9. Inherently Safe Reactor Designs

In the past nuclear power plants have been designed

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Unformatted text preview: e past, nuclear power plants have been designed, built and constructed on a plant by plant custom basis. This has led to costly delays, and in some cases project abandonment, because safety license could not be obtained without a lengthy review after plant completion. With standardization of the design: 1. Plant design is pre-approved, eliminating licensing delays during and after construction. 2. Standardization allows for plant completion in about half the time for a custom designed plant. 3. Final costs can be significantly cut through lower interest charges paid during the construction period. An example of a standardized station is the three units at Palo Verde, Arizona, USA, shown in Figs. 2 and 3. Several units are under construction in Korea and Taiwan. Another unique feature of the System 80+ design is the incorporation of an advanced control room design designated as the Nuplex 80+ Advanced Control Complex. As shown in Fig. 4, a 6x8 feet overview display lets all persons in the control room keep track of key status indicators simultaneously, minimizing the risk of human error. PASSIVE PLANT DESIGNS In these designs, the emphasis is on passive safety features that would negate the need for active safety systems which are prone to human error as occurred in the Three Mile Island and the Chernobyl accidents. Two plant designs stand out in this category: The AP600 design by the ToshibaWestinghouse Company, and the Simplified Boiling Water Reactor by the General Electric Company. THE AP600 ADVANCED PASSIVE PRESSURIZED WATER REACTOR A cooperative program between the Electric Power Research Institute (EPRI) and the US DOE, resulted in this concept. The Advanced Reactor Corporation (ARC), representing 16 US utilities chose it as the lead passive plant design for a next generation of nuclear power plants. Fig 6: The Passive Containment Cooling in the AP600 Pressurized Water Reactor Concept. The AP600 emphasizes a simplified plant design and safety by greatly reducing the number of small components that can fail during plant operation. Compared with a typical plant of the same size, it is designed to have 50 percent fewer valves, 35 percent fewer pumps, 80 percent fewer heating, ventilating and cooling units, 45 percent seismic building volume, and 70 percent less cables. Gravity and natural circulation are to cool the reactor core and transport heat through the containment vessel to the atmosphere. In case of a coolant release, internal condensation transfers heat from the flashed steam to the steel containment, as shown in Fig. 6. That steel structure would initially be cooled with gravity fed water from tanks on top of the containment. In addition, the steel containment is continuously cooled by natural circulation of air between the containment and the surrounding biological concrete shield structure. It is recognized that in existing PWR containment structures, a coolant leakage in the containment, without adequate cooling, would eventually breach the containment. The leakage would occur at the weakest points in the structure at the piping and instrumentation ducts seals. The concrete containment is not meant to contain any coolant releases, but is provided both as a biological radiation shield, and as protection from the outside elements, like tornado or hurricane driven missiles. Fig 7: The Simplified Boiling Water Reactor (SBWR) Assembly. The 1.7 inch thick steel walls of the containment vessel enclose a larger space than in conventional PWR designs, and can thus withstand a long buildup of pressure inside the containment. The reactor is placed at the bottom of the containment so that gravity fed water continually covers its core from emergency flooding tanks, in addition to the nitrogen pressurized accumulator tanks. Automatic depressurization valves on top of the pressurizer open above a setpoint venting steam into a quenching tank that condenses it into water. Instead of injecting water into the hot leg or cold leg or both like in current designs, Emergency water injection is directly into the core through a safety injection nozzle. High inertia canned motor pumps improve safety and reliability. These pumps are closely coupled to the steam generator to avoid small LOCA core uncovery. THE SBWR, SIMPLIFIED BOILING WATER REACTOR In this 600 Mwe design, there exists complete dependence on natural circulation for cooling the core. The recirculation pumps have been eliminated as shown in Fig. 7. Fig 8: Passive Safety Features of the SBWR System. Natural circulation cooling has been used in earlier BWRs such as the Dresden and Humboldt Bay plants. A 60 Mwe plant in the Netherlands, Doodeward used natural circulation over a 25 years lifetime at a capacity factor of 84 percent. In fact, most large BWRs operate with natural circulation up to 50 percent of their rated power or about 500-600 Mwe. To enhance natural circulation in the SBWR, the pressure drop through the core has been reduced from 25 psi to 5 psi. This reducti...
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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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