The Reactor Zoo
Yo, Dr. Z! So I keep hearing about different types of reactors – pressurized water, light water, heavy water, boiling water, and more – and I guess I don’t understand what makes each one different from the others. Can you help me out here?
Yo, Questioner – you came to the right place! And you’re right – it can get a bit confusing, so let’s see if I can help to shed some light on the matter! I’ll say, too, that there are even more types of reactors than you mentioned – there’s also high-temperature gas-cooled reactors, liquid metal fast breeder reactors, liquid salt reactors, and still more – enough that this might take two columns. But let’s get started! Oh – also, you might want to start with this piece I wrote a while back about how reactors work in general (https://www.ntanet.net/nuclear_reactor_fission_and_criticality.html as well as a piece from the American Council on Science and Health’s website – https://www.acsh.org/news/2021/05/05/let%E2%80%99s-build-nuclear-reactor-15526), so I won’t go over the basics again here beyond the big picture:
- Water is very good at slowing down neutrons
- Uranium atoms fission when struck by a slow neutron
- When a uranium atom fissions it gives off additional neutrons
- If one of those neutrons causes another fission then the reactor has a self-sustaining chain reaction – it is said to be “critical”
- Fission gives off energy, which heats the fuel, which heats the cooling water (the coolant)
- The hot coolant can be used to make steam, which is used to turn turbines to make electricity.
And this is where the similarities end. So let’s see what some of the differences are, one reactor at a time. Also – please note that the main differences in the types of reactors I’ll be going through are how the neutrons are slowed down (or moderated), and where the steam that drives the turbines is formed. Water (both light and heavy) and graphite are different media for moderating neutrons while pressurized water and boiling water reactors are two fundamental categories of water-moderated reactor plants.
Light- and heavy-water reactors
Neutrons are slowed down best when they bounce off of something that has about the same mass; since neutrons and protons both have a mass of about 1 atomic mass unit (amu), hydrogen (which is a single proton) has very nearly the same mass as a neutron and water, with two hydrogen atoms per molecule, makes for a great moderator. In addition, water does a good job of cooling the fuel to keep it from overheating.
It turns out that hydrogen comes in more than one “flavor” – the great majority of hydrogen has just a single proton and an atomic mass of 1 amu, but a small fraction contains a neutron as well and has a mass of 2 amu. For a variety of reasons, deuterium (as the heavier hydrogen is called) is especially effective at moderating neutrons; reactors that use this “heavy water” are able to sustain a critical chain reaction using less uranium than a comparable reactor using regular (“light”) water. Among other things, this means that, while a light-water reactor needs the uranium in its fuel to be slightly enriched in the fissionable U-235, a heavy-water reactor can operate using unenriched (natural) uranium. There are some commercial power reactors that use heavy water (Canada’s “CANDU” – Canadian deuterium – reactor is one), but most heavy-water reactors are used for making plutonium for nuclear weapons (more on that in the following section).
A quick summary is that water’s two hydrogen atoms make it an effective moderator; light water is common and inexpensive, but requires enriched uranium for the reactor fuel…most of the world’s reactors are moderated and cooled using light water. Heavy water is more expensive, but it doesn’t require the use of enriched uranium fuel.
Graphite – one form of elemental carbon – is another super-efficient moderator; like water, it will slow neutrons to energies that permit fission to occur efficiently but, unlike water, it won’t carry away much of the heat of fission. So graphite-moderated reactors still need water to be run through them – the complete name is graphite-moderated, water-cooled reactor. The very first reactor to achieve criticality (Fermi’s CP-1 reactor, located at the University of Chicago) was graphite-moderated, as was the Hanford “B reactor” used to produce plutonium for the Manhattan Project; although perhaps the best-known graphite-moderated reactor was the reactor that exploded at the Chernobyl nuclear power station in Ukraine in 1986.
Because graphite, like heavy water, makes it possible for natural uranium to sustain a critical chain reaction, there is no need for uranium enrichment. While this is only a minor advantage in the economics of reactor design and operation, it’s more important when it comes to producing plutonium. Specifically, the plutonium is produced when U-238 (which comprises 99.2% of the uranium atoms on Earth) captures a neutron to become U-239 and then decays to form Pu-239. More U-238 atoms (which we find in unenriched uranium) means more Pu-239 is produced – thus, plutonium production reactors tend to use graphite or heavy water moderators. And, in fact, even though the Chernobyl reactors produced electricity for the nearby area, they were built primarily to produce plutonium for the Soviet nuclear weapons program.
Other coolants (e.g. liquid metal, molten salt)
Water isn’t the only way to keep a reactor cool, and we’ve already seen it need not be the only moderator. In fact, water isn’t very thermally efficient when it comes to transferring heat from the fuel to make steam. Not only that, but to operate effectively at the high temperatures needed for good thermal efficiency, water has to be kept under pressure (we’ll talk about this a bit below). On the other hand, liquid metals and liquid salts can transfer more heat more efficiently than does water and they can both be operated at atmospheric pressures – a small leak need not be a catastrophe the way it can be in a water-cooled reactor.
Metals that have been used include liquid sodium, lead-bismuth, sodium-potassium, tin, and mercury. One problem is that some of these metals (especially sodium) will ignite if exposed to air and can explode if they come in contact with water – this latter issue led to the US Navy’s replacing an experimental liquid sodium submarine reactor with a more conventional water-cooled plant in the 1970s. Another issue is that some reactive metals can be corrosive, attacking the pipes or even the fuel itself.
