Zaporizhzhia nuclear reactor — how to keep it from going Kablooey!

[Dotini]

The situation at Zaporizhzhia nuclear power plant grows increasing desperate. Apparently evacuations have been ordered in the immediate area, as the plant is down to on-site emergency diesel power for reactor cooling. The reactors are possibly becoming unstable. Any nuclear physicists on the thread should look into this.

 

[UhClem]

I still do no understand how a facility designed to turn heat into electricity could melt down due to inadequate cooling resulting from a lack of electricity.

 

[ThirstyBarbarian]

I still do no understand how a facility designed to turn heat into electricity could melt down due to inadequate cooling resulting from a lack of electricity.

I think the problem is that even if it stops making electricity, it still keeps on making heat. Unfortunately a lot of reactor designs don’t fail in a safe way. If they stop working, they don’t automatically fall into a safe and stable state. I think there are designs that do, but not all.

 

[UhClem]

Reactors all make heat, a lot of it, after they are turned off because the fission products have short half lives and go on doing their thing. This has always been known.

So the reactor keeps producing heat. (Up to 6.5% of full power depending on operating history but dropping rapidly.) The entire purpose of the plant is to turn heat into electricity. Yet the reactor heats up because the excess heat can't be turned into electricity.

Why?

 

[Steve Shannon]

Reactors all make heat, a lot of it, after they are turned off because the fission products have short half lives and go on doing their thing. This has always been known.

So the reactor keeps producing heat. (Up to 6.5% of full power depending on operating history but dropping rapidly.) The entire purpose of the plant is to turn heat into electricity. Yet the reactor heats up because the excess heat can't be turned into electricity.

Why?

I don’t know if I fully understand why, but these are some of the issues:
Excitation current is external and requires connection to the grid. Generators trip off on over speed if they’re disconnected from the grid by the generator breakers. That happens very quickly Once that happens they can no longer provide their own electricity for cooling or excitation. For that reason all thermal plants have auxiliary generators, typically diesel.
I don’t know why they cannot be ramped down enough to provide their own cooling and excitation power. Maybe it’s just way below the range they can operate safely.

 

[Steve Shannon]

Maybe this will help:
https://www.researchgate.net/public...mes_in_Large_Thermal_and_Nuclear_Power_Plants

 

[UhClem]

While it has been a decade or three since my EE electrical machinery course, the lack of excitation current isn't a problem. There is always residual magnetization and this will result in power output from the generator. The generator output then feeds back into the excitation. Quickly reaching full power.

Generators are normally designed to run synchronized to the grid but it isn't impossible to design one to run isolated. You have to start them up isolated and then connect to the grid after matching frequency and phase.

It may be the case that the decay heat is insufficient to provide normal levels of auxiliary power but you don't need as much power when just trying to keep it from melting down.

 

[rharshberger]

Reactors all make heat, a lot of it, after they are turned off because the fission products have short half lives and go on doing their thing. This has always been known.

So the reactor keeps producing heat. (Up to 6.5% of full power depending on operating history but dropping rapidly.) The entire purpose of the plant is to turn heat into electricity. Yet the reactor heats up because the excess heat can't be turned into electricity.

Why?

Reactors are finely balanced machines, Russia was stupid taking Zaporihizia and should have left it alone...you damn sure don't shell an nuclear power plant and expect things to be ok, Russia is out to contaminate even more of the Ukraine just like they did in Chernobyl.

 

[Dan Griffing]

Reactors all make heat, a lot of it, after they are turned off because the fission products have short half lives and go on doing their thing. This has always been known.

So the reactor keeps producing heat. (Up to 6.5% of full power depending on operating history but dropping rapidly.) The entire purpose of the plant is to turn heat into electricity. Yet the reactor heats up because the excess heat can't be turned into electricity.

Why?

The heat from the fission products alone probably wouldn’t cause a meltdown. The “China Syndrome” is a runaway nuclear reaction where the fuel rods melt and you have a molten mass that remains past the point of fission criticality. That doesn’t happen with fission products alone that can boil water but not melt steel.

