5.20.
The recommendations in paras 5.21–5.27 apply to the design of the pressure control of the reactor coolant system and the secondary side for PWRs and PHWRs.
5.21.
The concept of defence in depth should be applied in the design of the pressure control of the reactor coolant system and the secondary side. According to this concept, systems and components with different capacities should be used for pressure control to ensure that the preventive measures are proportional to the severity of anticipated operational occurrences or accident conditions.
5.22.
In the design, the diversity principle should be applied between the pressure control system and the overpressure protection system to reduce the likelihood of common cause failures.
5.23.
If the pressurizer can be isolated from the reactor coolant system in certain operating conditions (e.g. during heat-up or cooldown of a PHWR), the pressure and inventory control system should include alternative means of controlling the pressure and inventory in the reactor coolant system, such as a set of automatically controlled feed and bleed valves. In such cases, the pressurizer should have independent safety devices and/or relief devices.
5.24.
Systems intended for the control of pressure should be designed to prevent, in normal operation and anticipated operational occurrences, the operation of the safety valves (or, for BWRs, the safety relief valves). The release of primary coolant should be minimized, and it should not be discharged into the containment.
5.25.
The settings and performance of systems designed to control operating conditions should be determined on the basis of a realistic response of the plant to such conditions.
5.26.
The pressure control system of the reactor coolant system should be designed to maintain the pressure within the limits that are set to ensure the cooling of the fuel in operational states (as long as two-phase conditions are maintained in the pressurizer).
5.27.
Pressure control in the reactor coolant system and the secondary side circuit should be ensured even in the event of a loss of off-site power.
5.28.
Paragraphs 5.29–5.41 provide recommendations on meeting Requirement 48 of SSR-2/1 (Rev. 1) [1] for PWRs, BWRs and PHWRs and also apply to the overpressure protection system of the secondary side for PWRs and PHWRs.
5.29.
The overpressure protection devices should include redundant safety valves (or, for BWRs, safety relief valves). The setting of the safety valves should be such that safety valves (or, for BWRs, safety relief valves) open in sequence for different levels of pressure to avoid unnecessary discharge of coolant.
5.30.
An overpressure protection system should be implemented to preserve the structural integrity of the reactor coolant pressure boundary by keeping, in conjunction with the reactor shutdown system, the pressure below the design limits specified for the different categories of postulated initiating event.
5.31.
For PWRs and PHWRs, an overpressure protection system should be implemented to preserve the structural integrity of the secondary side pressure boundary by keeping, in conjunction with the reactor shutdown system, the pressure below the design limits specified for the different categories of postulated initiating event.
5.32.
The discharge capacity of the overpressure protection system should be designed to meet the pressure limits prescribed by proven industry codes and should apply the design rules specified by these codes. The typical approach includes the following:
Analyses do not credit systems that are not safety classified unless the operation of such systems can aggravate the consequences of the initiating event.
With regard to the criteria to be met, safety classified systems are assumed to operate at a less favourable performance.
The discharge capacity of the safety valves is determined on the basis of the applicable design standard.
The total discharge capacity credited in the analysis is calculated taking into account the sequential opening of the safety valves and that at least one safety valve fails to open (or more safety valves fail, for systems with more safety valves). This independent failure need not be considered in the analysis of overpressure transients initiated by multiple failures.
Loss of off-site power is combined if it can aggravate the consequences of the initiating event.
5.33.
The overpressure protection devices should be designed to keep water hammer effects as low as possible.
5.34.
Equipment ensuring the integrity of the reactor coolant pressure boundary and/or the integrity of the secondary side pressure boundary should be supplied by the uninterruptible power source.
5.35.
Shut-off valves should not be placed in the discharge line of a safety valve (or, for BWRs, a safety relief valve), or between a safety valve (or, for BWRs, a safety relief valve) and the item being protected. When a relief valve is used for pressure control, its reliable closing should be secured by means of a shut-off valve on the relief line.
5.36.
Safety valves, safety relief valves and relief valves should be provided with a position indicator that is independent of the control equipment.
5.37.
The outlet of the steam relief valves should be monitored in order to detect leakages.
5.38.
Due consideration should be given to the layout of the safety valves, their pilot valves and connecting piping to prevent the accumulation of non-condensable gases and condensate, and thereby avoid the adverse effects of such accumulation.
5.39.
The valves used for overpressure protection and the associated piping should be designed to discharge steam, steam–water mixtures and water.
5.40.
The spurious opening of a safety valve (or, for BWRs, a safety relief valve) should be prevented, and the frequency of such spurious opening should not be higher than the frequency considered for loss of coolant accidents.
5.41.
Components that can increase pressure in the primary circuit (e.g. pressurizer heaters or make-up pumps for PWRs and PHWRs) should be equipped with a system that stops the operation of the component to prevent an inadvertent pressure increase.
5.42.
Paragraph 5.43 provides recommendations on meeting Requirement 20 of SSR-2/1 (Rev. 1) [1].
5.43.
The reactor coolant system overpressure protection system should also be designed to preserve the integrity of the reactor coolant pressure boundary in the event of postulated sequences involving multiple failures. Typically, the design of the overpressure protection system should be adequate to limit the pressure in the event of an anticipated transient without scram (if relevant).