Light Water Reactors

Pressurized Water Reactors (PWRs)


Like the first British power reactors, which were built to produce weapons-plutonium, the
first US power reactors also began under military auspices, albeit specifically as power
plants. The US Navy realized after the Second World War that a submarine powered by
nuclear fuel would not need to resurface to replenish oxygen supply, since the 'burning' of
nuclear fuel  - unlike that of oil - does not require oxygen. Spurred by this idea, and
constrained by the space limitations in a submarine, US designers developed a reactor
using a core of relatively high power density, with fuel elements immersed in a tank of
ordinary water  - called 'light water' to distinguish it from heavy water - under sufficient
pressure to keep it from  boiling. The 'first power reactor ever built', according to its
builders, went critical on 30 March 1953 in a land-based mock-up of a submarine hull at
the National Reactor Testing Station in Idaho. The following year saw the launching of
the USS Nautilus, the first nuclear-powered submarine, powered by a pressurized water
reactor system. In 1957 the submarine reactor came ashore, at the Shippingpoint power
station near Philadelphia, the first nuclear power station in the USA. In subsequent years
the pressurized water reactor, or PWR, has become the world's most popular.
The basic structure of a PWR is a large pressure vessel of welded steel with a lid held
onto the upper end by a ring of heavy bolts. The pressure vessel contains the reactor core,
and other so-called 'reactor internals' like control rods; the remaining volume is
completely occupied by ordinary 'light' water under a pressure of about 150 atmospheres.
The core is made up of fuel elements, each a faggot of 4m-long fuel pins. A PWR fuel
pin is a tube of zircaloy, about 1 centimetre in diameter, filled with stubby cylindrical
pellets of uranium dioxide. So far as neutrons are concerned the zircaloy cladding is
comparatively well-behaved - much more so than stainless steel  - albeit more expensive.
But the water in which the whole concatenation is immersed is - as noted in Chapter 2  -
an enthusiastic gobbler of neutrons, and to offset its distracting influence the uranium in
PWR fuel pellets is enriched to upwards of 3 per cent uranium-235. The water inside the
pressure vessel serves simultaneously as moderator, reflector and coolant. At the top of
the core it leaves through heavy pipes welded to the pressure vessel. PWRs can have two
or more 'loops' of cooling circuit. In each loop, the pipe through  which the water enters
the pressure vessel is called the 'cold leg', and that through which it leaves is called the
'hot leg'.
The hot leg of a PWR cooling loop carries the hot coolant water into a steam generator or
boiler. The hot high-pressure water from the reactor passes through thousands of tubes
immersed in more water, under considerably lower pressure. Although the pressurized
water inside the tubes cannot boil, the lower-pressure water outside them does. The
resulting steam is processed and piped to a turbo-generator set. The primary coolant
water returns through the cold leg to the reactor vessel, encouraged by a primary coolant
pump. One coolant loop also includes a 'pressurizer', in which an appropriate quantity of 41
the coolant water is evaporated or condensed, to maintain coolant pressure and to
compensate for the effects of thermal expansion and contraction as plant output varies.
The pressurizer can also help to offset unintended increases in system pressure resulting
from malfunctions. The electric immersion heaters in a pressurizer can generate 2000 kW
- a bit overwhelming for a household hot-water system.
Easily the most controversial feature of the PWR are the emergency core-cooling
systems, provided to prevent overheating of the reactor core in the event of an accident.
However, rather than describing them here, it will be more appropriate to defer their
description to Chapter 6; there can be few technologies which have been subjected to
such exhaustive - and inconclusive - scrutiny.

PWR control and instrumentation systems vary widely in design. But control rod42

assemblies are commonly suspended above the core, inside the pressure vessel lid, with 

drive mechanisms functioning through the lid from above. A PWR is refuelled off load  -

that is, with the reactor shut down. The reactor is allowed to cool. Then a pool-shaped 

chamber above the reactor - the 'reactor well' - is flooded with water, to provide shielding 

and cooling; the lid is unbolted and moved to one side, exposing the interior of the 

reactor. Since the whole procedure is time-consuming, a substantial proportion of the fuel 

charge is changed at each refuelling  - typically about one third of the core. PWR 

designers usually provide for one refuelling operation annually.

Needless to say a PWR is, like any power reactor, enclosed in heavy shielding. The 

reactor vessel itself is surrounded by two or more metres of concrete, extending upwards 

to form the side walls of the reactor well. The concrete also encloses the entire primary 

circuit - steam generators, primary pumps, pressurizer and piping  - because the primary 

coolant is commonly slightly radioactive. The reactor building itself is usually designed 

to serve as a secondary containment.

Some PWRs deliver close to 4000 MW of heat at a power density over 100 KW per litre. 

But the low coolant temperature attainable using water under manageable pressure - some 

150 atmospheres, as noted  - makes the PWR a comparatively inefficient  source of heat 

for electricity generation. Nonetheless PWRs greatly outnumber all other  types of power 

reactor.