Learn how to use concept maps to make learning more fun

Nuclear reactors - Types

The table below shows the main types of commercial nuclear reactors and their characteristics:

MAGNOX

The MAGNOX is a design created in the UK which is very different from designs used in most places. The British like to have their own designs and plans, like driving on the opposite side of the road and things like that…
This reactor uses CO2 as the coolant and graphite as the moderator (as seen on the table). It is curious that CO2 is needed in a nuclear reactor whose main advantage is to avoid CO2 emissions… The heat is transported by CO2 to the steam generator, where it is used to boil water and produce steam, which flows in a separate circuit leading to the turbines.
The use of CO2  cooling  is a safety improvement over the previous British designs like Windscale, which used air and caught fire. There is no possibility of a graphite fire when there is no air (and consequently oxygen) around.  Also, there is no need for water in the MAGNOX, so that the risk of a steam explosion is discarded.
Another advantage is the use of natural uranium, in the metallic form, so that no expensive enrichment is needed. That is possible because of the special alloy, used to hold the fuel pellets inside the reactor core, composed of Al, Mg and other metals. It is called MAGNOX (and lend its name for the whole reactor design) and has the important property of being quite transparent to neutrons, so that it doesn´t disturb the chain reaction by absorbing neutrons that are needed. The problem of using metallic uranium is that its fusion point is only 650 degrees Celsius so that the core temperature cannot get too close to that.
The main disadvantage of the MAGNOX design is that, because of the relatively low operating temperature, its power density is low and as a result a large reactor core must be used, what makes the project more expensive.
Another  disadvantage is the fact that the MAGNOX alloy deteriorates in water. Spent fuel is stored in a water poll at the power plant so that its level of radioactivity decreases before it is transported, in order to avoid hazards to the people involved in the process. Because MAGNOX cannot stay in the water pool,  it must be transported and reprocessed  when emitting high levels of radiation.

AGR

The Advanced Gas Reactor design is an improvement over the MAGNOX design. It is still cooled by CO2  and moderated by graphite, but the MAGNOX alloy was substituted by stainless steel and the natural uranium fuel was substituted by enriched  UO2 (uranium oxide). The use of stainless steel and UO2  allows the reactor to operate at higher temperatures (alleviating its main disadvantage that was the low operating temperature). Enriched fuel was needed because stainless steel is not as transparent to neutrons as MAGNOX was, and the use of enriched fuel compensates the loss of neutrons.

CANDU

The Canadian Deuterium Uranium design employs heavy water as a coolant. Deuterium is an isotope of hydrogen, weighing twice as much (it has a neutron in addition to the proton in the nucleus). Water made with deuterium is heavier than water made with “normal hydrogen” and it is called heavy water. A tiny proportion of the water we normally see is heavy water and it is expensive to separate the 2 kinds in order to have a large amount of heavy water to fill the CANDU reactor. This is compensated by the fact that this reactor design uses natural uranium which is a relatively cheap fuel because no enrichment is needed. That is possible because heavy water is much more transparent to neutrons than “normal” or light water.
Because neutrons hardly interact with the heavy water, a moderator is needed and it is graphite.

The heavy water, which is kept at high pressure to avoid boiling, transports heat to the steam generator.

An advantage of this design is that it permits online refuelling, which means that the fuel elements can be replaced without shutting down the reactor. A disadvantage is that the core is much larger than similar reactors that use light water and don´t need the graphite, like the PWR’s.

RBMK

This was the first type of reactor produced in the USSR and it uses pressure tubes, where the water flows, inside the graphite blocks., and then to the steam generators. More details of this type of reactor are described in the Chernobyl accident chapter.

PWR

The Pressurized Water Reactor is the most widespread design in the planet. It uses water (light water or “normal” water) for cooling and moderation, so that no graphite is needed (like in the designs above). This fact allows for a high power density and as a result the size of the reactor core can be very small and be installed in submarines, aircraft carriers, ice-breakers and other ships. In fact this reactor design was first developed to be employed in nuclear submarines.
Because of its small size it can be assembled in factories and it is produced by Westinghouse and Babcock and Willcox in the US, by Framatome in France and by Kraftwerk Union (KWU) in Germany.
The main characteristic of this design is that water circulates in 2 circuits. The primary circuit goes through the reactor core and is kept under pressure to avoid boiling of water. This water exchanges heat at the steam generator, where another circuit carries steam to the condensers. Then water is again liquefied (using cooling towers or sea or river water) and return to the steam generator.
The reactor core is made of thick metal and is very resistant. Problems normally happen in other components, like the steam generator. For this reason, the next reactor type excludes the steam generator.

BWR
Boiling Water Reactors promote the boiling of water inside the reactor core, so that no secondary circuit or steam generator is needed. Hence it has the advantage of reducing capital costs but it runs the risk of carrying radioactivity to the turbines and as a result workers in this kind of plant normally receive higher doses of radiation than in the case of the PWR plant.

This type of reactor is produced by General Electric in the US and also by ABB Atom in Sweden.

© Ricardo Esplugas. ricardochemistry@gmail.com