9. Inherently Safe Reactor Designs

With them compact containment designs are possible

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Unformatted text preview: hem compact containment designs are possible, greater excess flow capacity, absence of nozzles below the core and higher reliability are possible. Selection of materials in the ABWR eliminates cobalt from the design, since cobalt could activate in the crud into Co60, a strong gamma emitter. The condensers are made of titanium metal and valve seat use cobalt free alloys. The steel used in the primary system is low carbon steel alloys, which are nuclear grade materials. These alloys mitigate a source of worry in earlier designs concerning Intergranular Stress Corrosion Cracking (IGSCC). Fine Motion Control Reactivity Drives (FMCRDs) are being used. They perform with an electric stepping motor moving the drive in 0.75 inch increments. The control rods can be scrammed into the reactor core using the hydraulic system, but as a backup, the stepping motor can be used to scram them. Earlier Locking Piston Drives had 3 inches increments. A clean water purge is provided for the FMCRDs to keep radiation levels to low values. The ABWR Control and Instrumentation (C&I) system has four separate divisions of system logic and control, including four separate redundant multiplexing networks. These systems are made of digital and fiber optic technologies. Microprocessors in each system receive incoming sensor information and generate output control signals. The controllers are fault tolerant continually generating signals to simulate input data and compare them against the desired outcome. Controllers for both sensors and equipment are on cards. If the controller detects a problem, the malfunctioning cards can be replaced. Multiplexing and fiber optics have eliminated 0.4 million meters of cable and 3780 m3 of cable trays compared with the older designs. The four redundant divisions in the control system use a 2 out of 4 logic. On line repair of one of the systems while the others are still functional is possible. Solid state technology allows the control system to be enclosed in two cabinets, compared with 18 cabinets for the older design. Fig 5: Reactor Internal Pump for the ABWR. The control room is designed according to the wishes of plant operators who wanted to operate the plant from a single console with touch screen panels and Cathode Ray Displays (CRTs) displays, and not have to run around the control room looking for side panels. A system, its subsystems, and components can be called in a series of exploding displays. Touching the screen then can operate these. The entire system can be operated by a system master command. For instance, the Reactor Heat Removal (RHR) system has six operating modes, each of which the operator with a single touch, can invoke, and the computer will configure the system by aligning valves or turning on pumps. The designs are foreseeing the trend toward standardization. The plant is designed to fit any site in the world from the perspective of seismic potential. The containment building is a Reinforced Concrete Containment Vessel (RCCV) with a leak tight steel lining. The reactor building surrounds the containment, and doubles as a secondary containment. A negative pressure differential is maintained in the reactor building and directs any radioactive release to a gas treatment system. Large modules are prefabricated in a factory and assembled on site. The entire control room, weighing 400 tonnes, the steel lining of the RCCV, and the turbine generator pedestal are examples of these prefabricated modules. Three independent and redundant safety systems are available in the ABWR. The systems are mechanically and electrically separated. Each division has redundant sources of AC power and its own dedicated emergency diesel generator. Each division is located in a different quadrant of the reactor building, and they are separated with firewalls. Fires, floods or loss of power if affecting one division, will not affect the others. Each division has a low and a highpressure system. Each system has its own dedicated heat exchanger to control core cooling and remove the decay heat. One of the high-pressure safety systems are designed to keep the core covered at all times coolant injection systems, the Reactor Core Isolation and Containment (RCIC) system is powered by reactor steam. This provides a source of cooling in case of the Station Blackout (SB) accident, where it is assumed that both the local and outside sources of electrical power are not available to control and operate the plant. The safety systems have the capability of keeping the core covered at all times. In the event of a Loss of Coolant Accident (LOCA), operator action is not required for 72 hours, since the plant response has been fully automated. The operational transients leading to a plant shutdown have been reduced to a frequency of once per year. THE SYSTEM 80+ PRESSURIZED WATER REACTOR The system 80 design is characterized by being the first standardized system built in the USA. It obtained a Final Design Approval (FDA) by the NRC in 1994. In th...
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This note was uploaded on 06/16/2010 for the course NPRE 402 taught by Professor Ragheb during the Spring '08 term at University of Illinois at Urbana–Champaign.

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