Chernobyl HBO Chernobyl physics questions

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maybe someone in the net has given a summary which part of the series is true facts and false physics?
 
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Do not try to interpret any part of the series as any kind of 'true physics': it was intended to be a historically accurate authentic dramatic movie and not a scientific documentary/reconstruction. If you are interested in the scientific background of the incident, then go for it independently to the movie.

Of course there are various lists on the Internet about the 'errors' found in the movie. Each of them is good for just as long debates as the movie itself.
 

QuantumPion

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I believe a major reason for the graphite-ended control rods was to increase shutdown margin (the total amount of negative reactivity available to control). The total reactivity worth of the control rod is the difference in reactivity between fully withdrawn and fully inserted. If the negative reactivity worth of the fully inserted control rod is not enough, the only way to get more control rod worth is to increase the fully withdrawn positive reactivity worth - by replacing the channel with extra moderator. This seems fine in principle for normal operation but obviously has complicated transient behavior...
 
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@QuantumPion ,Isn't that like saying that in order to get better brakes on a car one must increase the total power of the engine?


Without the graphite tips the difference in reactivity between fully withdrawn and fully inserted would have been smaller but I think that would only impact the efficiency of the core at designed power rather than the very performance of the rods to kill the chain reaction, and closer to the end of the fuel lifetime you would end up with less burnup I believe. Maybe some other technical details would change probably, like asymmetric burn up of the fuel in the core etc.

What one wants from the control rods is to either decrease power or shut down the reactor completely, so the graphite tips in no way advance the main objective of the control rods (in Chernobyl case they indeed helped to sabotage the main objective of the control rods)

I think the rods themselves when fully inserted do the function of absorbing enough neutrons as to stop the chain reaction just fine , the graphite doesn't play any role in this it was just located there in the rod channels because the channels happen to be part of the reactor core and hence they also need to be designed such that they don't intervene negatively with the total efficiency of the reactor core.


PS. going back to the car analogy the control rods without graphite tips would be like driving a car at maximum designed speed down the highway and starving the engine of oxygen/neutrons at the same time, result is you still can drive but you have less power.
 
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QuantumPion

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@QuantumPion ,Isn't that like saying that in order to get better brakes on a car one must increase the total power of the engine?


Without the graphite tips the difference in reactivity between fully withdrawn and fully inserted would have been smaller but I think that would only impact the efficiency of the core at designed power rather than the very performance of the rods to kill the chain reaction, and closer to the end of the fuel lifetime you would end up with less burnup I believe. Maybe some other technical details would change probably, like asymmetric burn up of the fuel in the core etc.

What one wants from the control rods is to either decrease power or shut down the reactor completely, so the graphite tips in no way advance the main objective of the control rods (in Chernobyl case they indeed helped to sabotage the main objective of the control rods)

I think the rods themselves when fully inserted do the function of absorbing enough neutrons as to stop the chain reaction just fine , the graphite doesn't play any role in this it was just located there in the rod channels because the channels happen to be part of the reactor core and hence they also need to be designed such that they don't intervene negatively with the total efficiency of the reactor core.


PS. going back to the car analogy the control rods without graphite tips would be like driving a car at maximum designed speed down the highway and starving the engine of oxygen/neutrons at the same time, result is you still can drive but you have less power.
Your analogy isn't accurate in this case. I've been trying to come up with a car analogy for this issue but can't really think of a good one. Maybe consider a car that has no brakes, and can only slow down via engine-braking. In this case if you want more braking power, you need a bigger engine which provides more resistance when at zero throttle. That's not quite right either but it's closer.

Imagine you had two types of control rods. One made of boron, where inserting them decreases reactivity. And a second type of control rod that is made out of graphite. The second kind works in reverse, inserting it increases reactivity and withdrawing it reduces reactivity. Now if you combine both types into one rod, you get the RBMK design. When the control rods are inserted, they are simultaneously inserting boron rods and removing graphite rods. This proves more negative reactivity than either one on their own.
 
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I understand your point @QuantumPion and it is a fair point, the control rod graphite was essentially made to work as part of the core graphite when under full load in order to increase the neutron economy and fuel burnup efficiency. The problem I assume is that such rod material combining in an otherwise rather peculiar reactor design leads to instability during power changes, as already was noticed at other RBMK units well before 1986,

As for the car analogy, it's indeed not a good one but just as a sidepoint you don't need a larger engine in terms of ccm to get better engine resistance you just need a higher compression ratio, aka a diesel .. :D

I wonder how exactly were the rods modified after the Chernobyl accident in all of the other plants, maybe @Astronuc can say more on this.
 

Astronuc

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I wonder how exactly were the rods modified after the Chernobyl accident in all of the other plants, maybe @Astronuc can say more on this.
I discussed some modifications in previous posts in this thread. See the first page. I believe they shortened the graphite section and expanded the absorber section downward. However, I could not find complete details, in order to understand the full set of changes. I do mention that the designers decreased the drop time, but that is still too long by Western standards.