Its when a reactor can’t stop the fission process and the circulation system fails that removes the heat from the core to generate steam to make electricity. That system requires power to run but there are only so many layers of backup.

Soviet era nuclear power plants weren’t designed with as much of a safety concern, but the Fukushima nuclear disaster was a 2011 nuclear accident shows that there can be problems with safer reactor designs there can be problems.

And no nuclear reactor was designed to have its safety systems remain operational in a war zone while being bombarded.

 

[Dan Griffing]

Let’s not sidetrack the Ukraine War thread with a discussion about nuclear power plant design. I’m sure there’s a reason why it works the way it does, and it’s probably not a matter of the nuclear engineers just didn’t think to power the cooling systems with the electricity from the plant. Anyway, it would be a great topic for a different thread.

A nuclear power and atomic waste thread would be welcome. My wife and I each spent the first part of my careers at Hanford where nuclear waste management is their principle task. And others on TRF are currently working at Hanford too.

 

[heada]

A nuclear power and atomic waste thread would be welcome. My wife and I each spent the first part of my careers at Hanford where nuclear waste management is their principle task. And others on TRF are currently working at Hanford too.

My grandfather worked at Hanford in the 50s and 60s in nuclear engineering and design.

To somewhat tie it into on-topic (Hanford to Russia) while he was there he was in a program that was sampling high altitude air over the pacific. He told stories that from those samples he could tell that reactor #2 at (some) Russian plant was running 6% less efficiently that day.

The only thing I can think that makes sense about shelling near a nuclear plant is to shut it down and stop power generation. I don't think they want it to have an event or some of the contamination could impact them as well.

 

[Tyeeking]

Seems like if one wanted to take a nuclear power plant off line it would be easier and safer to take out the power lines going out of the plant rather than the plant itself. That would have the added benefit of being repairable after the ground was taken so that the victor would have an operational power plant.

Taking out the power plant reeks of Russia not believing that they will hold the territory and instead just want to create long term damage and havoc.

 

[Greg Furtman]

Here's some info on spent nuclear reactor fuel. And remember that most by-products of fission are radioactive for thousands of years.

https://www.ucsusa.org/resources/sa...fuel is uncovered,-137, into the environment.

 

[OverTheTop]

Seems like if one wanted to take a nuclear power plant off line it would be easier and safer to take out the power lines going out of the plant rather than the plant itself. That would have the added benefit of being repairable after the ground was taken so that the victor would have an operational power plant.

Taking out the power plant reeks of Russia not believing that they will hold the territory and instead just want to create long term damage and havoc.

The big problem is keeping the cooling water flowing around the fuel rods even as the reactor cools down after inserting the control rods and shutting down the chain reaction. Other radioactive byproducts decay and produce heat. Cooling them requires electricity which they normally rely on from outside the plant. They have a diesel generator but getting to that stage is really skating on thin ice. There is a significant amount of heat they need to get out of the reactor when it is shut down. If the cooling water shuts down at the same time as the reactor you can boil the coolant and melt things that were not meant to melt.

Adding to their problems is that the reactors don't run well at low settings. Plant 6 has been running at around 10-11% capacity to provide only cooling for the other reactors and itself and was the last remaining reactor on line. It was recently just shut down to cold, which is a great position to be in given what is going on around it, from a safety perspective. Not so good for the electricity supply in Ukraine and nearby though. They really had no choice but to shut it down completely.

 

[Funkworks]

This is very good news:

https://abcnews.go.com/International/wireStory/power-restored-zaporizhzhia-nuke-plant-shuts-89684121

 

[Greg Furtman]

This is very good news:

https://abcnews.go.com/International/wireStory/power-restored-zaporizhzhia-nuke-plant-shuts-89684121

I sure hope they can keep it connected to the power grid and that the Russians withdraw.

 

[afadeev]

This is very good news:

https://abcnews.go.com/International/wireStory/power-restored-zaporizhzhia-nuke-plant-shuts-89684121

Actually, that’s really BAD news.
The last reactor was generating electricity to power the coolant pumps for all 6 reactors. Now that it has gone offline, the power can only come from Ukrainian grid (that Russians keep blowing up), or backup generators.

