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Nuclear Safeguards Education Portal

Fuel Irradiation--Part 1

Reactors will then load these assemblies into the core when needed. Depending upon the reactor type, the reloading scheme for fuel assemblies can be quite different. However, there are two primary types: onload refueling and offload refueling. Onload refueling involves reloading the reactor core while it continues operating (the most notable example of this type of reactor is the CANDU reactor). Offload refueling is much more common and involves shutting down the reactor at which time typically about 1/3rd of the reactor core will be reloaded.

There are a variety of different types of nuclear reactors but they can generally be broken down into the following categories and sub-categories:

  1. Thermal Reactors
    1. light-water moderated [e.g., pressurized water reactors (PWR) and boiling water reactors (BWR)]
    2. heavy-water moderated [e.g., CANDU reactors]
    3. graphite moderated [of which there are both gas-cooled (e.g., MAGNOX) and light-water cooled (e.g., Hanford and RBMK) types
  2. Fast Reactors
    1. liquid-metal cooled (e.g., EBR-I and -IIPhenixMonju, etc.)
    2. gas-cooled
    3. No moderator

Each of these reactor types are operated differently, use different fuel, and produce different spent fuel. Of these types, the PWR and BWR types are by far the most common.

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