Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite?

Submitted by Figorama t3_11pzjx4 in askscience

In one episode of the series the protagonist, Legasov, explains the function of the safety protocol AZ5 that forces the boron control rods to descend into the reactor. That boron rods slows reactivity but he elaborates that the control rods are tipped with graphite which accelerates reactivity. The character opposite him in this scene asks "Why?" the control rods are tipped with graphite. He explains that it's "cheaper", but I find that explaination unsatisfying.

It sounds to me like a fireman explaining that the first few bursts from a fire extinguisher will dispense jet fuel before any kind of flame retardant.

Why would the control rods in this reactor be tipped with an accelerant of all things?

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Hiddencamper t1_jc1ir2b wrote

The design of the RBMK is fundamentally backwards. It’s all about the relative values of reactivity.

Coolant (water) goes in the bottom of the RBMK and boils as it goes up. Because this is a graphite moderated reactor, water has less moderation capability than the graphite. This is important because liquid water will reduce your neutron mean free path distance (how far the neutron travels before it is absorbed by something or lost from the reactor). As the water boils, it’s density drops significantly and the mean free path length for neutrons increases.

So let’s put this together. At the bottom of the reactor, you have neutrons which are more or less struggling to find graphite, get moderated, and get back into the fuel, before leaking out or being absorbed without causing fission.

At the top of the reactor, your neutrons have a very easy time getting to the graphite to get moderated and cause fission.

This also means the power generated at the bottom of the reactor is less than the top of the reactor (axial flux tilt is top peaked).

But the top of the reactor has less coolant (because much of the water has already boiled to steam). So the top of the reactor has a tendency to produce more power, with less coolant, which is inherently a risk to exceeding critical power ratio. While the bottom of the reactor, even with all control rods out, has little power production, and is also very sensitive to emergencies which cause rapid voiding since there are typically no control rods down there just to keep the bottom of the core running.

As a result, the RBMK has control rods which come in from the top. Backwards for a boiling type reactor but a necessity.

So what’s the problem here? Where the bottom of the reactor is going to not only barely have any power output, the fuel is going to be wasted down there, it’s more sensitive to certain transients, so what did they do? They put graphite followers on the rods. To help boost the reactivity in the bottom of the core. Yes this is a dumb idea, but on its own it’s not terrible. With the followers inserted in the core, they no longer have positive reactivity to add. They already have “done their damage” so to speak. So if you had a power spike, as the rods inserted, the graphite followers would be pushed down out of the core and be replaced with control rods.

This was a “win win” for this dumb backwards reactor.

Except….. if you ever find yourself pulling the followers out of the reactor, especially if you also have low reactor coolant flow and pressure and other conditions which could cause rapid boiling, and you have low control rod density, then the effect of a scram is to push the followers back into the core and cause a power spike.

Why there weren’t mechanical limits on the control rods equipped with followers or other system interlocks is beyond me. This design “feature” should never have existed without something in place to ensure those followers cannot be removed beyond a certain position. Or better yet, don’t build backwards reactor designs.

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BrightCharlie t1_jc1xudx wrote

>Why there weren’t mechanical limits on the control rods equipped with followers or other system interlocks is beyond me

To be fair to the designers, they did have to override a bunch of automatic and safety features that existed precisely to avoid accidents like that.

I'd argue that what happened in Chernobyl wasn't exactly an accident, because they deliberately put the reactor in a state where bad things would definitely happen -- as they did.

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BullockHouse t1_jc23dm0 wrote

It's definitely a mix of poor design and operator error. In general, if the reactor is beginning to show behavior you don't understand, that's an emergency and you need to execute a safe shutdown immediately. When they got into the xenon pit behavior and didn't know what was going on, trying to power through it without actually understanding what was happening was idiotic.

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Shadeauxmarie t1_jc2rlc1 wrote

“Back away from the reactor. It’ll save itself.” Navy nuclear power program.

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BullockHouse t1_jc3ltwc wrote

The naval reactors are rad. Some very neat architectures on both the US and Russian side, and extremely good service records.

