Passive emergency systems

The AP600 passive safety system includes subsystems for safety injection, residual heat removal, containment cooling, and control room habitability under emergency conditions. Several of these aspects are in existing nuclear plants such as accumulators, isolation condensers as natural-circulation closed loop heat removal systems (in early BWRs), automatic depressurization systems (ADS - in BWRs) and spargers (in BWRs). 

In what follows the principle of a passive safety measure is explained using the design and functioning of the passive emergency trip system shown in Fig. 4.4 as an example [15]. 

The availabilities of an emergency discharge system, an inhibitor system, a pressure relief system and a passive trip system are compared with one another. The pertinent fault tree models are established and quantified. 

Gravity driven cooling system X Passive emergency core cooling (3 independent divisions)... 

With respect to the redundant heat removal paths, the moderator can act as an emergency heat sink even with no water in the fiiel channels. Should the moderator heat removal system subsequently fail, the large water-filled reactor vault surrounding the calandria vessel provides an additional line of defence (Fig. 5.7.2). Its primary purpose is to provide shielding of the concrete reactor vault from neutrons and gamma rays. However it can also act as a passive emergency water reservoir in case of a severe core damage accident that is, should the primary coolant... 

On positive reactivity addition or loss of reactor heat removal following reactor trip by the electromechanical protection system or the emergency boron injection system, the core residual heat removal is effected by the passive emergency heat removal system. The amount of water in the tanks of the system ensures reactor cooling for at least 72 hours (seven days with two tanks and three days with one tank available). 

Passive emergency heat removal system (PEHRS) X Prevention core uncovery at primary circuit... 

Pnmary circuit pressure control CPS/ASS Passive emergency heat removal systems Passive Without discharge of primary coolant... 

Heat remov al Passive emergency heat removal system/guard vessel Passive ... 

The concept of the I C and electrical systems in HSBWR are basically the same as those in ABWR However, the electrical load is much smaller because the recirculation system is eliminated Furthermore, the I C and electrical systems are very simplified, e g the reduction of emergency diesel generator capacity, because passive safety systems are adopted... 

The use of passive safety systems and devices which do not require actuation (such as the containment, the independent heat removal system, etc.) or can be passively actuated (such as systems for the primary circuit and containment depressurization in emergencies) ... 

Passive safety systems providing the emergency shutdown, core cooling and aftercooling of the reactor ... 

The protective shell is equipped with a passive emergency pressure decrease system with heat removal to the ultimate heat sink. This system includes the following ... 

Specifically, it has been shown that the passive emergency cooldown system allows the complete removal of residual heat in case of SG isolation at temperatures in the reactor not exceeding nominal values. 

As long as the gas turbine serves as a passive cooling system, it can generate electricity for the NPP electric machinery and systems even after the reactor shutdown in other words, the gas turbine also provides the BN GT plant with a passive source of electricity. Electricity supply redundancy is achieved by placing an accumulator battery and an oil-fired emergency generator in the transportable module of the main equipment. 

Self-protection means that the reactor plant is capable of preventing damage to physical protective barriers in emergency situations in the event of failure of active safety systems and non-intervention or errors by the plant personnel, by virtue of passive safety systems and inherent safety features based on the action of physical laws of nature which result in selflimitation of the reactor power and temperature. 

The passive safety systems are emergency core cooling system totally passive, with only one non-static component (400% redundant, 200% + 200% in two independent trains, each having two redundant components [each with 100% capacity] manufactured under different conceptual designs). 

The use of passive safety systems for the emergency reactor shutdown, core cooling and reactor after-cooling. 

The CCR safety design philosophy is to increase the reliability of systems and to avoid design complexity, both with the use of passive safety systems, and through it to eliminate the need of emergency planning in the public domain. 

During BMN-170 development, the design principles to provide plant safety was aimed at an optimum combination of reliance on intrinsic safety features and application of engineered (active and passive) systems. To be specific, the protection system being developed employs hydraulically suspended reactivity control rods that effectively influence the reactivity and convert the reactor to a sub critical state when flow rate is reduced through the core [XXI-9]. Use of a passive emergency cool down system allows complete removal of residual heat. 

Thus, it is shown that a high level of BMN-170 safety is provided by intrinsic properties of the core, thermal-physical properties of the sodium coolant, use of natural circulation to organize core emergency cooldown, use of passive safety systems along with traditional active systems and maintained by the possibility of high quality fabrication of the equipment in workshops. 

The safety strategy for the AHTR is to use (1) passive safety systems for normal and accident conditions and (2) inherent safety features for beyond design basis accidents. This is to ultimately allow the AHTR to be licensed without off site emergency planning. Figure XXVI-6 shows the heat removal pathways for the AHTR. 

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