When the latter go, the reactors are liable to overheat and go into some form of a (minor) meltdown. And get damaged beyond repair. Which seams to be the Russians ultimate objective.

‘cause that’s how they roll in Mother Russia.

 

[Funkworks]

Actually, that’s really BAD news.
The last reactor was generating electricity to power the coolant pumps for all 6 reactors. Now that it has gone offline, the power can only come from Ukrainian grid (that Russians keep blowing up), or backup generators.

When the latter go, the reactors are liable to overheat and go into some form of a (minor) meltdown. And get damaged beyond repair. Which seams to be the Russians ultimate objective.

‘cause that’s how they roll in Mother Russia.

It's good news because now they have more options, as they wanted.

https://www.iaea.org/newscenter/pre...tor-general-statement-on-situation-in-ukraine

 

[OverTheTop]

Actually, that’s really BAD news.
The last reactor was generating electricity to power the coolant pumps for all 6 reactors. Now that it has gone offline, the power can only come from Ukrainian grid (that Russians keep blowing up), or backup generators.

When the latter go, the reactors are liable to overheat and go into some form of a (minor) meltdown. And get damaged beyond repair. Which seams to be the Russians ultimate objective.

‘cause that’s how they roll in Mother Russia.

It is actually good news as the reactor is cooling. The longer it cools for the less chance of a nuclear problem.

 

[afadeev]

It's good news because now they have more options, as they wanted.

https://www.iaea.org/newscenter/pre...tor-general-statement-on-situation-in-ukraine

I'm not sure if we are reading the same thing and drawing different conclusions, or if I'm missing something obvious.
By shutting down the last reactor, we are removing the only 100% reliable source source of (locally generated) electricity that can not be disrupted when lines going to Ukrainian power grid are shelled into oblivion. Which has happened repeatedly.

Note, that Ukrainians have zero incentives to disconnect their nuclear power plant from their grid (which they've never done before), so the only side incentivized to achieve this outcome are Russians. Who well positioned and well incentivized to blow up those lines again, and again, and again.

It is actually good news as the reactor is cooling. The longer it cools for the less chance of a nuclear problem.

Reactors (all 6) require power to run coolant pumps, to keep the core at safe operating temperatures. Even when the reactor is not generating power (control rods are fully inserted, aka reactor "turned off"), the fuel rods remain hot for weeks to months. That “decay heat” requires constant active cooling to prevent damage that could lead to venting of radioactive gas or a meltdown.

Thus, maintaining ongoing power to the coolant pumps is critical to avoid a meltdown.
There are 3 sources of electricity for those pumps:
1). Locally produced power from the plant (no longer available)
2). Power from the grid (lines keep getting blown up. Because of war)
3). Diesel back-up generators, of Soviet reliability. Which may, or may not, have adequate supply of diesel fuel to run.



Exactly my point. Of the 3 choices above, the only reliable source has just gone offline.
The availability of the other 2 can't be guaranteed.
How is that a good thing?

https://www.grid.news/story/science...reactors would,radioactive gas or a meltdown.

 

[rharshberger]

I'm not sure if we are reading the same thing and drawing different conclusions, or if I'm missing something obvious.
By shutting down the last reactor, we are removing the only 100% reliable source source of (locally generated) electricity that can not be disrupted when lines going to Ukrainian power grid are shelled into oblivion. Which has happened repeatedly.

Note, that Ukrainians have zero incentives to disconnect their nuclear power plant from their grid (which they've never done before), so the only side incentivized to achieve this outcome are Russians. Who well positioned and well incentivized to blow up those lines again, and again, and again.

View attachment 537356


Reactors (all 6) require power to run coolant pumps, to keep the core at safe operating temperatures. Even when the reactor is not generating power (control rods are fully inserted, aka reactor "turned off"), the fuel rods remain hot for weeks to months. That “decay heat” requires constant active cooling to prevent damage that could lead to venting of radioactive gas or a meltdown.