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Stillwater215 t1_jc5caks wrote

Passively safe reactors should be the future of electricity generation. Modern reactors are designed so that the job of the operators is to “fight” the reactor to make it more reactive. If they walk away or are incapacitated, the reactor brings itself into a steady, low-power state. But whenever people think of nuclear power, they only think of Chernobyl…

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Shadeauxmarie t1_jc5hki7 wrote

Common question for Navy nuclear watchstanders: “Everyone dies on the ship. How long will the reactor run and what will cause the shutdown?”

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ThatOtherGuy_CA t1_jc2aq85 wrote

The biggest issue with Chernobyl, which was also showcase in the show, wasn’t that the reactors had a potentially dangerous design, it’s that the Soviet Government hid the flaw from the reactor operators. So to their understanding an RMBK reactor couldn’t possibly blow up. Because the boron control rods would kill any reaction. And they either weren’t aware of the carbon tips, or at least the risks they posed.

So yes, the operators intentionally cooked the Chernobyl reactor to a point where it was a bomb, but they felt safe doing it because they had complete confidence that AZ-5 would kill the reaction. Not act as a detonator.

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Somnif t1_jc2td2d wrote

Yeah, something the show didn't really mention was that Chernobyl Unit 1 had actually suffered a similar (but much less severe) incident a few years earlier. It was not just a known issue, it was a known issue on site! And still, hushed up and hidden.

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Hiddencamper t1_jc2mhcl wrote

Exactly.

Today if you say the words “reactor safety limit”, that’s an inviolable parameter. If a reactor safety limit is exceeded the plant cannot restart without approval (10cfr50.36). And if there is a potential to exceed one, you are in a reportable event (for example is a safety system was found degraded such that it would actuate too late to protect the safety limit).

As reactor operators we are required to know them from memory.

The same level of deliberate caution around those limits likely did not exist with the USSR and the RBMK design, as evidenced by them withdrawing rods as much as they did. When rods are they far out in the RBMK, you not only get a positive reactivity spike on a scram, but you also magnify your positive void coefficient.

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RexStardust t1_jc27u7l wrote

Yes but they did so with the understanding that they had the ability to pull the plug with AZ-5. The designers and/or the overall operating authority knew that you needed to use AZ-5 earlier because of the initial reactivity of those graphite tips, but plant operators had not been informed.

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Y34rZer0 t1_jc4muxv wrote

That sounds fairly Soviet… “Let’s keep the reactor operators as in the dark as possible”

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[deleted] t1_jc2bg0n wrote

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[deleted] t1_jc2cdqt wrote

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[deleted] t1_jc2pjsj wrote

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Sythix6 t1_jc3eo36 wrote

I'd agree with that argument, everyone was warned, everyone still followed bad orders, all because Russia cannot look weak at any time, Soviet era Russia was rampant with these types of accidents, not as globally impactful though, most stemming from lack of quality materials caused by embezzlement from generals and other higher ups.

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pzerr t1_jc4ut9j wrote

Every time I read the events that led up to this I want to yell 'don't do it'. Even though I know the outcome I just feels if I yell loud enough they will hear me.

There were so many steps that led up to this. Had they stopped at any of them, this could have been averted. That design was just a disaster to happen all the same.

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PHATsakk43 t1_jc2698d wrote

You’re leaving out the xenon precluded startup conditions which allowed them to pull rods out farther than they should have.

It’s a pretty good design, economically speaking, as it’s the one reactor design that doesn’t require fuel enrichment or a fancy moderator like heavy water to function.

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Hiddencamper t1_jc2ms2l wrote

That’s true. They wouldn’t have attempted to do what they did if they weren’t flooded with xenon.

To be fair though, all light water reactors can overcome xenon except for the very end of the operating cycle. So you avoid issues related to xenon in most reactors out there which eliminates risk of potential power spikes. And the CANDU design simply doesn’t have enough reactivity to pull through a xenon peak (which is why their reactor protection systems will try to stabilize the reactor at 60% or 2% power when it is safe to do so, to allow the operators an opportunity to keep the unit running).

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PHATsakk43 t1_jc33vnn wrote

You’re answering your own question.

