Pressurized water reactor

Figure 1. The Watts Bar Nuclear Generating Station in Tennessee uses PWRs in its operation.

Figure 2. Nuclear submarines make use of the high power-to-weight ratio of PWRs in their operation.

The pressurized water reactor (PWR) is a type of nuclear reactor, used for the generation of electricity and for propulsion in nuclear submarines and naval vessels. They make use of light water (ordinary water, as opposed to heavy water) as their coolant and neutron moderator. It is one of three types of light water reactors, with the others being the boiling water reactor and the supercritical water cooled reactor.

It was originally designed for the U.S. Navy, however it quickly grew to become the most widely used reactor in nuclear power plants; with over 270 in operation around the world as of 2015. This makes them by far the most dominantly used reactor in the world, with the second most (the boiling water reactor) having "only" 80 in operation. Construction of PWRs diminished greatly after the Three Mile Island nuclear accident, mainly as a result of public support becoming weaker.

Their use on naval vessels and nuclear ships are of extreme importance to various militaries around the world, as nuclear power allows these ships to run for very long periods of time without the need to refuel. PWRs make a good reactor for these ships since they have a high specific power (high power for their mass) due to their use of high pressure. This allows the reactors to be fairly compact, especially with the use of highly enriched uranium.

Characteristics

Fuel

Pressurized water reactors must use enriched uranium as their nuclear fuel, because of their use of light water. This is because light water absorbs too many neutrons to be used with natural uranium, so the fuel content of fissile Uranium-235 must be increased. This is done through a uranium enrichment processes, in which the concentration of Uranium-235 is increased from 0.7% to around 4%.

The enriched uranium is packed into fuel rods, which are assembled into a fuel bundle, as seen in Figure 3. There are about 200-300 rods in each bundle for a PWR, with a large reactor containing 150-250 bundles in their core. This corresponds to 80-100 tonnes of uranium.

The bundles are arranged vertically in fuel tubes within the core. As the fuel is "burned" in the reactor, its density gradually increases, resulting in small voids to develop inside the fuel tube. These void spaces can cause a problem because high pressures could cause stress to the tubes, increasing the likelihood of a rupture. To avoid this problem, the tubes are pressurized with helium at about 3.4 MPa. As fission gas products accumulate over the fuel's lifetime, the pressure gradually balances with the high pressure of the core.

Figure 3. A nuclear fuel bundle for a PWR.

Coolant and Moderator

As mentioned before, light water is used as the coolant and moderator for a pressurized water reactor. Light water is much more abundant than heavy water, as it makes up 99.99% of natural water.

Light water does not make as good of a moderator as heavy water or graphite as a result of its relatively high absorption of neutrons. However its use as a moderator makes for an important safety feature; if there is a loss of coolant accident (LOCA), there will also be a loss of moderator causing the nuclear chain reaction to stop. Also if the moderating water overheats and turns to steam inside the bottom reactor core, there will be less moderator and therefore the chain reaction will stop.

Pressure, Temperatures and Water flow

Figure 4. The inverted U-tube bundle in the steam generator of a PWR.

As the name implies, the water in the reactor is pressurized. This is due to the fact that as the pressure gets higher, the boiling point of water increases with it. This means that at high pressures the water can operate at extremely high temperatures without boiling to steam. This is important for the reactor as higher pressures allow for greater power output and higher thermal efficiency. The pressure is maintained by the "pressurizer" (Figure 4), which acts to stabilize pressure changes caused by changes in electrical load.

Water enters the reactor at 290°C, and by the time it exits it is at around 325°C. In order for it to remain a liquid at these temperatures, the pressure must be 15 MPa, or about 150 times atmospheric pressure. By keeping the water in liquid form, the control rod system is simplified as they are able to be placed in from the top, rather than from the bottom like in a boiling water reactor. Therefore if the power is lost in the plant, the electromagnetic system holding the rods will give out, and gravity will cause the rods to fall into the core, stopping the reaction.

The hot water flowing from the reactor flows through inverted U-tubes (Figure 4) which acts as a heat exchanger, heating up a secondary loop of water in what is called a "steam generator". This secondary loop is at a lower pressure so it is able to boil to steam, which then passes through turbines in order to generate electricity. Large reactors have up to 4 steam generators, each of which may be larger than the reactor itself.

The basic operation of a PWR can be seen below.

Figure 5. The basic cycle and water flow of a PWR.