school energy&environment

Decommissioning

Beginning from the initial planning and design stages and the identification of the site, a nuclear power plant is a very complex plant to deal with. Each stage of its realization and each successive life-cycle of the power plant must be carefully controlled in order to guarantee maximum safety. After the operating period is over, a nuclear power plant still requires a great amount of attention because it is not sufficient to “shut down” the power plant in order to cancel all possible risks. The fuel, the waste products and the plant itself continue to be dangerous as they are radioactive for a very long period of time. The many procedures to be followed in order to dismantle a nuclear plant are known as decommissioning. The final aim is to restore the initial situation and therefore permit the destination of the area to any type of use.
The shutting down operations of a nuclear power plant are very long, complex and costly and can be summarized as follows:

• Initial plant shut-down.
• Decommissioning of used nuclear fuel present in the power plant.
• Processing of radioactive waste accumulated during plant operation and forwarding of the same to the deposit.
• Decontamination and dismantling of the equipment, plants and buildings.
• Processing (if radioactive) of materials deriving from the dismantling operations and forwarding them to the  deposit or disposal of the same.
• Characterization, requalification and release of the site for other uses.

According to the  IAEA scale, the following general decommissioning strategies can be used:

• DECON (Decontamination) – Immediately, after the plant is stopped,  dismantling procedures are begun in order to complete the decommissioning operations as soon as possible. Generally, this operation is chosen when a new power plant is planned on the same site, or if the site needs to be released for other uses in a short period of time.
• SAFESTOR (Safe Storage) – The fuel and waste products are taken away and the plant is kept safe for a few decades, awaiting the decay of the radioactivity to more acceptable levels in order to carry out the dismantling operations, which are then followed by the DECON strategy.
• ENTOMB (Entombment) – The radioactive parts of the plant are confined (i.e. encased  in a concrete material ) awaiting decay to environmental background levels.

At present, worldwide, there is no definite choice with regard either a Decon or Safestor strategy, while the Entomb strategy is practically never carried out for the high power nuclear power plants. In fact, opting for an ENTOMB strategy is the equivalent of transforming all nuclear sites into a site for the final dismantling of radioactive waste.
In some cases, for example Chernobyl, entombment became the only possible choice. Up to date it has only been used for some demonstrative low power reactors in the United States, but has never been considered for high power nuclear reactors.
In Europe and in the USA the two strategies, SAFESTOR and DECON, exist side by side. In Germany preference has been given  to the DECON strategy, while in France and in the United Kingdom,  usually the SAFESTOR strategy has been chosen  In the same country there can be changes in strategy during the course of time. In Italy for example,  the initial strategy was SAFESTOR, but later the choice fell on DECON, which however did not lead to advantageous consequences with regard to the strategy for the management of radioactive waste.