6.49.
The control of the reactor coolant system water inventory in accident conditions is performed by the emergency core cooling system in conjunction with depressurization of the reactor coolant system by the secondary side when necessary. The emergency core cooling system also performs some functions relating to the control of core reactivity. In general, the emergency core cooling system includes a combination of active and/or passive means of injection (e.g. pumps, piping and valves). The system can also include heat exchangers to remove residual heat from the containment. Recommendations concerning the sump filtration system are given in SSG-53 [22].
6.50.
The main function of the emergency core cooling system is to inject borated water into the reactor coolant system in order to ensure core cooling when the reactor coolant system water inventory decreases or in the event of loss of the residual heat removal by the secondary side. The emergency core cooling system is called upon to reach a controlled state in both design basis accidents and design extension conditions without significant fuel degradation, such as:
Loss of coolant accidents postulated as design basis accidents or design extension conditions;
Excessive and uncontrolled reactor coolant system cooling (piping breaks on the secondary side);
Steam generator tube rupture;
Total loss of feedwater with availability of reactor coolant system feed and bleed.
6.51.
Paragraphs 6.52–6.69 provide recommendations on meeting Requirement 52 of SSR-2/1 (Rev. 1) [1] in the event of design basis accidents and design extension conditions with a loss of primary coolant, and supplement the recommendations provided in Section 3 relevant for safety systems or for safety features for design extension conditions.
6.52.
The emergency core cooling system should be designed to cool the fuel in the event of a loss of coolant accident, in accordance with the relevant design criteria for fuel, fuel cladding and core geometry.
6.53.
In the event of a small pipe break, the energy removed at the break might not be sufficient for effective cooling of the fuel, and therefore complementary systems or equipment should be operated to achieve the appropriate cooling capacity (e.g. complementary decay heat removal by the steam generators or depressurization of the reactor coolant system to increase the water injection rate).
6.54.
The emergency core cooling system should be designed with adequate capabilities to prevent or limit the uncovering of the fuel assemblies in the event of primary piping breaks of different sizes and in different locations; different points of injection will usually be necessary.
6.55.
The possibility that injection flow rates could bypass the core or could directly flow to the break should be considered when designing the emergency core cooling system.
6.56.
The emergency core cooling system should be designed to restore and maintain an adequate coolant inventory in the reactor coolant system in order to recover the fuel cooling function.
6.57.
The boron concentration in the emergency core cooling system should be sufficient to achieve core subcriticality in an overcooling design basis accident (e.g. steam line break).
6.58.
The injection capacity of the emergency core cooling system should prevent boron crystallization in the core.
6.59.
In some designs, the emergency core cooling system includes capabilities to remove core decay heat in the long term when the integrity of the reactor coolant system is not ensured. These capabilities should be considered as part of the safety system.
6.60.
The emergency core cooling system should be designed to prevent or limit core uncovering in the event of the total loss of the residual heat removal capacities of the secondary side, taking into account the installed capacity for reactor coolant system bleeding.
Specific design aspects
6.61.
The pressure retaining equipment for the emergency core cooling system should be designed and manufactured in accordance with proven codes and standards widely used by the nuclear industry (e.g. Refs [24, 25, 27] and similar standards). For each individual component, the requirements to be applied should be selected with due account taken of the two effects resulting from its failure (safety function not fulfilled and a radioactive release occurring).
6.62.
The emergency core cooling system should be reliably isolated from the reactor coolant system by two isolation devices in series. In order not to decrease the reliability of the emergency core cooling system, these isolation devices should be designed to open quickly and without external service (e.g. check valves are widely used). The design of the reactor coolant system isolation should enable the leaktightness to be tested periodically. Moreover, the emergency core cooling system should be protected against overpressurization caused by leakages. Adequate means of monitoring (e.g. of pressure and temperature) should be installed to detect and warn about any leakages through the isolation valves.
6.63.
The emergency core cooling system equipment should be located outside the containment to the extent possible in order to limit the severity of the environmental conditions for which it should be qualified and also to facilitate maintenance and repair.
6.64.
The operation of the emergency core cooling system should limit the risk of causing overpressurization of the reactor coolant system. In particular, cold shutdown states should be considered in which the operation of the emergency core cooling system (spurious or not) could potentially damage the reactor pressure vessel or the residual heat removal system (e.g. owing to brittle fracture).
6.65.
Provisions should be implemented for the early detection of leakages in the parts of the emergency core cooling system that are located outside the containment in order to isolate the system before it causes the drainage of the water reserves.
6.66.
For accident management, the actuation of the shutdown and isolation of every emergency core cooling system train should be possible from the main control room. However, stopping the operation of the emergency core cooling system from the main control room should not be possible as long as a need for an emergency cooling of the core exists.
6.67.
The emergency core cooling system should be qualified to operate with radioactive water loaded with particles, in accordance with the capability of the filtration system.
6.68.
The minimum, net positive suction head for the normal operation of the emergency core cooling system pumps should be ensured at any time during design basis accidents, with account taken of limiting phenomena such as vortex, air entrainment and accumulation of debris at the surface of the sump filters. Accounting for the buildup of internal containment pressure should be specifically justified, if national regulatory provisions allow this.
6.69.
Mini-flow lines should be implemented to enable periodic tests and to prevent the failure of the emergency core cooling system pumps at low injection flow rates.