Thus, maintaining ongoing power to the coolant pumps is critical to avoid a meltdown.
There are 3 sources of electricity for those pumps:
1). Locally produced power from the plant (no longer available)
2). Power from the grid (lines keep getting blown up. Because of war)
3). Diesel back-up generators, of Soviet reliability. Which may, or may not, have adequate supply of diesel fuel to run.

View attachment 537357

Exactly my point. Of the 3 choices above, the only reliable source has just gone offline.
The availability of the other 2 can't be guaranteed.
How is that a good thing?

https://www.grid.news/story/science...reactors would,radioactive gas or a meltdown.

Reactors do remain hot for weeks or months after shutdown, but its the first 10-20 hours according to one source thats critical, during that time the Reactor Cooling Pumps (RCP) are required to provide a large amount of coolant to bring the core temps down steadily so as to induce less system stress. After the system hits a certain temp and pressure the required coolant flow can be generated by natural convection flow or by much smaller pumps (much less power required than the RCP's)than the Reactor Coolant Pumps.

 

[afadeev]

Reactors do remain hot for weeks or months after shutdown, but its the first 10-20 hours according to one source thats critical, during that time the Reactor Cooling Pumps (RCP) are required to provide a large amount of coolant to bring the core temps down steadily so as to induce less system stress. After the system hits a certain temp and pressure the required coolant flow can be generated by natural convection flow or by much smaller pumps (much less power required than the RCP's)than the Reactor Coolant Pumps.

<sorry for contributing to thread pollution>

The demands for pumping coolant through the core may decrease as the core's temperature drops, but it does not drop to zero.
Thus, the dependency on external energy source to keep the pumps going is critical.
Has one of the reactors remained functional, it could have continued to generate electricity to run the coolant pumps autonomously. Now that all 6 are shut down, that source of electricity has been removed. But not the need to keep coolant pumps running.

From my earlier source:

Nuclear plants are designed to have external power for cooling with managing this balance in mind, said Jacopo Buongiorno, a professor of Nuclear Science and Engineering at MIT. A reactor that is shut off and no longer generating electricity cannot provide the power needed to keep itself cool. “That is the pickle they are in right now,” he said, at the Zaporizhzhia facility, without outside power. “Cooling must be provided even to a shutdown reactor for a long time to prevent the nuclear fuel from overheating, potentially getting damaged, and then having a release of radioactivity.”​

​

On the subject of Russian invasion itself, and the implications from the most recent developments:

 

[OverTheTop]

Reactors (all 6) require power to run coolant pumps, to keep the core at safe operating temperatures. Even when the reactor is not generating power (control rods are fully inserted, aka reactor "turned off"), the fuel rods remain hot for weeks to months. That “decay heat” requires constant active cooling to prevent damage that could lead to venting of radioactive gas or a meltdown.

So you would rather have the remaining reactor run at 10% of its power output to cool all the remaining reactors and itself? There are far too few safety systems between that and a major disaster. If it shuts down inadvertently then there is no power to cool itself or its friends, except diesel generators. If there are problems with the generators or associated switchgear, or perhaps some other common equipment in the complex system, they would be in a world of pain. Imagine a big lump of heat, around 100MW, with nowhere to go. Bad things would happen, commencing with boiling the cooling water.

Also note that maintaining a reactor at such a low power is a very delicate dance. One wrong move and the reactor could suffer Xenon poisoning and kill the nuclear chain reaction very quickly. Recovery from this is very slow. This is more likely as control rods are moved in. Note that the unstable reactor would be having control rods moved out and in on a fairly frequent basis to maintain the equilibrium, exacerbating the risks.

Shutting it down and letting it get as cool as possible is the right thing to do. I guess that is why they did it, regardless of what armchair commentators think.

 

[afadeev]

So you would rather have the remaining reactor run at 10% of its power output to cool all the remaining reactors and itself? There are far too few safety systems between that and a major disaster.

I never advocated for any power output level below 100%.
Neither did IAEA.
Neither did I see IAEA advocating for shutting down the reactors.