One would not consider graphite as a 'control element', since in the case where it displaces water, it reduces parasitic absorption, which adds positive reactivity to the core, especially at the moment, as in the case of Chernobyl unit 4, one wishes to scram/shutdown the reactor. The long/slow drop time added to the problem.

At the very low power that the test was conducted, there was not much Xe-135 to override, so the graphite tip exacerbated an already precarious situation, and of course, the results were catastrophic.
 

QuantumPion

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As far as the TV show goes, one bit that really bugged me was the "2 to 4 megaton explosion" that was claimed to be a potential result of the molten core coming into contact with water collected in the basement. That is just nonsense. Maybe it was supposed to be hyperbole to scare the politicians into action, but they made it sound legitimate. Have you ever seen footage of volcanic lava flowing into the ocean? It does not explode. Even if you somehow instantaneously converted all of the stored heat energy of the molten core into water, the total thermal energy is on the order of 0.1 tons of TNT, not megatons.
 
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But the even more interesting thing was that the creators of the show did not make this claim up, in fact it really happened back in 1986 by some rather unknown scientists who made that claim.

The USSR was rather mesmerizing from many different often weird reasons so I don't think it is even noteworthy to discuss this claim, if not for HBO Chernobyl it would have been forgotten long ago.
 
You are asking good questions...

> 1. He says water reduces reactivity but steam increases it. How does this really work?

You're first sentence is really saying the same thing. In the case of the Chernobyl reactors, water acts as a neutron absorber. if you add more water, you absorb more neutrons and reactivity decreases. If the water boils, you have less water, so reactivity increases. Why this matters is that when the power increases, the reactivity increases, and this is a really bad design feature (called a positive reactivity coefficient).

Another thing to remember is that neutron moderation is performed with graphite blocks, which are stationary at all power levels.

Contrast this to light water reactors (LWR's) where the water is both the coolant and the moderator. When the power increases, the water density decreases, which decreases the moderation as well as the coolant. Therefore, LWR's have a negative power coefficient. When the power increases, the reactivity decreases.



> 2. He says heat reduces reactivity. (Completely hypothesizing here, is it something to do with phonons interacting with neutrons? I might be out of context though, if so please correct.)

This is something called the "Doppler effect". In the figure in comment:2, you can see "spikes" in the cross sections. These spikes are called resonances. When the temperature increases, the resonances increase in width, and absorption increases. This causes a decrease in reactivity.

So when the power goes up, the temperature goes up, and the reactivity decreases due to Doppler. This is also a negative reactivity coefficient and another very important safety feature.

The Doppler effect occurs in every reactor that has uranium or plutonium fuel (i.e. every reactor)


> 3. The deadly flaw about the rods: their graphite tips. He says graphite increases reactivity. But isn't it used for "moderating" purposes in the reactors all over the globe? An elaborate explanation I would appreciate.

Yes, you are correct. Graphite is a moderator. The key here is that the graphite is located on the tip of the control rod. Therefore, when you insert a control rod, you are inserting graphite and increasing reactivity. This is a bad thing, when you insert a control rod you expect that the reactivity will go down, not go up.

The graphite tips are complicated because the total reactivity depends on where the graphite tip is, and where the absorber regions are located relative the core. Almost always, the control rod is located where the neutron absorber has a much bigger affect than the tip, so the control rod is always negative reactivity. However, in the accident, they had the control rods withdrawn too much and the tip was located at the very top of the core and the absorber was completely withdrawn. Therefore, when the initially inserted the control rod back in the core, the tip was dominate and positive reactivity was inserted. This is what initiated the accident, but it was due to the reactor being in a very unsafe initial condition. The operators had to do a lot of things wrong to get the reactor into this condition.
I think the problem with the Graphite tips on the control rods was that the Graphite was less of a moderator than the water it was replacing. As the control rods were activated and returned into their control tubes to moderate a chaotically unstable reactor, the first action, as the leading graphite section of the control rods entered the boiling water column in the control rods steel tube, was to displace the water with an even LESS effective moderator (the graphite tip), it may also have instantly reduced the effective instantaneous coolant volume, increasing the meltdown thermal factors. So, in a nutshell, the first effect of lowering the control rods made the reactor more unstable, exactly the opposite of what you would wish to acheive in a SCRAM control event.
 
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@franklyamazed I don't think I can agree with your postulate that "graphite is less a moderator than water"

https://energyeducation.ca/encyclopedia/Neutron_moderator

See the table for scattering and neutron absorption, water is a good moderator but it absorbs neutrons far more than graphite so from the two graphite is more efficient from a neutron perspective.
So when the rods were lowered into the RBMK in fact what happened is the less efficient water moderator was replaced with a more efficient graphite one which lead to the additional increase in reactivity. This is the reason why the blast happened right after the SCRAM was initiated.

