Summary Report for Conduct of Kozloduy NPP Stress Tests

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“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests Specific technical solutions applied in the design of safety systems are multichannel structure (redundancy), physical separation and diversity. The combination of these solutions ensures resistance of safety systems to common cause failures, i.e. total loss of the ability of safety systems to perform their function. 1.3.2.1 Reactivity control 1.3.2.1.1 Systems ensuring core criticality with the reactor pressurized Reactivity control both during reactor operation and in shutdown mode is performed by two principally independent ways of action: · Moving the control rods by the core height; · Changing the concentration of dissolved boric acid in the primary coolant. In normal operation the core criticality in hot shutdown mode with the reactor pressurized, cooling from hot to cold state, maintenance outage are ensured by injecting control rods in the core and the primary circuit blowdown and makeup system. Following the reactor shutdown by changing the concentration of boron solution change in the reactivity due to xenon decay and coolant cooling to the cold state is compensated and also the necessary sub-criticality during refuelling, which is not less than 2% (with reactor control and protection system control rods fully extracted) is provided. In emergency situations with loss of offsite power supply and/or isolation of containment system the core criticality is ensured by inserting the control rods, emergency boron injection system - high and medium pressure, and the emergency core cooling. At large loss of coolant accident core criticality is ensured by passive part of essential components cooling system, emergency boron injection system - high and medium pressure, and the emergency core cooling. 1.3.2.1.1.1 Reactivity fast control systems Reactor control and protection system has the following key functions: · reactor emergency protection; · fast reduction of reactor control at tripping of the unit main equipment; · unloading and limitation of reactor power at tripping the unit main equipment; · operational control of unit power. The system control rods are designed so that at normal operational they are maintained in extreme upper or intermediate position from electromagnets. In case of actuation of emergency protection or at loss of the system power supply due to any reason, the electromagnets release the control rods and the latters enter the core due to gravitation, i.e. passive actuation of protection takes place. 27/202
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests Effectiveness of the emergency protection is selected based on the need to compensate for abrupt changes in reactivity associated with bringing the reactor to sub-critical condition of any power level, and considering the reduction in efficiency due to postulated sticking in the extreme upper position of most effective control rod and use of the rest for operational control and balancing the energy release field providing sub-criticality of not less than the minimum allowable (required) value equal to 0.01 Dк/к, i.e. 1% (section 4 of [7],[8]). The design provides for movement of control rods at a constant speed of 2 cm/s and dropping them at the emergency protection signals within max.4 s. 1.3.2.1.1.2 Reactivity smooth control systems The reactivity smooth control systems include all systems that influence on the reactivity by changing the boric acid in the primary circuit. They combine the functions of both normal operation systems and safety systems. Normal operation systems used for reactivity control. Makeup and Blowdown System. One of the key functions of the system is to perform operational control of primary circuit boron concentration and thus precise reactor power control. The system can be used both at normal operation and in other, accidents modes where the containment system is not isolated. At loss of offsite power, for the system to be used operation of the additional diesel-generator and at least one of the safety systems is required. Through the primary circuit makeup-blowdown system primary circuit boric acid removal is performed during reactor transfer to critical state, at power increase, at compensation of intoxication and other processes associated with smooth introduction of positive reactivity. Through the system also boron injection is performed in load reduction or reactor attenuation mode, at reactor intoxication compensation and other modes associated with smooth introduction of negative reactivity. Boron concentrate system The system is designed to supply boric acid solution with concentration of 40 g/kg, to the suction side of makeup pumps in primary circuit boric acid concentration increase mode. Reactor building boron containing water system The system is designed to collect coolant removed from the primary circuit. The system also serves for storage and supply to the primary circuit of coolant with boric acid standby concentration. Emergency systems used for reactivity control. High pressure emergency boron injection system. The system is designed for emergency injection of emergency boric acid solution to the primary circuit with concentration of 40 g/kg in case of loss of off-site power or in case of failure to 28/202
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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