Environmental Engineering Reference
In-Depth Information
helium or CO 2 ). The accident at the Three Mile Island power plant near Harrisburg, Pennsylvania,
in 1979 occurred because after shutdown (full insertion of the control rods), the reactor was com-
pletely drained of its coolant, so that the residual radioactivity in the fuel rods caused a meltdown
of the reactor.
The heat removed by the coolant, in the form of steam or pressurized hot water, is used in
conventional thermodynamic cycles to produce mechanical and electrical energy. In addition to the
control rods position, the mass flow of the coolant also determines the plant's power output.
6.4.1
Boiling Water Reactor (BWR)
About one-quarter of the U.S. nuclear power plants are of the BWR kind. The schematic of a
BWR is depicted in Figure 6.2; the reactor assembly itself is depicted in Figure 6.3. Most BWR's
use 3-4% enriched 235 U as the fuel. Light water serves as both the coolant and moderator. As
the control rods are withdrawn, the chain reaction starts and the coolant-moderator water boils.
The saturated steam has a temperature of about 300 C and pressure of 7 MPa. After separation
of condensate in the steam separator, the steam drives the turbine that drives the generator. After
expansion in the turbine, the steam is condensed in the condenser and returned via the feed water
pump to the core.
The direct cycle has the advantage of simplicity and relatively high thermal efficiency, because
the steam generated in the core directly drives a turbine without further heat exchangers. The thermal
efficiency of a BWR is on the order of 33%, calculated on the basis of the inherent energy of the
nuclear fuel that has been consumed in power production.
In a BWR there is no need for an extra moderator, because the coolant light water slows
down the fast neutrons to thermal velocities that can engage in further fission reactions. Another
advantage of a BWR is the fact that it is self-controlling. When the chain reaction becomes too
intense, the coolant water boils faster. Because of its low density, steam has little or no moderating
Saturated
steam
Containment
vessel
Steam
separator
Load
Turbine
Water-steam
mixture
C ooling
Condenser
Downcomer
recirculation
water
(saturated)
Core
Feed water
pump
Condensate
pump
Figure 6.2 Schematic of a boiling water reactor (BWR) power plant.
 
 
 
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