The heat released by radioactive disintegration is a key consideration in high-level waste management strategies. The heat is generated inside a fuel assembly or vitrified waste package, as these materials absorb most of the emitted radiation.
Like radioactivity and radiotoxicity, the amount of heat released decays over time. Heat release is particularly intense when the fuel has recently been unloaded from the reactor. As a result, in France, irradiated fuel is stored in a pool near the reactor for an initial period, followed by several years in another pool in La Hague before being reprocessed.
Vitrified waste obtained after removing uranium and plutonium from spent fuel emits less heat. It cools more rapidly.
In both cases, however, heat is released constantly. When the time is right to dispose of the fuel assembly or vitrified waste package in a repository, these materials will be surrounded by rock with no cooling or ventilation provision. The host rock will be heated.
To be effective, the waste management strategy must aim to prevent the temperature from reaching 100°C, ensuring that any water present does not evaporate. Accordingly, the most highly radioactive materials are held in storage prior to disposal. They must be allowed to cool sufficiently before moving them to a repository.
This storage applies to both spent fuel assemblies (in pools or dry storage) and vitrified waste (in ventilated shafts). Waste that is thoroughly cooled can be disposed of in smaller depositories.
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