01:03:24 · Isaiah Taylor, Ty Morse
We use that heat for useful things. Now, the danger is that an exponential growth function is exponential, and so you need that curve to taper. Right? You need to get up to a certain rate of fission and then go no further so that you don't have an exponential growth of heat. So that's the first principle of nuclear safety is reactivity control. Now reactivity control has many factors, there are lots of different ways to control the rate of reactivity, but in traditional nuclear power plants, the control rods are the main way to control reactivity. I I hesitate to to talk negatively about the traditional nuclear industry because the fact is, traditional nuclear is the safest form of energy on earth. Even with all of the flaws I'm about to describe, it is still the safest energy on earth. But I think we could do a lot better. [01:04:14] S0: In
01:04:15 · Isaiah Taylor
[01:04:15] S1: traditional nuclear, the control rods are really, really important. Right? If you run, you know, take those rods all the way out, and you're not able to put them back in to shut the reaction down, the plant can get into a very dangerous state where you're producing much more energy than you can safely handle, and that can lead to an explosion. Now we do a lot of engineering and they've done a lot of engineering to make sure that that's not possible, and they use other effects like Doppler broadening in u two thirty eight to moderate that. But it's still, you know, a piece of engineering that you have to treat very carefully and get right. This type of reactor is actually totally different. We do have control rods here, but the control rods are actually not for nuclear safety, they're for plant shutdown. So this is a really interesting thing. The way that we guarantee that this reactor maintains a reactivity level that is not going to result in a runaway reaction or a meltdown is actually through the inherent physics of the plant itself. And how that works is that as uranium two thirty eight heats up, two thirty eight by the way is the non fissile isotope of uranium, so there's two thirty five and two thirty eight, and two thirty eight is the one that doesn't split, And there's always a bunch of two thirty eight in every reactor, this is what uranium enrichment is, you enrich up to a certain percent, but not all of it. And as uranium two thirty eight heats up, it actually captures neutrons more effectively. And so there's actually this natural response curve where the hotter the reactor gets, the less reactive it gets. And so it's a self regulating principle. And specifically in a graphite reactor, graphite has really, really high thermal inertia, and it's really good at spreading that heat out, it's thermally conductive. And so when you have this growth of neutron flux and you have this growth of heat, that heat rapidly gets spread out and you have this moderating effect in the Doppler broadening, and then actually the graphite itself also becomes worse at scattering neutrons back into the core. So what you call this is you have strongly negative thermal feedback of reactivity, and what that means is there's a very strong negative relationship between temperature and reactivity.
01:06:28 · Isaiah Taylor
As the reactor core gets hotter, it gets significantly less reactive, and that's all physics. We don't have to do anything in the plant control to make that true, it just happens because of physics. So, again, we have control rods to make sure that we can shut the reactor down, but if the control rods broke for whatever reason and got frozen and, you know, they're gravity fed, so, you know, if the plant turns off or loses power, they'll just drop it in the core. But let's say for some crazy reason their controllers get stuck up, that is not going to lead to a runaway event. So that's the first aspect of nuclear safety. The second aspect of nuclear safety is meltdown. So most people don't know this, but meltdown is actually something that generally happens after a reactor has been turned off. So it's a post shutdown meltdown, this is what happened in 3 Mile Island in Fukushima. A meltdown is when you've already turned the reactor off, the rods are down, but there's still heat being produced in the core, and that's because after uranium splits, you still have recently split daughter products, back to these fission products, that are unstable isotopes of various atoms, and they are themselves still decaying into other things, and they produce heat when they do that, and so after you turn a reactor off, you still have about 7% of the active heat production still present in decay heat, and it goes down to about a percent and it kind of tapers off over about a twenty four hour period. And so immediately after shutdown, in a traditional nuclear power plant, in order to prevent that heat from just building up to an unacceptable level, you have to keep running the cooling loop. So in a normal nuclear reactor, after shutdown, you actually keep running the pumps, and you keep pulling that heat out for about twenty four hours after shutdown, And that's the condition of failure that leads to what we call nuclear meltdown.
