How a PWR reactor works

Between 150 and 200 fuel assemblies of uranium dioxide, enriched to 3 to 5% U-235, sustain the chain reaction. Each fission releases about 200 MeV: the heat passes from the ceramic pellet to the cladding and from there to the water flowing between the rods.

The control rods, of silver-indium-cadmium or boron carbide, enter from above to regulate power or stop the reaction in seconds. The water also moderates the neutrons; if it gets too hot it moderates less, a natural brake on power.

Water enters the vessel at about 290 °C, flows down the annular downcomer, rises through the core and leaves at 315 °C. At 155 bar it never boils: that is why the reactor is called pressurized water.

The pressurizer, a tank with electric heaters and sprays, sets that pressure with a steam bubble. The reactor coolant pumps move about 20,000 m³/h per loop. It is a closed, radioactive circuit that never leaves the containment.

It is the boundary between the two circuits. Hot primary water runs through thousands of Inconel U-tubes and gives up its heat to the secondary water, which bathes them from outside at lower pressure, about 70 bar, and does boil.

The steam is dried in separators and leaves through the top at about 285 °C. A 1,000 MWe plant has two to four steam generators, one per loop; their tubes are the barrier between the radioactive water and the clean steam.

The steam expands first in the high-pressure turbine and then, after being dried and reheated, in the low-pressure ones. The set spins at 1,500 or 1,800 rpm, half the speed of a fossil plant, because of the large flow of wet steam.

The shaft drives a synchronous generator producing 1,000 MW or more at about 20 kV; a transformer steps that up to 400 kV for the grid. Only a third of the thermal energy becomes electricity: the rest is heat that has to be rejected.

Below the turbine, the exhausted steam condenses on thousands of tubes carrying cold water from the tertiary circuit. The vacuum created, about 0.05 bar, increases the usable pressure drop. The condensate returns to the steam generator, pumped and preheated to about 230 °C.

The cooling water never touches the steam: it is cooled by evaporation in a hyperbolic tower, whose white plume is clean water vapor, or returned to a river or the sea a few degrees warmer.

Between the fuel and the environment there are successive barriers: the ceramic pellet retains the fission products; the Zircaloy cladding encloses them; the roughly 20 cm steel vessel and the primary circuit contain the radioactive water; and the containment building, more than a meter of prestressed concrete with a steel liner, withstands internal pressure and external impacts.

On top of that come redundant emergency injection systems and, in the newest designs such as the AP1000, passive cooling by gravity and natural convection that depends on neither pumps nor electricity.