Nuclear-Fueled Power Plants - Energy and the Environment - page 131

Pressurizer

Containment
vessel

Steam

Load

Pressurized
heated
water

Turbine

C ooling

Condenser

Heat
exchanger

Reactor

Pump

Feedwater
heaters

Feed-
water

Pump

Figure 6.4 Schematic of a pressurized water reactor (PWR) power plant.

steam even though the temperature is in the 340-350 ◦ C range. At that temperature, the pressure
of the water exceeds the vapor pressure, so there is only liquid phase. The hot, nonboiling water
is pumped into a heat exchanger, which is located together with the reactor core inside a heavy
steel and reinforced concrete containment vessel. In the heat exchanger, feedwater is boiling into
steam at about 7 MPa in a secondary loop. The steam drives a turbine that drives the electric
generator in a conventional Rankine cycle. The steam turbine and condenser are located outside of
the containment vessel.
One advantage of the PWR is that because of the single phase of the coolant water, the moder-
ating capacity of the (light) water can be precisely adjusted, unlike in a BWR, where the coolant is
in two phases, liquid and vapor. Usually, boron in the form of boric acid is added to the coolant to
increase the moderating capacity. In such a fashion, fewer control rods are necessary to maintain
the reactor at the design capacity. The other advantage is that the steam which is generated in the
heat exchanger never comes into direct contact with the coolant water. Thus, any radioactivity
that may be present in the coolant is confined to the primary loop inside the containment vessel.
Because of the heat exchanger the overall thermal efficiency of a PWR is somewhat lower than
that of a BWR, on the order of 30%.
A special type of PWR is the Canadian Deuterium Uranium (CANDU) reactor. This type of
reactor is also used in Argentina, India, Pakistan, and Korea. CANDU uses natural uranium fuels
(without enrichment) and heavy water (D 2 O) as the moderator. Heavy water absorbs practically no
neutrons; thus its neutron economy is superior to that of light water. On the other hand, its moderating
capacity is less than that of light water; therefore neutrons have to travel twice as far to be slowed
down (become thermal) as in light water. The fissile material is 235 U as in enriched uranium reactors,
but some of the fertile 238 U is converted into fissile 239 Pu, which can participate in the chain reaction
or can be extracted for fuel recycling or for weapons production. Instead of uranium enrichment
facilities, the CANDU-type reactor requires heavy water production stills. Depending on which