01:08:20 · Isaiah Taylor
When those pumps fail, for instance, in Fukushima, the backup generators were flooded from a tsunami, right, the cooling pumps fail, the heat starts to build up because you still have fission product decay heat, and eventually those temperatures exceed the structural temperatures in the core, and you have any number of things happen including the pressure boundary can get breached due to high temperatures and you can have steam escape that's radioactive and all these sorts of things. So how do we avoid that? Well, when I said at the beginning that this plant is a 100 times safer than existing nuclear power plant, what I mean is, we already talked about thermal runaway, right, so this is a very strongly negative thermal feedback of reactivity, but it is also incapable of meltdown through decay heat, and what that means is there's a certain amount of decay heat in the core, but the reactor's also just really small, and it's made of graphite. And the fact that it's small and made of graphite are massive advantages. The first advantage is being made of graphite. Graphite has very high thermal inertia. So as this decay heat gets released from the fission products, it starts heating up the graphite, and it turns out that most of the decay heat is actually just going to get absorbed in getting the graphite hotter, because it takes a huge amount of energy to heat graphite up, and so a lot of the decay heat in a graphite reactor gets absorbed with just making the graphite hotter. By the way, graphite has a melting point, it actually doesn't melt, it sublimates, but it has a sublimation point of around 3,000 degrees Celsius. So it can get extraordinarily hot. The other important aspect of this is that we use a fuel called triso. So Triso is a particular fuel that can get extremely hot without compromising itself. So normal nuclear fuel can't get that hot before it starts to crack and leak and eventually burst, and you have fission products leaking around inside the reactor. Triso can get very, very hot before it gets compromised, over 2,000 degrees. And so the combination of these things means in a shutdown scenario, the core can just get really hot, and that's actually just fine, that's okay. And a lot of the k heat is taken care of by just allowing the core heat up. And then the last thing is the size of the reactor.
01:10:35 · Isaiah Taylor
The reactor being very small means that we have a high surface area to volume ratio. As a cylinder gets bigger, the walls of a cylinder, the surface area of a cylinder scales with the square, whereas the volume of cylinder scales with the cube. Right? So what that means is a really big cylinder has a small surface area and a huge volume, and a small cylinder has the opposite. Right? So what this is behind us is a small cylinder. What that means is we actually have a lot of surface area, and ultimately the heat from the decay products just escape through the walls of the vessel passively. Right? They just get convected and radiated from the walls of the vessel. And the combination of all of these three factors means if we were to have a circulator shut off of our reactor, exactly like Fukushima, what would happen is that the core would slowly heat up, heat would start escaping out the sides of the vessel, it would get to an equilibrium temperature, which is below the temperatures at which nuclear fuel would be compromised, or where the graphite would be compromised, and then it would start to taper off. And what's awesome about Valar is that that statement is not a theoretical statement. That is something that we've tested in real life. We had this exact reactor built exactly as it is in Los Angeles, we even built a fake building around the vessel to mimic this nuclear shield and to mimic the Citadel, and we insulated it with three times the amount of insulation that you would have in this concrete citadel, and we did exactly that. We actually ran the plant at full temperatures for about a week, and then we actually have a video of this, we turned off all safety systems. We just shut everything off, and we had temperature sensors inside the core and on the exterior of the vessel, and we said, let's see if what we've designed for actually happens. And exactly what I said happened, the temperature rose, it reached an equilibrium point where the temperatures are very stable, where there's heat being slowly, passively removed from the vessel walls, and then over about a two day period, the heat, the temperature fell again. So really what all of this leads to is that when we talk about nuclear safety in the terms of the plants that Valar Atomics builds, it's not engineering safety, it's physics safety. Right? The safety of our plant comes from physics.
01:12:53 · Isaiah Taylor, Ty Morse
It comes from the basic choices that we've made in the physics and materials and geometry of our plants, not whether or not a single pump works or a single valve works or does it get flooded or any of these things. [01:13:05] S0: So there's this amazing chart that I absolutely love looking at, which is basically the launch cadence over time, And you see effectively all of these different countries competing and The US kind of like tapers off in the early two thousands. [01:13:17] S1: Yep. [01:13:17] S0: And we stopped launching. And then suddenly you have SpaceX and they invert this entire graph and suddenly like 95% of all launches or something globally are just SpaceX launches. Yeah. And then there's, like, Rocket Lab in China. And I kind of think this is the same position that we are in right now, you know, twenty years ago is exactly what we're in right now, where there's 30 reactors getting built in China. There's, like, two getting built in The US. Yep. Except for maybe there's three now. [01:13:41] S1: Yep. [01:13:41] S0: How does the scale up look like for your reactors to get to a point where you're actually matching or exceeding the SpaceX growth rate