非能动安全.doc

Bu Qingyang 09300200015
Passive Safety System
ABSTRACT
In order to improve the security of reactors, the concept of passive safety system was raised in 1980s. It pertains to the field of pressurized water reactor type and is concerned with the fluid systems which mitigate consequences of events. These systems provide emergency cooling of reactor core, reactor building heat removal and pressure reduction, and reactor building fission product control. Passive safety system just relies on natural forces, like gravity, natural circulation, compressed gas and known laws of physics. So it needn’t rotating machinery or external power.
BACKGROUND
Fukushima Daiichi nuclear disaster made the public draw attention to nuclear safety. Since 1952, there have been 14 nuclear accidents, including 6 accidents with local consequences (Level 4), 5 accidents with wider consequences (Level 5), 1 serious accident (Level 6) and 2 major accidents (Level 7). The reason of most nuclear reactor accidents is that operator errors or design flaws caused cooling system failure and core meltdown, such as Jaslovské Bohunice in 1977 and Three Mile Island accident in 1979.
When an emergency occurs, the reactor is supposed to shut down safely to prevent radioactive leak. However, reactors continue to produce heat from radioactive decay products even after the main reaction is shut down, so it is necessary to remove this heat to avoid meltdown of the reactor core. Current nuclear safety, which is called active safety system, is based on external power to drive the cooling system. Once the external power was damaged or operated wrong, the nuclear reaction would fail to slow down, even it might accelerate. For example, in Fukushima Daiichi nuclear disaster, the tsunami broke the reactors' connection to the power grid and also resulted in flooding of the rooms containing the emergency generators. Those generators ceased working and made the pumps circulating coolant water in the reactor cease to work, which caused the reactors to overheat. At last, reactors 1, 2 and 3 experienced nuclear meltdown and several hydrogen explosions.
SUMMARY
In order to improve the security of reactors, the concept of passive safety system was raised in 1980s. It pertains to the field of pressurized water reactor type and is concerned with the fluid systems which mitigate consequences of events. These systems provide emergency cooling of reactor core, reactor building heat removal and pressure reduction, and reactor building fission product control. Characterized by the absence of active components for driving fluid to effect heat removal, passive safety system just relies on natural forces, like gravity natural circulation, compressed gas and known laws of physics. So it needn’t rotating machinery or external power. In passive safety system, there’re some valves with failure protection, the capability that needs energy to hold the valves in closed position. Once the energy was lost because of emergency, the valves would open automatically, and the system would integrate and make an adjustment.
Also, passive safety system will provide for alternate heat removal from the reactor containment to the environment, containment pressure reduction and reduction of containment atmosphere fission product concentration.
The passive safety system is usually made up of for subsystems. The first subsystem includes a heat exchanger for cooling water flowing from the first branch, which guides heated water from reactor vessel into the steam generator, and prior to being introduced into the second branch, which guides cooled water from the steam generator into the reactor vessel, and a valve for allowing flow of water from the first branch to the second branch solely in response to parameter value pertaining to operational safety. The second subsystem includes a valve for allowing the stored cold water to be introduced into the reactor vessel solely in response to parameter value pertaining to operational safety. The third subsystem includes a valve for depressurizing the reactor coolant circuit in response to parameter value pertaining to operational safety. The forth subsystem removes heat from the containment by natural air circulation externally of and in contact with the entrainment shell.
DESCRIPTION OF REPRESENTATIVE TYPE
Now the passive safety system has been used in some Generation III reactors in construction. The representative types of reactor with passive system are AP1000 and European Pressurized Reactor (EPR). The AP1000 is a two-loop pressurized water reactor developed by Westinghouse Electric Company, and its design is the first Generation III+ reactor to receive final design approval from the U.S. Nuclear Regulatory Commission.

Fig. 1 AP100 Passive Core Cooling System
In the AP1000, Westinghouse's Passive Core Cooling System uses multiple explosively-operated and DC operated valves which must operate within the first 30 minutes. This is designed to happen even if the reactor operators take no action. The electrical system required for initiating the passive systems doesn't rely on external or diesel power and the valves don't rely on hydraulic or compressed air systems. The design is intended to passively remove heat for 72 hours, after which its gravity drain water tank must be topped up for as long as cooling is required. The Passive Core Cooling System includes 4 important subsystems: passive residual heat removal system (PRHR), passive safety injection system, automatic depressurization system (ADS), and passive containment cooling system (PCS).
The PRHR is designed for 100% decay heat removal, and it protects plant from upsets in normal steam generator feed water and steam system. The PRHR HX with passive containment cooling system provides infinite decay heat removal capability without operator action.
The AP1000 passive safety injection system uses three sources of water for RCS make-up. CMTs (Core Make-up Tanks) provide coolant at full system pressure to down comer through DVI (direct vessel injection) line. Two accumulators provide coolant at high flow rates once RCS pressure is lower than 4.9 MPa. IRWST (in-containment refueling water storage tank) supplies borated water to RCS once the primary system has depressurized to low pressure.
Purpose of the ADS is to progressively decrease RCS pressure to allow passive systems to inject. ADS actuates when CMT level decreases below 67.5%. Some valves open and begin depressurization, discharging from top of pressurizer through spargers into IRWST. Others actuate at 20% CMT level and vents directly to containment. There is a ADS connected to RCS at top of HLs.
The PCS provides the safety-related ultimate heat sink. Steel containment shell provides heat transfer surface area to remove heat by continuous natural circulation. Air cooling is supplemented by evaporation of water on external surface.
Unlike the simplified and innovative design of AP1000, EPR design, developed mainly by Framatome (now Areva NP), Electricité de France (EDF) in France, and Siemens AG in Germany, adds safety redundancy to previous PWR designs to increase safety and get enhanced economic competitiveness.
The EPR design has several active and passive protection measures against accidents. Digital technology and a fully computerized control room with an operator -friendly human-machine interface improve the reactor protection system.
Four independent emergency cooling systems provide the required cooling of the decay heat that continues for 1 to 3 years after the reactor's initial shutdown. Independence is important because, if a safety system provides protection against the failure of a control system, then they should not fail together. In fact, each of cooling systems is designed to be capable of performing the entire safety function on its own.

Fig. 2 EPR engineered safety system
A core catcher allows passive collection and retention of the molten core should the reactor vessel fail in the highly unlikely event of a core melt. The extremely robust, leaktight containment around the reactor is designed to prevent any external radioactive release. The arrangement of the steam generator bunker inside the containment and the use of passive hydrogen catalytic recombiners prevent the accumulation of hydrogen and the risk of deflagration.
An outer shell covering the reactor building, the spent fuel building and two of the four safeguard buildings provide protection against a large commercial or military aircraft crash.
Although AP1000 and EPR stand for different ideas of passive safety system, both show outstanding safety level. I’m sure that nuclear power plants, with passive safety system will generate safer electricity. If public approval of the nuclear energy option continues to grow, the reactors with passive safety system in years to come could prove to be a strong contender in the global energy market.
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