The low reactivity of the RBMK is why the moderator tipped control rods existed. The xenon-precluded startup is inherent in any reactor, regardless of enrichment as the xenon is from fission of U-235 and is a function of time at power (equilibrium xenon) or for a startup, time after shutdown (peak shutdown xenon.)

Repetitive startup/shutdowns that were being performed at Chernobyl would create the same xenon problems with any reactor, at any point in operating cycle if done excessively.

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Hiddencamper t1_jc35h9s wrote

BWRs never are xenon precluded. They always, at all parts of the operating cycle, have sufficient hot excess reactivity to have xenon override capability. They also naturally stabilize spatial and axial xenon tilt based on their design and the boiling boundary effect.

Pwr plants have total xenon override until the last 5-8% of cycle, when they are essentially at max dilution. They do not have natural flux tilt stabilization so the operator has to manually make adjustments to control tilt within limits.

I have personally started up a commercial BWR in peak xenon. It was very weird to have the reactor go critical moving a corner rod from 00 to 04, not see the criticality (power actually appeared to be going down at the time we notched it out), then as xenon burnout started happening we saw only one SRM period on scale. The PPC displays, when you have period in trend mode, you can see an inflection when critical occurs, and we saw the signature only on one instrument which didn’t make much sense. So we stopped pulling rods to watch, as a minute or two later the second SRM started to come on scale, then the third and fourth, as xenon burnout reduced shielding around the SRMs and allowed the core to finally couple. Then reactor period advanced over the next 12-15 minutes to about 82 seconds, when we finally hit point of adding heat and everything stabilized. Not a common evolution and I can see where other operators pulled too far and tripped their units, because you don’t see the core go critical on peripherals for quite a while.

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PHATsakk43 t1_jc3bqm4 wrote

I've worked around BWRs but never been an operator at one (my fleet had a two unit station BWR, while the rest were PWRs.)

I stand corrected in that case. Does make perfect sense when you game it out, as you can basically put a shitload of positive reactivity into a BWR.

I have heard of situations similar to Chernobyl at naval plants (all rods out, waiting on xenon decay, below POAH), but again, that was all stories as I only operated new naval plants as well.

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Hiddencamper t1_jc3czqr wrote

A full power BWR has a void defect around 40% of your total reactivity. When you scram, those voids go away, and you recover all of that reactivity. Voids are dominant in a BWR. The rule of thumb is Doppler 10^-5, moderator temp 10^-4, void coefficient 10^-3. So you always have enough to start back up in a BWR. And actually, especially if it’s a fast restart, more xenon helps a lot with getting to target rod pattern as it’s one of the things that impacts thermal limits and PCIOMR.

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LudSable t1_jc2pnng wrote

After the Chernobyl disaster all the current active RMBK were fitted with safety features and actual shielding in the walls. Better late than ever...

>Following the accident at Chernobyl, all remaining RBMK reactors were retrofitted with a number of updates for safety. The largest of these updates fixed the RBMK control rod design. The control rods have 4.5-metre (14 ft 9 in) graphite displacers, which prevent coolant water from entering the space vacated as the rods are withdrawn. In the original design, those displacers, being shorter than the height of the core, left 1.25-metre (4.1 ft) columns of water at the bottom (and 1.25 metres [4.1 ft] at the top) when the rods were fully extracted.[3] During insertion, the graphite would first displace that lower water, locally increasing reactivity. Also, when the rods were in their uppermost position, the absorber ends were outside the core, requiring a relatively large displacement before achieving a significant reduction in reactivity.[40] These design flaws were likely the final trigger of the first explosion of the Chernobyl accident, causing the lower part of the core to become prompt critical when the operators tried to shut down the highly destabilized reactor by reinserting the rods. The updates are:

>* An increase in fuel enrichment from 2% to 2.4% to compensate for control rod modifications and the introduction of additional absorbers.