Keep 5 shut down (if necessary) run the remaining 6th reactor at the safe and steady output level, whatever that maybe, to provide steady source of electricity for cooling all 6.

The entire emergency has been kicked off by Russia insisting on shutting-down these reactors to deprive Ukrainian grid of electricity. Safety be damned. Everything else has been a logical (and unfortunate) outcome of that.

If it shuts down inadvertently then there is no power to cool itself or its friends, except diesel generators.

Right. And that is right where we are at today.
Except we seam to be happy about that now, somehow?
Where is the logic in that?

Also note that maintaining a reactor at such a low power is a very delicate dance. One wrong move and the reactor could suffer Xenon poisoning and kill the nuclear chain reaction very quickly. Recovery from this is very slow. This is more likely as control rods are moved in. Note that the unstable reactor would be having control rods moved out and in on a fairly frequent basis to maintain the equilibrium, exacerbating the risks.

Indeed.
The above use-case is VERY close to the scenario that precipitated Chernobyl.
Excepted that they could not move the control rods back in fast enough due to the primitive and full-on-defective reactor design.

Shutting it down and letting it get as cool as possible is the right thing to do. I guess that is why they did it, regardless of what armchair commentators think.

The right thing to do from whose perspective?
Russians?
What was the downside to keeping it running, and keeping military campaign away from its walls? Oh right, that didn't suit Russian political ambitions to f*@k Ukrainians over.
And nuclear reactor safety is just enough tool to throw into battle.


On the constructive side, here is the latest GoodFellows episode, this one with one fellow sharing observation during a trip to Kyiv:

 

[ThirstyBarbarian]

The Zaporizhzhia nuclear reactor in Ukraine has even under Russian control for some time during the war and subject to combat damage. The plant needs electrical power to run its cooling systems and to keep its reactor cores from overheating. Usually that power can come from the plant itself, from the electrical grid, or from backup generators, but all of those systems have been compromised to some degree by the war. This thread is about how to keep the reactor safe.

 

[Funkworks]

I'm not sure if we are reading the same thing and drawing different conclusions, or if I'm missing something obvious.
By shutting down the last reactor, we are removing the only 100% reliable source source of (locally generated) electricity that can not be disrupted when lines going to Ukrainian power grid are shelled into oblivion. Which has happened repeatedly.

If it's attacked while shut down, the damage won't be as bad as if if it's running. We just don't want it to be running when nuts with guns and missiles are around it. The good news is that it's being shut down. In the worst case, there are diesel generators to cool it.

 

[Zbench]

My grandfather worked at Hanford in the 50s and 60s in nuclear engineering and design.

To somewhat tie it into on-topic (Hanford to Russia) while he was there he was in a program that was sampling high altitude air over the pacific. He told stories that from those samples he could tell that reactor #2 at (some) Russian plant was running 6% less efficiently that day.

The only thing I can think that makes sense about shelling near a nuclear plant is to shut it down and stop power generation. I don't think they want it to have an event or some of the contamination could impact them as well.

Considering winds blow west to east you can take it to the bank that the Russians don’t want to see that reactor melt down Chernobyl style.

 

[Hobie1dog]

Considering winds blow west to east you can take it to the bank that the Russians don’t want to see that reactor melt down Chernobyl style.

I would have thought they would not gone near the plant for that very reason. This seems to me that they are displaying how ignorant that they really are.

 

[Zbench]

And no nuclear reactor was designed to have its safety systems remain operational in a war zone while being bombarded.

Actually there is one, the molten salt reactor. If the containment vessel is compromised the molten salt runs out and hardens and immediately stops fissioning. Oak Ridge experimented with Thorium which is many times more plentiful that Uranium. It wasn’t commercialized because Thorium reactors can’t produce Plutonium.

Fortunately, there are many companies that are currently working to commercialize the MSR. You want a green solution? This is it. Even existing stocks of nuclear waste can be burned in one of these beauties.

I hope it’s time will soon come.

 

[afadeev]

Just came across this interview with General Ben Hodges.
He has been exceedingly on the money with his analysis and predictions about Russia-Ukraine war, to date.

Draw your own conclusions: