4.167. The mechanical features of the containment comprise the mechanical components of the outermost barrier and the mechanical parts of the extensions of this barrier (i. e. piping, valves, ducts and penetrations). Together with the containment structure, these features comprise the containment envelope.

4.168. The leaktightness criteria for mechanical features of the containment and its extensions should be consistent with the assumptions used in the radio­logical analyses for design basis accidents.

Provisions for containment isolation of piping and ducting systems

4.169. To ensure containment isolation, piping and ducting systems that penetrate the containment envelope should have appropriate provisions for isolation (i. e. valves and dampers). Requirements for containment isolation are established in Ref. [1], paras 6.55-6.57.

4.170. In the provisions for containment isolation, two barriers should be provided for each penetration. Annex II elaborates on means of isolation for piping and ducting systems.

4.171. Each line penetrating the containment that is not part of a closed loop[10] and that either (a) directly communicates with the reactor coolant during normal operation or in accident conditions or (b) directly communicates with the containment atmosphere during normal operation or in accident conditions should be provided with two isolation valves in series. Each valve either should be normally closed or should have provisions to close automatically. Where the line communicates directly with the reactor coolant or the containment atmosphere, one valve should be provided inside the containment and one valve outside. If two valves either inside or outside the containment structure can provide an equivalent barrier (i. e. can meet all the design requirements) in certain applications, then this may also be an acceptable arrangement. Each valve should be reliably and independently actuated. Isolation valves should be located as close as practicable to the structural boundary of the containment.

4.172. Loops that are closed either inside or outside the containment should have at least one isolation valve outside the containment at each penetration. This valve should be an automatic valve, a normally closed valve or a remotely operated valve[11]. Where the failure of a closed loop is assumed as a postulated initiating event or as a consequence of a postulated initiating event, the recom­mendations in the previous paragraph will apply to each line of the closed loop.

4.173. Loops that are closed both inside and outside the containment envelope should have at least one isolation valve, an automatic valve, a normally closed valve or a remotely operated valve outside the containment envelope at each penetration.

4.174. Exceptions to the above recommendations are permitted for small dead-ended instrumentation lines that penetrate the containment. For these lines a single manually operated valve outside the containment is sufficient. Instrumentation lines that are closed (i. e. not in communication with the atmosphere) both inside and outside the containment are acceptable without isolation valves provided that they are designed to withstand design basis accidents for the containment. The rooms where these lines emerge should be equipped with a filtration-ventilation system to maintain subatmospheric pressure. Such rooms and the equipment within them should be designed to withstand increased levels of temperature and humidity due to possible leakage from these lines.

4.175. The need for isolation of the containment in accident conditions and the need for operation of the safety systems that penetrate the containment envelope may result in contradictory design requirements. In such cases, consideration of the isolation provisions should be balanced against the need for the availability of safety systems and the need to avoid escalation of the accident conditions. Check valves may be used for the inner isolation barrier to resolve this issue, but the use of two check valves in series should not be considered an acceptable method of isolation.

4.176. Overpressure protection should be provided for closed systems that penetrate the containment and for isolated parts of piping that might be overpressurized by the raised temperature of the containment atmosphere during design basis accidents.

4.177. The extensions of the containment envelope should be designed and constructed to levels of performance that are at least equivalent to those for the containment barrier itself.

4.178. For the systems or piping that are normally closed to the containment atmosphere, but which might be opened in some reactor shutdown states (i. e. opening of the steam generator envelope in shutdown states or of the fuel transfer tube when the spent fuel pool is located outside the containment), and for which isolation can be provided by only one means,

— The leaktightness of the existing means of isolation should be demonstrated.

— A qualified mobile device should be used as a means of isolation.

— The system concerned should be opened only when the risk to safety is sufficiently low.

4.179. Particular consideration should be given to the containment isolation features of the following systems:

— Those systems, such as safety injection lines and emergency cooling lines, that are connected with the primary circuit and that can transport radio­nuclides outside the containment in design basis accidents;

— Those systems that can transport airborne radionuclides from the containment atmosphere to outside the containment in design basis accidents (i. e. systems used in some designs to mix the atmosphere inside the containment in order to prevent the ignition of hydrogen);

— Those systems that support systems important to safety (inside the containment) for which, in the event of leakage, fluids with a high activity might be released outside the containment (i. e. in some designs the component cooling water system, the containment sump purge system or the sampling systems).

4.180. Systems connected to the primary circuit in normal operations (i. e. primary circuit filtration systems or in some designs the chemical and volume control system) and systems connected to the containment atmosphere should be automatically isolated in accident conditions when they are not necessary for safety.

4.181. If valves used for normal operations are also used for containment isolation, they should meet the same design requirements as the containment isolation system.

Isolation valves

4.182. To achieve the objective of limiting any radioactive release outside the containment, the isolation devices should be designed with a specified leaktightness and closure time. In specifying the leaktightness and closure time, the amounts of potential radioactive releases should be taken into account. In making the choice between motorized and pneumatic valve operators, the requirement for the valve to reach a safe position in the event of loss of its motive force and the required closure time of the valve should be taken into account. It may be necessary to limit the closing speed of valves or dampers, particularly for larger penetrations, to ensure their proper functioning and tight sealing.

4.183. Design provisions for leakage tests (such as nozzles and instrumen­tation test lines) should be made such that each isolation valve may be tested. Any possible exceptions should be fully justified.

Penetrations

4.184. Containment penetrations should be designed for the same loads and load combinations as the containment structure, and for the forces stemming from pipe movements or accidental loads (Ref. [1], paras 6.51-6.54).