  • Manual control rod count increased from 30 to 45.
  • 80 additional absorbers inhibit operation at low power, where the RBMK design is most dangerous.
  • AZ-5 (emergency reactor shutdown or SCRAM) sequence reduced from 18 to 12 seconds.
  • Addition of the БАЗ or BAZ* system,[41] (rapid reactor emergency protection) which would insert 24 uniformly distributed rods into the reactor core via a modified drive mechanism within 1.8 to 2.5 seconds.
  • Precautions against unauthorized access to emergency safety systems.

>In addition, RELAP5-3D models of RBMK-1500 reactors were developed for use in integrated thermal-hydraulics-neutronics calculations for the analysis of specific transients in which the neutronic response of the core is important.[42]

>*BAZ button is intended as a preemptive measure to bring down reactivity before AZ-5 is activated, to enable the safe and stable emergency shutdown of a RBMK.

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Guilherme_Sartorato t1_jc4jg8k wrote

The control rod modifications and additional absorbers reduced the positive void coefficient from +4.5 (far higher than any architecture other than RMBK) to +0.7, hence the need for enriching the uranium a bit more.

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Cthulhu625 t1_jc2sptq wrote

Most of the reactor control rods are inserted from above; 24 shortened rods are inserted from below and are used to augment the axial power distribution control of the core. With the exception of 12 automatic rods, the control rods have a 4.5 m (14 ft 9 in) long graphite section at the end, separated by a 1.25 m (4 ft 1 in) long telescope (which creates a water-filled space between the graphite and the absorber), and a boron carbide neutron absorber section. 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. When the control rod is fully retracted, the graphite displacer is located in the middle of the core height, with 1.25 m of water at each of its ends. The displacement of water in the lower 1.25 m of the core as the rod moves down could cause a local increase of reactivity in the bottom of the core as the graphite part of the control rod passes that section. This "positive scram" effect was discovered in 1983 at the Ignalina Nuclear Power Plant. The control rod channels are cooled by an independent water circuit and kept at 40–70 °C (104–158 °F). The narrow space between the rod and its channel hinders water flow around the rods during their movement and acts as a fluid damper, which is the primary cause of their slow insertion time (nominally 18–21 seconds for the reactor control and protection system rods, or about 0.4 m/s). After the Chernobyl disaster, the control rod servos on other RBMK reactors were exchanged to allow faster rod movements, and even faster movement was achieved by cooling of the control rod channels by a thin layer of water between an inner jacket and the Zircaloy tube of the channel while letting the rods themselves move in gas.

​

To answer the question of why they didn't have more safety features, likely money.

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Blaizzzzzed t1_jc2oyak wrote

Man. All you need are those blue and red cards and it’s like the series. ;)

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Hiddencamper t1_jc35kpz wrote

My wife was watching Chernobyl with me and she said the red and blue cards were the first time it made sense…… she doesn’t like hearing me talk I guess : )

They did a great job in the show with the cards. There’s a little more nuance to the what and why but it was a great explanation that incorporated a lot of technical details.

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chronoboy1985 t1_jc3scp2 wrote

What would the fundamentally correct way to design the reactor if the RBMK was backwards?

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Hiddencamper t1_jc4gdls wrote

I would have preferred to make it a pressurized water reactor similar to a CANDU. Or minimize boiling in the fueled region similar to the ESBWR. The issue is you are voiding your coolant in the reactor. If you never let it boil you don’t have the strong positive void coefficient (and you can design around loss of pressure transients like the CANDU design). Then it doesn’t matter as much if rods go in the top or the bottom.

Honestly the CANDU design is a far better answer. You still have some positive void coefficient but you can design around it.

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Y34rZer0 t1_jc4mn8h wrote

What a comprehensive answer, thank you.
One thing I remember hearing, and I’ve always wondered if it was true, is that water is such a good ‘insulator’ for radiation that you could actually swim around in the water at the top of a modern reactor and not be harmed?

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Hiddencamper t1_jc4sknq wrote

So there are a few different things that we use water for.

Water is a great coolant.

It also makes a good moderator in many designs.

It is an effective shield for radiation sources. About 7 feet of water will reduce the lethal radiation levels in nuclear fuel down to levels we can work under. The spent fuel is typically under 23 feet of water to act as a buffer in case a fuel rod leaks or splits open to act as a dissolving agent for radioisotopes that leak out.

