America’s Nimitz-class aircraft carriers are powered by two small on-board pressurised water reactors (IPWR), which drive the ship’s four steam turbines that not only power four five-bladed propellers, but also generate electricity to power everything on the ship.
The nuclear reactions taking place in the PWRs can generate enough power to reach a top speed of 30 knots. Inside the reactor cores a huge amount of energy is given off due to fission – the splitting of large atoms (uranium) into smaller ones and releasing kinetic energy in the process.
There are two separate loops inside each aircraft carrier’s nuclear power station. The primary loop contains superheated water, or coolant (in liquid form). This coolant is pumped through the uranium- fuelled reactor – where it reaches up to 900?F – under high pressure so it doesn’t boil inside the core.
This superheated water is then passed through a steam generator. The heated water in the primary loop doesn’t get converted to steam itself (because it’s under high pressure), rather the intense heat of it is used to convert the cooler water in the separate secondary loop into turbine-driving steam to power the generator. And because the loops are separate and the water never mixes, the radioactivity is safely contained in the reactor of the primary loop.
The steam from the turbine is then cooled and condensed, converting it back into liquid water ready to run the cycle again.
The reactors rarely require refuelling – maybe once every 25 years – offering carriers practically unlimited range. Refuelling involves removing the used core and replacing it with a shiny new one replenished with enriched uranium nuclear fuel.