electricity. Some gross generation is consumed by pumps and other station services; the remaining output is net generation. These distinctions matter when assessing generation costs and the benefits and limitations of nuclear power.

Shutdown does not remove the need for cooling

Stopping the chain reaction does not instantly eliminate heat production. Radioactive fission products in the fuel continue to release decay heat. Necessary heat-removal functions therefore continue after reactor shutdown.

Safety assessments treat reaction control and cooling as different functions. Lessons from serious accidents underline the importance of protecting heat-removal systems and the support they require. Potential environmental consequences also belong within that assessment.

What measurement and control contribute

Temperature, pressure, flow and water level are monitored. Our guides to temperature sensing, pressure measurement and level control help explain the underlying measurement principles. General automation knowledge does not replace nuclear protection-system design and qualification requirements. Those requirements are addressed through the relevant standards and regulation.

The fuel's responsibilities outlast its time in the reactor

Spent fuel remains radioactive after removal and can continue generating heat. Cooling, suitable storage and subsequent management must be planned. Other radioactive waste from operations is managed according to its characteristics. Waste and spent-fuel management remain ongoing responsibilities of power production.

Fusion research has a different electricity-production challenge

Fusion joins light atomic nuclei. Research must address not only energy release but also sustained plasma conditions, heat extraction and the energy balance of the whole facility. Energy gain in an experiment and net electricity supplied to the grid are different measures of success.

ITER is not designed to generate electricity. It studies science and technologies needed for future fusion systems. Some proposed plants would transfer reaction heat into a power cycle, but today's PWR diagram should not be presented as a fusion-plant design. Explore how fusion works and future nuclear technologies for the wider picture.