Molten salt offers similar thermal efficiency advantages and they, too, can be operated at low pressures. Unfortunately, they can also be corrosive; in addition, they can solidify if not kept warm when the reactor is shutdown (as can the liquid metal reactors).
Pressurized water reactors (PWR)
The reactors I learned about and operated when I was in the Navy’s nuclear power program were pressurized water reactors. What this means is that the water is kept at a high pressure to make sure the water in the reactor core doesn’t boil – the reason for this is that, while water is fairly effective at carrying away heat, steam is much less so, and steam in the core can lead to the fuel melting down. The way the reactor core is kept under pressure is by having a special tank with powerful heaters connected to the reactor plant – this tank is called the pressurizer. The pressurizer is kept only partially full of water and the rest is filled with steam – the hotter the water is kept, the higher the pressure in the reactor plant. Pushing the water through the reactor’s core is accomplished with reactor coolant pumps – powerful pumps that can reliably move a lot of water, absorbing the heat from fission.
This is great for the reactor – but we still need to make the steam to turn the turbines. And, come to think of it, we also need to transfer heat from the hot reactor coolant so that it can pick up more heat from the reactor fuel; this happens in the steam generators. The coolant is pumped through thousands of tubes; on the other side of the tubes is the water of the steam plant (also called the “secondary side”), which is at a lower pressure than the reactor plant. The lower pressure makes it possible for the water on the secondary side to boil, producing steam to drive the turbines. After giving up its energy the water is cooled to near room temperature and has condensed to form water that can be pumped into the steam generators to go through the process again.
The advantage of the pressurized water reactor is that the water that passes through the reactor core never comes in contact with the water that goes through the turbines – the primary and secondary sides are separate and the secondary side is neither contaminated nor radioactive. The primary disadvantage is that PWRs are more complex than boiling water reactors – the pressurizer, the secondary loop (including the additional pumps), and other odds and ends make for a plant that’s more complex than the simpler (but more-contaminated) boiling water reactors.
Boiling water reactors
And this brings us to the other side of the coin! There is no steam generator in the BWRs; instead, the water boils as it’s passing through the reactor’s core, and the plant is designed so that the fuel can experience this without melting down. The good and bad about BWRs are the opposite of the PWR – they’re simpler and less expensive and there’s less to go wrong, but contamination from the reactor can make its way through the entire steam plant. On the submarine I knew it was unlikely that I’d run into any contamination at all in the turbines, condensers, or pipes and bilges on the steam side of the plant – if we’d had a BWR I’d have expected to run into contamination everywhere. I should hasten to add that this does not mean that either type of reactor plant is better or worse than the other – both are designed to be exceptionally safe and both can be operated safely – they’re simply two different approaches to solving the same problem.
Fast breeder reactors
Among the ways to characterize nuclear reactors is the speed of the neutrons that are prevalent in the reactor core. In general, relatively slow-moving neutrons (also called “thermal neutrons” because they are at the same temperature as the fluid they are in) are more effective at causing fission, so most nuclear reactors use slow neutrons to cause fission. Fast neutrons will cause fission, but not as efficiently as thermal neutrons. Fast neutrons, on the other hand, are readily captured by U-238. More on this in a moment.
Isotopes other than U-235 will fission, and Pu-239 is just as good at fissioning as U-235. Pu-239 is created when U-238 captures a neutron (forming U-239) and undergoes two beta decays. Pu-239 is formed in any reactor, but breeder reactors are designed to maximize Pu-239 production. Then, when the reactor is refueled, the plutonium is removed from the spent fuel and used to make fuel for other reactors. And, since U-238 is so much more common than U-235, a breeder reactor can produce more fuel than it uses up.
“Normal” nuclear reactors use water or gas (such as helium) to moderate the neutrons, and the thermal neutrons efficiently cause nuclear fission (in such reactors, the moderator is also the coolant). Pu-239 is formed, as are heavier isotopes of Pu, but these fission as the reactor operates. In fact, a nuclear reactor can derive a significant amount of energy from plutonium fission. But the fact that this plutonium is fissioned reduces the amount in the fuel. To maximize plutonium production, for nuclear weapons production or for making further reactor fuel, it helps to maximize the amount of plutonium that is formed and to minimize that which is fissioned.
One way to do this is to reduce the number of thermal neutrons in the core by using a coolant that is not an efficient moderator. Liquid sodium, molten salt, and similar materials fulfill this function admirably.
A fast reactor is one that uses fast neutrons to cause fissions. A breeder reactor is one that produces more plutonium than it uses in fission. If commercial reactor fuel contains 6% U-235, then 94% of the fuel is U-238; 94% of the atoms in the fuel are potential neutron capture targets and potential future plutonium atoms. This is one reason why a breeder reactor can generate more fuel than it uses; in some cases, up to 30% more.
One problem with fast breeder reactors is that the plutonium produced is a nuclear proliferation hazard. Another problem is that, to extract the plutonium, the fuel must be reprocessed, creating radioactive waste and potentially high radiation exposures. In addition, in the US, spent fuel reprocessing was halted, making the use of breeder reactors problematic.
I think this is a good place to wrap up – there are more reactor types (especially the gas-cooled designs and some of the newer generation plants), but we can save them for later. I hope this starts to answer your question – and the next installment ought to finish the job.