Technology Jargon Series: Thermal vs Fast

There are so many phrases, acronyms or words that have unique meanings in the nuclear industry and it, like most other specialized industries, unfortunately, seems impenetrable to the layman thanks to this jargon. In this series of posts, I’ll attempt to peel back some of the most common jargon and hopefully allow people to have enough information about this field to start knowing which questions to ask.

Today we are going to take a look at the differences between the most fundamental distinction in how nuclear reactors are designed. What is a Thermal Reactor and what is a Fast Reactor?

At it’s most basic, the difference between a fast reactor and a thermal reactor is how fast the neutrons are moving in the core. Where a fast reactor uses neutrons that move at the same speed that they have upon being released from a fission event of an atom, a thermal reactor uses neutrons that have been slowed down to approximately the same speed as all the other atoms in the reactor core. 

So why is there this difference? It’s because of something called the Neutron Absorption Cross-Section of an element. It determines how likely an atom is to absorb a neutron that hits it at a specific energy, and some elements prefer fast neutrons to thermal neutrons. This preference for specific kinds of neutrons affects a reactor “neutron economy” which is the balance of how many neutrons get absorbed by elements that will fission and create more neutrons (we’ll call this F for Fission neutrons),  plus how many neutrons get absorbed by atoms that do Not fission (we’ll call this P for Poison neutrons as these elements are known as reaction poisons), compared to how many neutrons are produced by a fission event (and finally C for Created neutrons). Ideally, you want P to be as close to zero as possible as this means that you aren’t wasting any more neutrons than absolutely necessary.

One of the ways of minimizing the number of lost neutrons is by having the neutrons move at a speed (known in the industry as “moving in a spectrum”) that these reaction poisons cannot absorb. Which reaction poisons are present in a specific reactor is mostly determined by what element you are using as your power generating fuel such as uranium plutonium, thorium, or even americium. Each one will produce a different batch of reaction poisons which makes designing around them something that requires a great deal of thought and planning beyond simply what spectrum of a reactor to use. 

Now you may be a bit disgruntled with me at this point because I said I was going to explain one bit of jargon in this post but instead I added in what seems like 5 more and I’m sorry. But just like falling down a Wikipedia hole, one thing just leads to another which leads to another and so on Ad nauseam. But I only have one more new word to add and it has to do with how we actually separate thermal reactors from fast reactors. Since all neutrons that are initially emitted from the fission of atoms are considered fast, we need a way to slow them down to make them into thermal neutrons, and we do that with Moderators.

Moderator is a word to describe any material that is fairly light and has a very low chance to absorb a neutron at any velocity. This allows the neutrons to ping pong off the moderator and loses speed with each collision until they are in the appropriate spectrum. If you have ever heard of someone talk about control rods in a reactor, those are moderators that can be raised and lowered into a reactor to control how slow the neutrons in the reactor are going. Common moderator materials are water, heavy water (cheerleading for CANDU reactors from the Canadians in the audience), graphite and boron. Most low atomic number materials can work in a pinch (except lithium metal, it does some fun stuff with neutrons that aren’t very conducive to safe operation of a nuclear reactor)

But what’s the point of using these different spectrums? Why not just use the fast spectrum if the thermal spectrum costs more due to having to have moderators and control systems for them. The answer is two-fold. First, fast reactors are slightly more temperamental than thermal spectrum reactors and require more careful control over the operation of their facilities. Or at least they were/did back in the early days of the nuclear age before learning from accidents and general improvements in safety give them enough redundancies and back up safety mechanism as to make them nearly indistinguishable from thermal reactors. All of the mechanisms for shutting down a thermal reactor work just as well on a fast reactor. The only real difference is the possibility of worst case scenarios.

If you have a complete and total meltdown of a thermal spectrum reactor, the reaction will most likely stop once the molten core gets away from the core and into the containment vessel because the fast spectrum neutrons are the wrong kind to continue the reaction of the fissile elements being used at the reactor. Whereas a fast reactor has the possibility to continue using these fast neutrons to keep the reaction going even after exiting the reactor core into the containment vessel that all reactors have now. Now, remember, this is an absolute worst case scenario so it is not a valid reason to base the construction of a power plant off of. Especially considering that fast reactors are significantly more commonly used as the design for breeder reactors which is a topic for another day because I promised I only had one more jargon word to explore in this post and I used that up with Moderators. 

If you have any suggestions for future entries in this series leave a comment below, or feel free to point out any inaccuracies I may have had. 

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