The last part though, is the water can have radioactive material dissolved in it. So yeah it would shield you from the radiation from fuel rods 23 feet deep. But if there are dissolved fission products in the fuel, when you jump in the water those products are now coated on your skin, causing direct radiation impact. If you ingest it or your body absorbs it, you can have internal effects.

So while it’s not going to be lethal like hugging a fuel rod, it’s still harmful and we need to decontaminate you to not only protect you, but keep it from getting out of the plant.

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Y34rZer0 t1_jc4sqey wrote

thank you for your response, sounds like I need to rethink my summer vacation next year 😁

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galqbar t1_jc4lgm7 wrote

What a superb and informative comment. I thought I knew a fair bit about RBMK reactors and the accident but there was a lot of interesting new information here.

Boiling coolant inside of the vessel seems like a Bad Idea when one of the effects is to vary the amount of moderation at different depths in the core.

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Hiddencamper t1_jc4rz0u wrote

We do that in a boiling water reactor. But in that case the water is the coolant and the moderator, so as you boil, you get less moderation, which protects the upper portion of the core. We also “shape” the flux profile by adjusting enrichment and gadolinium content (burnable poisons) in the fuel.

Normally in a BWR, power leaks in the bottom 1/4 of the core, and as you deplete the fuel in the bottom later in the cycle, the water is able to “climb” further up the core before boiling, which improves the moderator in the upper portion of the core. By the end of core life, the power peak is in the top 1/3rd of the core.

So it can work when designed right.

But yeah in nearly all other cases, you want to keep your coolant and moderator in a single phase (for the most part)

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tauofthemachine t1_jc4ravu wrote

>Why there weren’t mechanical limits on the control rods equipped with followers or other system interlocks is beyond me.

I believe the answer to that is in the book "Atomic accidents" By James Mahaffey. Apparently there actually were safety systems like that, but in preparation for the safety test they had a special switch installed which disabled them.

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Hiddencamper t1_jc4rk1f wrote

They disabled the reactor trip system. But still typically with actual reactivity controls you have some other interlocks.

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Ridley_Himself t1_jce1e0z wrote

So, if I'm reading this correctly, the design flaw is that, if you need to insert the control rods to kill a reaction, the graphite tips have to move past the portion of the fuel with the highest rate of reaction?

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Hiddencamper t1_jcewc2g wrote

The “design flaws” are:

Reactor design with active boiling and positive void coefficient, such that your power profile is essentially inverse to your normal control rod position. Additionally you are severely impacted by things like trips of a reactor coolant pump.

No mechanical limits on location of the graphite followers (not just control system limits, but a physical hard stop)

The graphite followers having to move past the fuel is the result of the two above plus the operators making some very dumb decisions.

If those graphite followers were never removed as much as they were, if they essentially stayed in the lower portion of the core, they would have been fine. But when you pull them out far enough, then during the reactor trip they will initially add reactivity.

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Montrama t1_jc1elu5 wrote

I think tipped with graphite is kinda misleading. When we say it like this it feels like a small portion of the control rod is made from graphite at the tip. In reality there is a slightly smaller graphite rod which is connected to the boron rod. So when you raise the boron rod it gets replaced by graphite rod which is also called "Displacer".

Why they have graphite rod than? Two main reasons. One is to increase the efficiency of control rods. When you raise the control rod it gets replaced by displacer graphite rod which accelerates the reaction. When you push the boron rod back, displacer got removed from the reactor and boron rod takes it place and slows down the reaction. So your delta power is much higher between two states and this gives you better control ability over the reaction. Second reason is to increase capacity of the reactor. Graphite accelerates the reaction so increases the maximum power that the reactor can create with same footprint.

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radioactive_dude t1_jc1usah wrote

This is probably a decent representation of the ratio of the boron part of the rod to the graphite "tips".

https://i.stack.imgur.com/Ai7Yp.jpg

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Hiddencamper t1_jc2nqpi wrote

That’s a great picture.

And just so people are thinking about this the right way. From a safety perspective We don’t care about the graphite as long as it is in the fuel region or below the fuel region, because during a scram they go down which means graphite will be exiting the fuel region and control rods will be coming in.

