Online refuelling has been provided for three main reasons:
- To allow low-burnup fuel to be extracted for the production of plutonium suitable for nuclear weapons.
- To allow frequent rearrangement of fuel within the core for the purposes of balancing the thermal load and allowing higher fuel burnup, reducing both the fuel requirements and the amount of high-level nuclear waste for disposal.
- To improve availability and economy by reducing the downtime required for refuelling and other maintenance.
Power reactors provided with online refuelling facilities include:
- The AGR. However, problems with the mechanism led to the discontinuation of full load refuelling of this class.
- The CANDU.
- The RBMK.
- Most pebble bed reactors.
- All molten salt reactors using liquid fuels.
Reactors provided with online refuelling have to date been one of two types:
- Reactors cooled by gas such as the Magnox.
- Reactors cooled by water in pressurised channels, rather than in a pressure vessel. Channel-type reactors include the CANDU and RBMK.
Refuelling a water-cooled, pressure-vessel type reactor has, to date, always involved a major shutdown in which the coolant is depressurised to allow for dissassembly of the pressure vessel. These types of reactors include the popular PWR and BWR types and their generation III descendents.
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