HKNIC - Inside a Nuclear Island - Nuclear Reactor

Inside a Nuclear Island - Nuclear Reactor
The reactor contains nuclear fuel which undergoes a fission process - the splitting of atoms -- to produce heat. At Daya Bay, uranium 235 with an enrichment of 4.45% is used as nuclear fuel. It is loaded as sintered fuel pellets of uranium oxide inside fuel rods to make up a fuel "assembly". Each square prismatic assembly is about half a tonne in weight and its 17 X 17 grid contains 264 fuel rods, 24 guide tubes for control rods and a tube for instrumentation. An assembly is about 4.4 metres tall and just over 20 centimetres wide.
Image Credit : DNMC

Each reactor uses 157 fuel assemblies grouped into a core of about 3.7 metres high and 3 metres in diameter, producing just under 3,000 MW of heat.

Although each fuel assembly has a set of 24 guide tubes for the control rods that help control the reaction process, not all of the 157 sets of guide tubes are used. At Daya Bay, 61 sets of control rods are enough to maintain the safe operation of the reactor and the unused guide tubes are plugged.

Image Credit : DNMC

A fuel assembly stays inside the reactor for between three and four-and-a-half years. A change of fuel, known as refuelling, takes place roughly every 18 months and each time about 40% of the fuel assemblies are replaced.

Refuelling can be done in slightly over a week, but the opportunity is usually taken to conduct maintenance of the power station. This period typically takes 30 days, although more time may be taken if additional inspection and maintenance are considered necessary