It’s only a problem when those followers are all the way up and partially out. They will raise power below them as they drive down, right into the power peak.

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jobblejosh t1_jc4z2bf wrote

Also gives a great look into why it's named RBMK. Reaktor Bolshoy Moshknosti Kanalnyy; High-power Channel-type reactor; since the cooling water is sent through individual channels as opposed to being in one large pressure vessel.

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CleverNameTheSecond t1_jc2oogs wrote

To me this looks like the equivalent of having a car with just one pedal that handles both the acceleration and the braking depending on how hard you push down on it.

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The_Real_RM t1_jc5vpcw wrote

You just described one-pedal driving in electric cars. We in fact have exactly this

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PHATsakk43 t1_jc27opb wrote

It’s “cheaper” not in the sense of the control rods themselves, but the reactor.

The graphite tips were key to getting an RBMK to operate without either enriched uranium (expensive) or a fancy moderator (heavy water, likewise expensive.)

By putting a couple inches of positive reactivity on the control rod, you can create a localized higher reactivity which can get the neutron flux high enough to “jump start” the reactor. This is necessary in natural uranium reactors, as the amount of fissile U-235 is very low.

While Russia absolutely had the capability to enrich uranium, there are other “benefits” from the RBMK design which make natural uranium reactors preferable, specifically their ability to produce plutonium for weapons. This seemed to have been left out of the discussion in the miniseries as well.

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Accelerator231 t1_jc2t4di wrote

You know the surprising thing I learned today is that reactors can work without enriched fuel. I thought all reactors needed enriched fuel before they can work.

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Crizznik t1_jc30n4k wrote

There are natural nuclear reactors in the world. Pits of natural uranium that get super hot from fission reactions.

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echawkes t1_jc3uari wrote

There aren't been any natural nuclear reactors any more. Over a billion years ago, the natural enrichment of uranium was much higher, because U-235 and U-238 have different half-lives.

The only place a natural reactor was ever thought to have operated was at Oklo, and it can't happen anywhere now.

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Accelerator231 t1_jc2txj0 wrote

>The graphite tips were key to getting an RBMK to operate without either enriched uranium (expensive) or a fancy moderator (heavy water, likewise expensive.)

Wait a moment. Where's the source on this?

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VorAbaddon t1_jc38zt6 wrote

Pretty much the design of the reactor. It doesnt use enriched uranium nor heavy water. So it has to have another source of moderation to get the reaction going from a less fissile fuel. Hence, graphite.

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Hiddencamper t1_jc3b51m wrote

The graphite blocks are the key.

The tips are there to help levelize axial flux tilt (get power more uniform across the core) in a safe manner… when done correctly (and by safe, I mean in a way that when executed as intended allows you to get enough power from the bottom 1/3rd of the reactor without risking other transient conditions causing core damage).

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Accelerator231 t1_jc3bgem wrote

So... You don't need to refine material to get nuclear reactors?

Wow. I did not know that. So by adding graphite, the hurdles with using normal uranium can be overcome?

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insta t1_jc3gufu wrote

This was a major benefit to RBMK. You can get limitless, carbon-free power from clean water and rocks you dug out of the ground. There's still 8 of them kicking around today, the design works well if you don't intentionally disable every single safety system at once.

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Accelerator231 t1_jc3mrcy wrote

This changes my understanding. I always thought part of the reason why nuclear wasn't used was because of difficulty of refining fuel.

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echawkes t1_jc3vk15 wrote

Not at all. In fact, unlike power plants that use fossil fuels (like coal), fuel costs aren't a huge part of the cost of running a nuclear power plant anywhere, regardless of enrichment. (Caveat: nuclear power plants use relatively low fuel enrichments, like 5% or less. If you had an NPP with a very high enrichment, the cost could change, but NPPs don't need high enrichments.)

