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Unformatted text preview: INHERENTLY SAFE REACTORS DESIGNS
© M. Ragheb
3/11/2009 1. INTRODUCTION
The electrical power production industry worldwide is faced with a need for new
generating capacity. In the USA, the need is for an increase of 1/3 of the existing capacity. The
utilities in the USA are currently building gas turbine and combined cycle plants to meet peaking
and intermediate needs. Of a forecast need of 150 GWe, 65 Gwe are for peaking and
intermediate capacity, and 85 Gwe are needed for uncommitted baseload capacity.
A new generation of nuclear power plants possessing passive inherently safe safety
features is replacing the older designs, and are being constructed or in the conceptual design
stage. The inherent safety features are a response to the understanding that human factors
significantly contributed to the Three Mile Island and the Chernobyl reactor accidents. These
plants would contribute to the reduction of the environmental costs of emitting nitrogen oxides
(NOx), sulphur dioxide (SO2), and carbon dioxide (CO2) from fossil fuels. As these emissions
become economically monetized, nuclear electricity is expected to offer both an environmental
and a cost advantages.
The safety improvements in the new generation of plants are quantified by the Core
Damage Frequency (CDF), resulting from Probabilistic Risk Assessment analyses (PRAs). The
design goals for CDFs for the new generation of nuclear power plants are set at 10-7 per
reactor.year. This is compared to the value of 10-4 for existing power plants. For existing plants
the frequency of 1/10,000 translates into one severe core accident per 200 years for a world with
500 reactors (500 x 200 = 10,000). For the new generation of reactors, even with a world with
more than double the number of reactors as today (434 plants as of the year 2001) at 1,000
plants, the frequency of 1/10,000,000 translates into one severe core accident in 10,000 years
(1,000 x 10,000 = 10,000,000).
The new design also satisfy the Nuclear Regulatory Commission's (NRC) Severe
Accident criteria, which require that these new plant possess the capability to protect the public
from radiation releases, even in the improbable situation of a severe accident. 2. ADVANCED LIGHT WATER REACTOR (ALWR)
The USA Department of Energy (DOE) has launched cost share programs with the USA
electric utilities, and vendors to develop advanced LWR reactor designs. On the basis that
reactor safety is a global concern, government agencies and companies from about 20 other
countries participate in the program. The development of a new generation of reactors has
moved in two directions:
1. Evolutionary plant designs.
2. Passive plant designs EVOLUTIONARY PLANT DESIGNS In these designs, evolutionary changes in the design are incorporated in the sense that the
designs started from existing plant designs and built into them improved safety margins. Modern
safety systems, advanced instrumentation and controls, and simplified operations and
maintenance are incorporated in these designs.
Two large designs have been developed with an electrical capacity of 1,350 Mwe and have
received final design approval from the NRC.
The first one shown in Fig. 1 is the Advanced Boiling Water Reactor ABWR designed and
built by the General Electric Company. Plants are being built in Japan following this concept. Fig 1: The ABWR pressure vessel design.
The second concept that has received final design certification from the NRC is the ABBCE System 80+ which evolved from the Combustion Engineering (CE) Pressurized Water
Reactor System 80 plants, shown in Fig. 2. The System 80+ plus plants are being built in Korea, and are the basis of the first
standardized system used in the three units at the Palo Verde Nuclear Generating plant in
Arizona, shown in Fig 3. Fig 2: The plant layout of the ABB-CE System 80+ evolutionary design. Fig 3: The three standardized System 80+ units at Palo Verde Nuclear Station in Arizona,
USA. Fig 4: The Control Room of the System 80+ features human factors engineering features.
THE ADVANCED BOILING WATER REACTOR (ABWR)
The ABWR reactor pressure vessel is 21 meters high and 7.1 meters in diameter, and is
designed for a 60 years lifetime. The vessel is mostly made of a single forging including the 4
vessel rings from the core beltline to the bottom head.
The external recirculation loops in older designs using jet pumps, and subject to leakage
failures have been eliminated. The 10 canned rotor Reactor Internal Pumps (RIPs) shown in Fig.
5 replaced them. As a result the vessel has no nozzles greater than 2 inches in diameter
anywhere below the top of the core. Over 50 percent of the welds and all the piping and pipe
supports in the primary system have been eliminated. This eliminates the largest source of
occupational exposure in the BWR earlier designs. These pumps are improved versions of those
used in Europe with significant operational experience. The RIP pumps are continuously purged
with clean water to keep the crud from settling into them from the vessel, reducing the radiation
levels around them. With t...
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