Fuelling a Nuclear Reactor
The Refuelling Process
Nuclear reactors do not burn fuel constantly like coal plants; their fuel lasts for a long time but eventually loses its efficiency and must be replaced.
- Shutdown Refuelling: Most common reactors (like Light Water Reactors) must be completely shut down to open the reactor vessel. This refuelling occurs at intervals of 12, 18, or 24 months. During this time, only a quarter to a third of the old fuel assemblies are removed and replaced with fresh ones.
- On-Load Refuelling: Certain advanced reactor designs—such as CANDU (Canada), RBMK (Russia), and AGR (UK)—do not use a single massive pressure vessel. Instead, they use individual pressure tubes. This allows operators to disconnect and refuel individual tubes while the reactor is still running and generating electricity.
Types of Nuclear Fuel (Uranium)
The core fuel for almost all commercial reactors is Uranium. However, its composition dictates what type of reactor can be used:
- Natural Uranium: When mined from the Earth, Uranium consists of over 99.2% Uranium-238 (U-238) and only about 0.7% Uranium-235 (U-235).
- Only the U-235 isotope is easily fissile (capable of splitting).
- Reactors that use highly efficient moderators like Heavy Water or Graphite (such as India’s PHWRs) can successfully run on this unenriched, natural Uranium.
- Enriched Uranium:
- Because regular “Light Water” absorbs some neutrons, a Light Water Reactor (LWR) requires a higher concentration of the fissile U-235 to maintain a chain reaction.
- The fuel undergoes a complex industrial process called enrichment to increase the U-235 proportion to 3.5% – 5.0%.
- (Note: Some newer small modular reactors require High-Assay Low-Enriched Uranium (HALEU), which is enriched to nearly 20%).
Fuel Assemblies and Materials
Nuclear fuel is not put into the reactor as a raw metal; it is carefully engineered.
- Ceramic Pellets: The Uranium is converted into a ceramic material, Uranium Oxide (UO2), which has an extremely high melting point (2800°C). It is shaped into small pellets (about 1 cm in diameter).
- Zircaloy Tubes (Fuel Cladding): These pellets are stacked inside long metal tubes to form fuel rods. These tubes are made of a special Zirconium alloy (Zircaloy).
- Why Zirconium? It is exceptionally hard, highly resistant to corrosion in superheated water, and most importantly, it is “transparent” to neutrons (it does not absorb the neutrons needed for the chain reaction).
- Nuclear Grade: Raw zirconium naturally contains Hafnium (a strong neutron absorber). For nuclear use, it must be refined into extremely pure “nuclear grade” zirconium to remove all traces of Hafnium.
- Fuel Assemblies: Hundreds of these rods are bundled together in an open metal lattice to form a fuel assembly, which is lowered directly into the reactor core.