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jadebenn t1_jcao7ll wrote

Fuel enrichment was originally an extremely expensive service: Part of the advantage to the CANDU and RBMK designs was they didn't need it (CANDU still doesn't, modern RBMK does). Back then, uranium enrichment was primarily accomplished through an extremely energy-intensive process called 'gaseous diffusion' that required large facilities and infrastructure. Then the gas centrifuges arrived, and cut enrichment costs by an order of magnitude.

The last US gaseous diffusion plant was shut down in 2013, but it was uneconomic far prior (IIRC, it was kept around for DoE weapons purposes since that uranium can't be civilian-procured). Modern enrichment is relatively cheap now, which is part of the reason the nuclear industry is interested in boosting enrichment rates (which would have been prohibitively expensive originally).

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Hiddencamper t1_jc3d3bq wrote

Correct. Graphite is a stronger moderator than water.

So is deuterium, which is why CANDU reactors can use natural uranium.

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Accelerator231 t1_jc3mv3g wrote

Actually come to think of it you got a paper or anything similar?

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saluksic t1_jc2m1ci wrote

So a regular reactor with uranium enriched beyond natural levels can’t make plutonium?

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Hiddencamper t1_jc2nwl9 wrote

All uranium based reactors produce plutonium.

It’s a feature! We use U-238 as the filler material in the fuel, knowing we will get some breeding and use that plutonium to extend the fuel cycle.

When you pull fuel out of a LWR after three cycles, it’s running on about as much Pu-239 as it is U-235.

We have to account for that in fuel cycle analysis, hot excess reactivity / shutdown margin, and the Beta factor (fast/thermal fission ratio). It also can impact moderator temperature coefficient and cause it to shift to zero or even slightly positive.

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Gunnarz699 t1_jc2nula wrote

It can and does, but makes less of it.

It would be fine in normal times, but the Soviets were stockpiling warheads in the tens of thousands.

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PHATsakk43 t1_jc31lte wrote

While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.

Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)

So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.

This is a gross simplification and there are other things that can affect this, but it’s part of the equation.

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UnamedStreamNumber9 t1_jc2o0o9 wrote

It can, but by enriching the 235 relative to the 238, you reduce fraction of the fuel that can be jumped from U 238 up to Pu 239 vs the fraction of U 235 that breaks down into barium, krypton and 3 neutrons. There’s still U238 in the fuel rods but with enriched uranium, there’s less of it available to be transmuted

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PHATsakk43 t1_jc32sb1 wrote

While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.

Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)

So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.

This is a gross simplification and there are other things that can affect this, but it’s part of the equation.

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mrmonkeybat t1_jc2dxa6 wrote

When the control rods are fully down the graphite tips are safely under the fuel rods. Then when you want to increase reactivity you raise the rods getting the boron out the way and putting the graphite in at the same time which is cheaper than separate rods. On the day the control rods were fully retracted something that was never really intended to be done. So when the control rods were dropped for a crucial moment the graphite tips were accelerating the reaction before the boron could get in the way.

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token-black-dude t1_jc2kmq4 wrote

>which is cheaper than separate rods.

Not just cheaper. Fuel rod distance is important, if there is made room for both boron and graphite side-by-side, you'll need fuel rods with a lot more uranium in them and overall efficiency would drop a lot.

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Greyswandir t1_jc2dik8 wrote

Perhaps a more instructive version of your metaphor might be a fire extinguisher which emits a flame retardant for the first second which explodes when it contacts a certain chemical. Would it be better if it didn’t do that? Sure. But the flame retardant which doesn’t explode is expensive and changing the design of the fire extinguisher is even more expensive. Besides, the fire extinguisher would still work fine, you just have to not use it on that one particular type of chemical fire.

As others have covered in a lot more detail, the operators of reactor four had to take a number of extraordinary measures to put the reactor into a state where the graphite tipped control rods could cause a catastrophic failure. Worse, they didn’t even know there was a catastrophic failure state they needed to look out for. To go back to the fire extinguisher metaphor, no one ever warned them about the chemical.

That’s supposed to be the big revelation at the end of the show. Legasov knew about the potential for an RBMK reactor to explode, but the people running Chernobyl that night didn’t. Even though the operators where running the reactor in reckless and dangerous manner, they only thought they were risking a shutdown. They didn’t know they were courting utter disaster because they had never been told that was a possibility.

