Systems in safe enclosure

A facility in safe enclosure will typically still have systems to support surveillance and maintenance activities, including a ventilation system, electrical supply, fire detection equipment, etc., as well as a security system. Individual systems may be those that were used during the operational life of the facility, but it is more likely that these systems would be reduced to minimize the maintenance burden, or a new, smaller system would be substituted to support the safe enclosure phase. A safe enclosure strategy may be defined as passive or active;

however, individual systems may also be active or passive regardless of the intended designation of the facility as a whole. Specific considerations relating to managing the systems at a safe enclosure facility are provided in Section 6.

2.3.1. Passive systems 2.3.1.1. Ventilation

Where the safety assessment does not require ventilation for the control of radioactive material or humidity, the ventilation system may be switched off, taken out of service or even removed prior to the start of the safe

enclosure period. A passive ventilation system may utilize a differential pressure regulator or a breathing vent where air can enter the safe enclosure via valves if the outside pressure is higher than the inside pressure; conversely when the inside pressure is higher than the outside, air will leave the safe enclosure. An example of a reactor with a shutdown ventilation system is shown in Fig. 1.

A filtration system may be required to prevent radioactive materials from leaving the buildings. A wind catcher system can be used to force air through the facility to regulate the humidity in the reactor safe enclosure;

this is possible in low residual contamination facilities. In all cases, care needs to be taken when entering a passive safe enclosure, as oxygen levels might be low in some areas and airborne contamination can be present.

In some cases, the ventilation system is switched on a few hours before people have to enter the safe enclosure for periodical inspections.

2.3.1.2. Electrical supply

If an electrical supply is only necessary for limited surveillance and maintenance activities, the high voltage power lines could be removed and a suitable power supply provided. In the absence of an installed power supply, an alternative power supply will be utilized to meet safe enclosure requirements. System conditions should be assessed to ensure that they will function as expected throughout the duration of safe enclosure.

2.3.1.3. Conventional water system

Where a water supply is not necessary for surveillance and maintenance, a portable water supply may be sufficient. If water supplies are only necessary for the welfare of personnel, water may be brought to the site and wastes removed from the site by road during periods of planned works on a passive safe enclosure facility.

Storm water systems may need to be reconfigured to meet safe enclosure requirements.

FIG. 1. Gentilly-1, Canada, in a cold and quiet state within the safe enclosure; all ventilation has been shut down and a dehumidifier installed to maintain the safe enclosure and systems properly (photograph courtesy of Atomic Energy of Canada Limited (AECL)).

2.3.1.4. Aqueous wastes

If very low levels of loose contamination exist that may result in low volumes of low activity liquors being produced, as may be achievable with passive safe enclosure, it may be appropriate to transfer this liquor to a holding tank for disposal from site by road rather than discharging to the environment. More information on this item can be found in Section 6.2.

2.3.1.5. Security and site access

A safe enclosure facility will always need a security and site access system. When the safe enclosure is located on a multifacility site, this may be provided by other site staff. For a single site safe enclosure with a passive intent, the security arrangements may be provided remotely. The presence of personnel for a significant proportion of the time enables surveillance arrangements to be simpler. Where the safe enclosure is not associated with another facility, the cost of a continuous or semi-continuous site presence can be a significant contribution to the overall safe enclosure budget, particularly where local regulations preclude lone working.

2.3.1.6. Fire detection and fighting

Passive fire detection and fighting systems may be appropriate where significant effort has reduced the fire loading material in the building enough to justify the removal of active fire detection and firefighting systems. Such justification would be subject to appropriate safety assessment. Combustibles around the structure (e.g. vegetation and storage sheds or similar structural attachments) also need to be removed, and vegetation needs to be maintained on a regular basis. Deactivated electrical transformers that pose a fire hazard to the main structure need to be removed or drained of their combustible oils. Oil lines and oil carters in pumps and valves have to be drained.

The turbine oil carter needs to be drained as this may be the largest volume of oil on-site. Storage of flammable or combustible liquids needs to be eliminated. Storage containers, including above ground and underground tanks, need to be emptied and secured either by purging or removing the container, or by filling with sand or concrete.

More information on this item can be found in Section 4.

2.3.2. Active systems 2.3.2.1. Ventilation

An active ventilation system may be permanently or periodically operated where required by safety assessment. The cost of maintenance of these systems over the period of safe enclosure, as well as the wastes requiring disposal, should be recognized when this strategy is selected. Ventilation may, however, be required where loose contamination is present or for other reasons, for example the control of humidity within a building.

2.3.2.2. Electrical supply

If an installed power supply is necessary, the system only needs to be sufficient to manage the safe enclosure period and not final dismantling, unless the safe enclosure period is relatively short (10–20 years) and a system assessment has been completed to prove the integrity of the system. This may mean reducing the size of the existing system or installing an overlay. It is unlikely that high voltage will be present on single facility safe enclosure sites.

2.3.2.3. Conventional water system

Maintaining a conventional water system, whether for plant operation or personnel welfare, introduces a significant maintenance burden, e.g. to ensure continued compliance with Legionella regulations. As with the electrical system, the system should be sufficient to manage only the safe enclosure period.

2.3.2.4. Aqueous wastes

Where aqueous wastes are generated during the safe enclosure period and there is no provision to remove them from the site by road, an aqueous waste disposal system will need to be maintained. The extant operational life of the plant may be reconfigured during the preparations for safe enclosure to ensure the continued operability of the plant for the deferral period. More information on this item is found in Section 6.2.

2.3.2.5. Security and site access

An active security and access system is more likely to be associated with a remotely managed safe enclosure.

While the cost of maintaining the hardware is more expensive than a passive system, the labour costs are typically substantially lower, thereby making this the preferred configuration where a positive safety assessment can be made.

2.3.2.6. Fire detection and fighting

Where there is significant fire loading within a facility, or a safety assessment requires it, a fire detection system needs to be maintained. A wireless detection system is demonstrably cheaper than a wired system and involves less operational activity, but detection coverage and reliability needs to be assured. More information on this item can be found in Section 4.