This section considers various decommissioning strategies and the options available.

Generally, the ultimate objective in decommissioning is to return the site to a state whereby it can be used without restriction (de la Ferte, 1996). The IAEA have defined three stages of operation, see Table 6.3. The timescale for carrying out these activities will depend on the decommissioning strategy. Work may proceed from one stage to the other relatively quickly or may take place over many decades, perhaps over as many as a 100 years.

Table 6.3. Stages of decommissioning

Stages Potential periods of activity

1. Removal of nuclear fuel removes 99.9% of the radioactivity

2. Dismantling of structures, e. g. other than

the reactor itself and its surrounding biological shield

3. Total dismantling, removal of all materials

with radioactivity exceeding natural background

Table 6.4. Decommissioning options

Options Features

1. Safe enclosure Following defuelling as rapidly as possible

enclose the active inventory without immediate dismantling

2. Safe enclosure together with partial dismantling Similar to Option 1 except with

partial dismantling and storage of components on site for total dismantling later

3. Immediate total dismantling Total dismantling with removal off site

of all waste materials

EBmann (1990).

On this basis, a number of options are available to the operator. Table 6.4 summarises these options. Option 1 or the ‘safe enclosure’ option leaves the plant essentially unchanged after the completion of Stage 1. Once the entire operating medium, e. g. the fuel has been removed, all the nuclear plant equipment is sealed. The objective of safe enclosure is to enclose any remaining activity as soon as possible without immediate dismantling and then when this has been achieved to wait for the inventory radioactivity to reduce by natural radioactive decay.

Option 2 or partly dismantling with safe enclosure involves placing certain active components of the plant obtained by dismantling along with other plant components in a safe store. The principle of this approach in terms of environment protection is similar to that of Option 1. Total dismantling will be completed at a later date, once the inventory has reduced sufficiently by natural decay.

Option 3 is based on the premise of total dismantling. Here all active and inactive waste materials are removed from site directly after the end of operational life.

There are various important technical, safety and economic issues that need to be addressed in all decommissioning programmes. These are summarised in Table 6.5.

Table 6.5. Important issues to be considered in decommissioning

Technical aspects — structural integrity issues, inventory management and volume of material, degree of automation, remote handling requirements, decontamination arrangements, health physics and available dose minimisation techniques, material re-usage following decommissioning

Decommissioning policy — regulator requirements, licensee decommissioning strategy and workplan, timing of operations

Safety and environment — control of hazardous releases during decommissioning operations, waste treatment, temporary or permanent storage, repository storage

Radiological issues — adherence to ALARA principle for personnel exposure, advantage in delay in plant dismantling (safe enclosure)

Public relations — management of waste disposal concerns

Economics — relative benefits/disadvantages of ‘safe enclosure’ versus ‘immediate dismantling’

The technical aspects of various activities that need to be considered in the planning of plant decommissioning are discussed in this section. The resolution of these issues will generally be site dependent and depend on the infrastructure for decommissioning that already exists, both at the national and local level.

Assurance of structural integrity fidelity and effective management of radioactive inventory are key pre-requisites towards ensuring the safe and efficient management of decommissioning operations.