Last time in this series, we wrote about how SMRs deal with heat, and how their small size makes it possible for them to be potentially immune to the dangers of meltdowns. While true and fundamentally accurate, that explanation left a lot to be desired in expanding understanding of how heat is actually moved around a reactor. This is because it dramatically simplified the topic of interest into basically one uniform lump of homogenous material. So while it was able to get the correct and pertinent information across, it skimped on the subtlety that would allow readers such as yourself to apply the information you learned to more common and varied experiences so you could see how the same rules affect not just a nuclear reactor but even simple, everyday items such as your laptop or toaster. That is what we are going to try and do here, so that you my dear readers can feel confident in understanding not just the Why, but also the How a reactor operates and controls itself.
Radiation is Energy, Energy is Motion, Motion is Heat.
At the most derivative, there really isn’t much difference between a toaster and a nuclear reactor. They are both devices that use a heat source to produce an effect via transfer of that heat through a medium by one or more mechanisms. Really by that description, the biggest difference between them is that the nuclear reactor’s heat is derived internally, while the toaster has to be plugged into an outlet to get the energy to produce its heat. Though some might argue that there is a massive difference between nuclear radiation and the heat from the heating coils in a toaster, I beg to differ. As I’ve described in several articles before, radiation is simply other, more common things that have higher energy than normally experienced.
Alpha particles are just fast Helium, Beta radiation is just fast electricity, and gamma rays are just infrared scrunched up real tight because it’s already moving at the fastest possible speed so it just crammed more of itself into the same space. And as they travel they bump into all the other matter around them, and with every bump, they lose some energy, giving it to whatever they bumped into. And fundamentally this is how all heat transfer occurs, no matter which macroscale mechanism you talk about.
Now that we know what it is, how do we move it?
When we talk about ways to move heat, generally we mean one of three categories. Conduction, which is when the atoms bumping into each other are in direct contact such as in solids that are attached to each other or part of the same object. Convection, where heat is transported into a fluid that changes the density due to the temperature which causes a flow to occur in the body of fluid. And radiation, where the atom shakes so hard a little photon pops out and flies away through the void until it hits something else that it gives all that energy to.
But since all of these categories are based on the transfer of energy by collisions, either between atoms directly or via emitted photons, there is a common aspect that acts as a baseline method of knowing how good something will be at transferring heat. And if you remember the first part of this series (it was several months ago so I’ll forgive you if you don’t) you’ll know that it is the ratio of the surface area to the volume of the object in question. This is because volume denotes how many atoms there are available to store heat (and how much heat those atoms can store is more properly considered as the Heat Capacity of the volume) in their vibrations and motion, while the amount of surface area is the measure of how many atoms are available to transfer that heat Out of the object.
Energy Gradients and Tobogganing
The second most important thing to keep in mind when talking about heat transfer is, ironically, heat. Specifically how hot something is compared to its surroundings, as heat is kind of like a 4-year-old kid with a toboggan. The higher the difference between the top of the hill and the bottom, the faster that little maniac will go, with no regard for how dangerous things might get.
The steeper the hill, the more energy is available to pull from it, but if you have enough then there is no material on the planet that can actually harness it. Thus now is when we start seeing the true artistry required to play with heat and convince it to power our homes and our world. Everything that we use that makes use of heat, doesn’t matter if it’s a toaster, a soldering iron, a nuclear reactor, or a fridge has to be designed with this balancing act in mind. How much volume do you need to hold the amount of heat it needs to function, how much surface area does it need to shed that heat fast enough to prevent overheating, and how hot you can let it get relative to its surroundings to determine how much energy is coming out or going in.
And here’s the best part. Because as we said earlier, and in previous articles, that alpha, beta, and gamma radiation are just more concentrated form of heat, there is no functional changes that happen because of them. The only type of radiation that adds considerations is neutron and that is putting upper limits on the distance that specific objects in a reactor can be from each other, otherwise the reaction wouldn’t sustain itself. Now you can add all the extra considerations on top of these fundamentals that you want, like insulation, or counter flow heat exchangers, or different fluids with different heat capacities and conductivities. Heat transfer can become infinitely complicated (Did you know that adding too much insulation to something can actually Increase the rate of heat loss? See if you can figure out why that might be on your own. Email, tweet, comment or facebook message us for the answer to check your work.) but at the basis of it all is always the considerations of surface area, volume/heat capacity, and temperature difference between the object and its surroundings.
First Principles are First Steps to Understanding
Our world is a vast and complicated thing, with many amazing phenomena that happen every day. How heat moves through it is a fundamental piece of why the world is the way we see and as we understand it more and more we learn more about how to harness the world around us for not just our benefit but for the benefit of the world itself. Knowledge of fundamental components of the world such as this allow more people to constructively participate in the discussions of what direction we should take for our species and our planet.