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.

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.

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)

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

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.

Correct. Graphite is a stronger moderator than water.

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

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.

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).

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.

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.

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.

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.

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).

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.

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.

Part of this issue, is laws meant to force CEO pay to be public has only resulted in the CEOs getting more negotiating power with each other. So their pay has skyrocketed.

The public outage and shareholder outrage apparently isn’t big enough for them to care.

This in particular is due to climate change affecting a region in a way that it wasn’t designed for.

Pressure in the pipes forced things to leak OUT, not IN.

If you lose water pressure, you have risk of bacteria moving into the pipes and intermingling until you get appropriate system flushing. So until the system is flushed out, you issue boil orders.

It was crazy. The administration was adamant it wasn’t airborne. Then immediately after said they are converting. My sister was like “this is nuts….. is it airborne or not”.

The understanding I had, was that someone with environments experience heard that and was confused because when it comes to pollution, the particle size is 100 microns that they have to worry about. It didn’t line up. And then we come to find out that medical science was potentially inconsistent here based on some specific / older assumptions.

I remember my sister, a nurse in a major Chicago hospital, telling me in April 2020 that they were finding Covid virus matter outside of patients rooms / in hallways and that didn’t make sense since it couldn’t be airborne. She was like “they keep saying it’s not airborne, but clearly something is allowing it to leave patients rooms”. The hospital at the time decided to convert all Covid floors to negative pressure after that.