A question I have after finishing my annual rewatch of Chernobyl. The simplified reason given for the reactor exploding is that when they SCRAMed the reactor, reactivity went up instead of down because the control rods were tipped with graphite. But the only explanation for why the control rods were tipped with graphite was “it’s cheaper” which doesn’t really explain anything. So why were the rods, which were supposed to slow down reactions tipped with the things that would speed up a reaction?
Graphite slows nuclear reactions. It doesn’t increase them. See, e.g. Open Knowledge Wiki - Graphite in Nuclear Industry
There’s an explanation here, that you may or may not be able to understand:
The role of the graphite section, known as “displacer”, is to enhance the difference between the neutron flux attenuation levels of inserted and retracted rods, as the graphite displaces water that would otherwise act as a neutron absorber, although much weaker than boron carbide. A control rod channel filled with graphite absorbs fewer neutrons than when filled with water, so the difference between inserted and retracted control rod is increased.
The “It’s cheaper” applies to the use of graphite in the first place, not just to the parts of the control rods. Other moderator systems are well known to be better than graphite, but as said, they are also more expensive. They made a design choice intended to save money, which led to a design flaw that caused the accident. Had they used some other system, this particular flaw would not have entered the design.
No, graphite increases nuclear reactions by decreasing the speed of the neutrons. It’s called a “moderator” not because it moderates the reaction, but because it moderates the speed of the neutrons. When the neutrons are slowed down, they are able to cause fission and create more neutrons. When they are going too fast, they pass right through the Uranium without doing anything.
As for why the control rods were “tipped” with graphite, it’s because each rod has an accelerator (graphite) and a brake (boron carbide). The boron absorbed the neutrons, thereby decreasing the reaction. The graphite slowed the neutrons which increases the reaction. The “it was cheaper” comment simply refers to the fact that they’re using graphite in the first place. It wasn’t about the fact they were “tipped” with graphite. It’s that they were using graphite as the moderator instead of heavy water which is much, much more expensive but safer.
ETA: Here is a video that explains it. I’ve started it from the part where he directly answers the OP’s question. https://youtu.be/hIGtTImeYU4?t=462
I really hate that the graphite portion of the control rods is called a “tip”. This gives the impression that there’s a short piece of graphite, maybe ground into a conical shape, sitting atop the boron rods that can stop the reaction.
The graphite portion is 4.5 meters long. That’s almost 15 feet of graphite.
Thanks, that really cleared it up for me.
Except that even ordinary water is also a very good moderator, not quite as good as heavy water, but better than graphite. I’m sure there’s some advantage of graphite over normal water, but it’s certainly not cost, and I don’t know what it is.
There’s other costs beyond that of the cost of the raw materials. Water requires plumbing and increases corrosion in some materials, for example. However, I suspect a very important factor is that a rod can quickly be moved in or out as needed and you can’t move water that fast.
Because in addition to moderating them, water will also absorb a high percentage of the neutrons. That isn’t an ideal quality in a moderator.
No, it is the costs. Not the cost of the moderator, but the cost of the fuel. The higher costs associated with using just water vs graphite is in the Uranium itself. Graphite allows for the use of less-refined, cheaper uranium.
Keep in mind that they also didn’t build a containment building either because it was cheaper not to. The lack of such containment is a key reason why it turned out so much, much worse than other nuclear accidents (and demonstrates exactly why such containment buildings are normally built in the first place). I think that’s clear evidence they considered cost cutting more important than safety when designing it.
I think that absolutely everything about Chernobyl is clear evidence that they didn’t really consider safety, at all, in any step of the process. I mean, if you brought together an expert team of nuclear technicians and asked them to come up with the absolute least safe way to do anything, they still probably couldn’t have gotten as bad as Chernobyl.
Disagree. Look into the Kyshtym Disaster and the Semipalatinsk Test Site. The Soviets were sloppy about radiation beginning, middle, and end.
Ironically, they were doing a safety test. They wanted a way to power the water pumps if the main power went out and starting the diesel generators was too slow. The test itself wasn’t the problem. The problem was how they executed it.
And to be fair, they thought that the worst case would be that they would press the scram button (AZ-5) and everything would shut down. The operators had no idea that under the conditions at Chernobyl the scram button would cause an explosion.
The way they presented it on the Chernobyl TV show was that the rods had graphite tips, so I don’t blame folks for thinking that the graphite section was small. In reality, when you pressed the scram button the rods would move at about a foot per second or so, which meant it took a good 20 seconds or more for the rods to fully insert.
A grad student with an axe and gravity wasn’t good enough for them? I mean, seriously, if you have a scram process that takes 20 seconds to take effect, you don’t have a scram process. Nothing in a nuclear reactor allows for that kind of timescale.
Right? Whatever happened to 32 ft/sec^2? Were the control rods designed to come up from the bottom?
The space between the fuel rods in the reactor would be filled with water. I don’t know how much space there is between fuel rods, control rods, and other items in the reactor to provide escape paths for water as the control rods descend, but if there’s not much room for water to get out of the way, then you need something besides gravity to force those rods down and make the water get out of the way.
Hank Green’s video is great, but I like to pair it with Scott Manley’s video which goes into a bit more detail.
