How a nuclear power plant works

It all starts in the reactor core. There, on the order of 10²⁰ uranium-235 nuclei split every second in a typical 3,000 MWt reactor: each fission releases heat and two or three new neutrons, which in turn split other nuclei in a chain reaction that sustains itself at constant power. The fuel is uranium dioxide, pressed into ceramic pellets that are stacked inside metal cladding to form the core's fuel assemblies; depending on the reactor, that uranium is enriched to 3 to 5% or used as it comes out of the mine. Control rods that absorb neutrons raise and lower the power, and, fully inserted, stop the reaction within seconds.

That heat passes to the water (or, in other designs, the gas or the liquid sodium) that circulates in direct contact with the fuel: the primary circuit. In a pressurized water reactor, the most common type in the world, that water sits at about 155 bar and never boils; it hands its heat, without ever mixing with it, to a second water circuit inside a steam generator, and it is that second circuit that boils. A boiling water reactor skips that intermediate step: the water boils directly inside the vessel and the steam heads straight to the turbine.

The steam, whether from the steam generator or from the vessel itself, pushes the blades of a turbine that spins an electrical generator: the same principle as any thermal power plant, coal or gas. That is where the core's thermal energy turns into mechanical energy and then into electricity; a transformer raises the voltage at the generator's output and that electricity enters the high-voltage grid. The thermal power the core produces and the electrical power that reaches the grid are different numbers, measured differently, MWt and MWe: conversion efficiency runs around a third, so a 3,000 MWt reactor delivers on the order of 1,000 MWe.

The other two-thirds of the heat cannot be put to use, a consequence of the second law of thermodynamics rather than a design flaw, and it has to go back to the environment. The steam leaving the turbine, no longer under pressure, condenses against a third cooling circuit that is itself cooled by a river, a lake, the sea or, when no source that large is nearby, a cooling tower; the white plume rising from those towers is, again, water vapor condensing in the air, unrelated to radioactivity. And even after the reactor shuts down, the fuel keeps generating decay heat for days, which is why every plant has emergency cooling systems built to work even without power from the grid: the first of several layers of defense in depth that protect the fuel, the vessel and the containment.

Once at power, the economical choice is to keep it there as long as possible, which is why reactors run as baseload, at full power 80 to 90% of the year.

Fuel spends three to six years in the core before it is replaced, in partial batches, during a refueling outage. What comes out is not ordinary waste: it is the starting point of the back end of the nuclear fuel cycle, which continues with cooling, storage and, depending on the country, reprocessing or final disposal of what were, at some point, those same uranium pellets.