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aaeme t1_jc2icou wrote

Maybe a simpler adaptation of the fire extinguisher analogy: a fire extinguisher that shoots a second of water before carbon dioxide. Operators were playing with deep fat friers thinking it was fine because they had CO2 fire extinguishers.

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[deleted] t1_jc2g85j wrote

The RMBK was basically all about cheap and simple from a construction point of view, and about dual purpose - it can produce plutonium.

The whole design is dangerous and relies technical solutions to problems that shouldn’t even exist.

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HedonisticRush t1_jc2582n wrote

The rods consisted of the 7.5m boron portion and a 4.5m graphite portion. There is a 1.5m water gap on either side of the graphite part when pulled out.

​

Regular water is slight neutron absorber. When they were trying to raise the power they had pulled out almost all the control rods. When they hit AZ5 it pushed the water out of the bottom of the control rod tubes. Displacing the all the water with graphite greatly accelerated the reaction locally in the bottom of the reactor. This caused the rods to get jammed due to massive thermal expansion. The reaction accelerated uncontrolled once this happened.

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axloo7 t1_jc2z7z5 wrote

It's not much of an answer but: because that's how it was designed.

Reactors are very complex machines with so very many things that effect the way they operate.

Running a reactor is like balancing a stick by its tip. There are things that make it more reactive and things that make it less reactive. One of those things are the control rods (but they are not the only mechanism) The people who designed the RBMK reactor decide to design the reactor to have more "powerful" control rods. The graphite ends are just a way to assert more control over the reactor.

This design decision made it possible to put the reactor in to an unstable state.

There probably exist other reactors where Nuance of their design allows the same thing but we don't hear about them because those operating regimes are never used because they are not safe. I would asume most have safeguards (like the rbmk has) to prevent it.

It's worth mentioning that just because it's posible to break a thing by using it incorrectly doesn't mean that the thing is poorly designed.

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hunguu t1_jc2lsux wrote

This is a very simple question to answer actually. Reactor power is all about controlling the number of neutrons. Control rods absorb neutrons to help control and lower power. But if you pull out a rod to raise power, it doesn't work well if water fills in that space because water is great at absorbing neutrons too. So the designers had a very bad idea of adding 4.5 meters (over 14 feet) of graphite to the bottom of the control rod because graphite adds positive reactivity compared to water. When the rod is pulled out, graphite takes its place. I find saying the "tip" is graphite causes confusion because it's actually a long rod of graphite nearly the height of the core entire.

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Eltuine t1_jc3qcml wrote

Hank Green has a really good explanation about this in a video on the Vlogbrothers youtube, starting at about 7:33 - https://youtu.be/hIGtTImeYU4?t=452

The short answer is, since the reactor was cooled with regular instead of heavy water (hence, cheaper), pulling the control rods out didn't speed the reaction up as much as it would if they were to use the (more expensive) heavy water, as regular water absorbs more neutrons. So, instead of just "removing" control rods, the reactor pulled in graphite rods behind the control rods. (Graphite being a substance that would "speed up" the reaction, thus counteracting the slowing of the water).

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Ayad3 t1_jc5920j wrote

Why? For the same reason our reactors do not have containment buildings around them, like those in the West. For the same reason we don't use properly enriched fuel in our cores. For the same reason we are the only nation that builds water-cooled, graphite-moderated reactors with a positive void coefficient. - It's cheaper.

  • Valery Legasov
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forbiddenthought t1_jc3vuo7 wrote

The way it was explained to me, is that the graphite would typically slow the reactivity, but because the tips were in the reactor they were themselves irradiated. When they're inserted they bring it down, but there would be an initial surge from the tips going in. This could be fixed by not having the ends of the rods in, but would be more mechanically complicated.

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braize6 t1_jc557p6 wrote

Off topic, as someone who operates kia nuclear power plant, the Chernobyl incident absolutely infuriates me every time I hear about it. The RBMK is just..... wrong. It's bass ackwards, and completely dangerous (obviously)

That said, Chernobyl is a great show. "Go on, tell your lie"

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