Design for safe enclosure and deferred dismantling

It is important to recognize that parts of the nuclear power plant will remain in situ for some years following the shutdown of the plant, regardless of the decommissioning strategy [30]. During this time, which may include a safe enclosure period, the plant components, systems and buildings or other structures will need to be retained in a safe condition, and their integrity and functional capability will need to be maintained at an appropriate level. The environmental conditions within a shutdown plant will be different from those associated with operational plants, and this will need to be taken into account when considering the post-shutdown requirements.

In addition, the decommissioning strategies of some Member States may include a period of safe enclosure prior to the completion of dismantling of a nuclear power plant [30]. The periods considered vary from a few years to several decades. If a safe enclosure period is likely to be included in the decom-missioning strategy, it is then appropriate to consider this issue at the design stage, so that any special requirements can be addressed.

The primary requirement during a safe enclosure period is to preserve those buildings to be retained, and the plant they contain, in a safe and secure condition, and to maintain their integrity throughout the whole period. If a safe enclosure period is to be considered, the full design life of the residual plant and buildings, including the safe enclosure period, may be significantly longer than the operational period. Furthermore, the environmental conditions (e.g.

temperature and humidity) during the safe enclosure period may be signifi-cantly different from those experienced during the operational period. This means that the degradation mechanisms that require consideration are different from those during the operational period; for example, corrosion may become a significant issue.

The residual radioactivity remaining on the site during a safe enclosure period will be predominantly contained within various plant components, for example, the reactor vessel and heat exchangers. Corrosion of these components will be the primary mechanism behind any potential reduction of their containment integrity. Corrosion could result from increased humidity created when the reactor is shutdown and cold. Therefore, a sufficient corrosion allowance should be included in their design to cover both the operational and the safe enclosure periods. Alternative approaches could be to maintain low humidity conditions and/or use protective coatings. Attention must also be paid to the long term integrity of the supports for items such as vessels, pipework and ducts to ensure that they do not fail during the safe enclosure period.

The capability to fully drain, flush and dry out the vessels and associated pipework also needs to be considered at the design stage to ensure that any liquids, chemicals or contaminants are removed before the start of the safe enclosure period. Similarly, insulating materials that may be hygroscopic and may result in enhanced corrosion during the safe enclosure period are to be avoided if possible, or designed to be readily removable before the start of the safe enclosure period. Means for performing surveillance of the critical components and features, where required, during the safe enclosure period need to be considered, for example, design of corrosion samples relevant specifically to this period.

In addition, it is preferable to avoid the need for plant systems to remain in service during the safe enclosure period. However, it may be necessary to operate some systems during this period and/or during the deferred

dismantling. This requirement could be addressed by utilization of the existing systems or may require installation of a new service plant. It is also important that consideration be given at the design stage to possible long term require-ments for any plant systems and for incorporation of helpful features, where appropriate.

Fire detection and suppression, radiological and environmental monitoring, compressed air, drainage and waste treatment facilities, lighting and power supply, ventilation and service water are some of plant systems whose availability the regulatory body may require during different decommis-sioning stages. Design of these systems needs to be reviewed accordingly, and any limitation on their operational lifetime is to be clearly specified.

One particular design criterion for plants subject to extended periods of safe enclosure is related to the electrical distribution system that will be needed for decommissioning. Some thought needs to be given to providing for a separate system for supply of the essential services needed during decommis-sioning — for example, cranes, sump pumps, ventilation, lighting and any other requirements. While the main electrical distribution systems of the plant are being removed, which could be as early as the commencement of decommis-sioning, the system identified for decommissioning will need to remain in service without any interference from the other systems being removed.

Several shutdown nuclear plants have had to rebuild electrical distribution systems to accommodate decommissioning requirements.

It will also be necessary to maintain the building structures in a sound condition throughout the safe enclosure period. These structures are likely to incorporate various construction materials with a variety of potential long term degradation mechanisms (e.g. carbonation and chloride penetration of concrete as well as corrosion of steelwork). The structural components of buildings may be required to perform a variety of functions including structural support, containment, weather protection and protection against water infil-tration. These functions will be required to continue throughout the safe enclosure period.

Consideration needs to be given at the design stage to the potential long term integrity of the building structures that will be required during the safe enclosure period. Structural weakening of the buildings needs to be avoided, radioactivity containment must be maintained and the buildup of water that could potentially result in the spread of contamination needs to be minimized.

Water infiltration could result from rain or surface water and from groundwater in-leakage through floors and basements, all of which are to be avoided.

Features for continued water collection and drainage must be included. It may be possible to prevent groundwater in-leakage into buildings by ensuring that any susceptible features are above the local groundwater table.

Periodic monitoring of all safety related components of the plant must be incorporated into the decommissioning plan [42].

4.6. DEVELOPMENT AND IMPROVEMENT