34
Water for Energy and Fuel Production
taBle 3.1
Water-Based nuclear Power Plants (nPPs) in Commercial Operation
reactor type
total nPP (%)
GWs
Coolant/moderator
PWR
65
270
Water/water

BWR
20
81
Water/water

PHWR (CANDU)
11.5
27
Heavy water/heavy water

RBMK (light water)
2.5
10
Water/graphite

Other
1
0.04
Water/graphite

Source: Nuclear Engineering International Handbook, 2011.

Note: The total number of reactors in the world is 438 with 399.3 GW.

PHWR, pressurized heavy water reactor.

a zone containing a large array of fuel rods where it picks up the heat generated by
the fission of U 235 present in the fuel rods. The coolant transfers heat to turbine and
returns back to the reactor core. This loop is called primary circuit. It is the pressure
at which coolant flows through the reactor core that makes the distinctions between
PWR and BWR. In both types of light water reactors, about one-third of the fuel is
replaced every year (implying life cycle for any given rod to be about three years)
because by that time the concentration of fission fragments produced as a result
of fission reactions absorbs enough neutrons to interfere with the chain reaction.
Thus, before U 235 is exhausted, fuel rods are periodically replaced to maintain high
efficiency of the fission process.
3.2.2 Boiling WATer reACTor
The BWR does not have a steam generator [1–3]. Instead, water in the BWR boils
inside the pressure vessel and the steam–water mixture is produced when very pure
water (reactor coolant) moves upward through the core absorbing heat. The uranium
core in the reactor vessel creates heat. The control rods enter the reactor from below.
The water boils and produces steam that is passed through a turbine, which in turn
drives the electric generator. While the BWR has many similarities to the PWR, there
is only one circuit with water at lower pressure (about 75 atm) in the BWR so that it
boils in the core at about 285°C. About 12%–15% of water is in the upper part of the
core as steam and this has a lower moderating effect. BWR units can operate in the
load-following mode more readily than PWR. The steam passes directly to the turbines
before being condensed and recycled. Both water and steam are thus a part of a close
reactor circuit. The entire close loop along with the BWR is illustrated in Figure 3.1 [2].
As shown in the figure, the core of the BWR contains 3.5–4.0 m-long fuel rods
(90–100) and assemblies (up to 750), which hold up to 140 tons of uranium. In most
nuclear reactors, the fuel is enriched ceramic uranium oxide (UO 2 with melting point
of 2800°C). The fuel pellets (usually about 1 cm in diameter and 1.5 cm long) are
typically arranged in a long zirconium alloy tube to form a fuel rod. A fuel assembly
is an open lattice that can be inserted and withdrawn from the reactor core.