Also the reason why CANDU reactors use heavy water is because ordinary light water has a lesser neutron economy compared to heavy water.
All in all graphite vs water in the same situation with all being identical , graphite will produce a larger reactivity increase simply because it will allow for more neutrons to do their job, at least that is what I get from all the tables and literature.
 
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Sorry if it isnt the most proper thread for this (in this case, i would be glad for a link), but i wondered about the scene when robot circuits fryed due to gamma radiation, so they sent biorobots to clean the roof...
Any data about their fate, how could their cells handle so intense radiation? As far as i know, the worst effect is swallow the radioactive dust (people who did this, almost surely died) and unstable iodide takes place in the body. Gamma radiation itself has a much less worse effect on organic bodies?

Does present day robots handle much better such a situation? As far as i know, the ones sent to Fukushima also melt down.
 
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Gamma is nothing more than very energetic photons or very high energy EM radiation. Since the particles that carry the EM radiation have no mass and no electrical charge they can penetrate very far into a material, unlike alpha or beta which have both mass and charge.

I don't have any data here about the doses of liquidators and I doubt someone here does but this is not so straight forward as you would think, I would even say measuring doses and understanding the effects of radiation to biological bodies is the hardest part of nuclear physics. Swallowing or taking in with air small radioactive particles is not necessarily the end much like being in a gamma background isn't it all depends on the amount of dose absorbed and the time period that it took. This is the reason the guys were up on the roof for only 2 minutes max.
I think breathing in a few hundred radioactive particles does nothing much to ones body on average as I am sure most of the cleanup guys in Chernobyl did just that as it was summer and respirators were rarely used.
Now breathing or drinking in a whole lot of radioactive substances causes acute radiation sickness and death , see Alexander Litvinenko, the Russian KGB spy who decided that he will dare to open his mouth about his former work place, they poisoned him with a small amount of Polonium 210 which emits alpha radiation, they added the substance to his tea , a single cup was enough to kill him in two painful weeks.


i am not an expert on radiation doses etc so I can only say the overall picture, just remember your body is nothing more than a sack of cells that are made from atoms just like any other thing you are surrounded. Whether radiation will cause cancer or death solely depends on how many of the total amount of cells are destroyed in your body and where. Cells die all the time and new ones get created by the body as long as you don't kill too many you are fine , eating and sleeping recovers the body.
The supervisor at the reactor 4 at the night of the accident Anatoly Dyatlov received what is estimated to be a lethal dose that night , at morning he was vomiting, feeling sick and loosing his thinking he was taken to hospital , he recovered and lived almost another 10 years until he died a little over 60 years old (63 I think) so no one can say for certain , different people perform differently under severe circumstances.
 
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Oh one more thing, the same Dyatlov that I mentioned in my previous post was a chainsmoker for most of his life, he smoked something like 1 to 2 packs of cigarettes a day and he did not die from cancer but from heart failure, surely his almost lethal dose at Chernobyl probably contributed to his death at 63 but it could also very well be the case that his unhealthy lifestyle, stress and nonstop smoking of red labeled cigarettes caused his early demise.
I just want to point out that it is really complicated to determine what will happen to a person after a certain dose of ionizing radiation, unless the dose is lethal or close to that it might well be that the person is fine and maybe only minor injuries result like pain in joints, more frequent headaches and some other otherwise minor symptoms that are reported by the surviving liquidators. It is a myth that all of them have died or suffered from classic cancers.
Surely there is an increased cancer rate among the liquidators as well as on average they look and feel older than their physical age. The same happens with alcoholics, basically any long term abuse of health results in the body wearing out faster. I know this because plenty of men went to Chernobyl from my country and over the years I have read about their stories and medical records. Some I knew personally.
 
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So the test they made actually wanted to improve security, to deal with the consequences of a power shortage.

I wonder how could a present day nuclear power plant deal with such a situation?
It is good that my country has PWR type reactor with negative void coefficient.
 
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Yes the test was meant to determine whether the turbine inertia is enough in order to power the core cooling pumps at reactor shutdown power levels for the time needed before backup diesels were capable of producing the required power.
In older RBMK reactors this time span was roughly 1 minute, it was so long due to the diesels. In modern reactors and modern emergency power systems this time is much much shorter.

The test in itself wasn't dangerous , the way in which it was carried out and the reactor type on which it was carried out was what made all the difference and history.
 
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I wonder how could a present day nuclear power plant deal with such a situation?
Modern "passive" designs such as Westinghouse AP1000 do not rely on AC-electric motor driven equipment (big pumps) to maintain core cooling and heat removal. Operation of the diesel generators in these designs is therefore not required to prevent core damage.
 
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So the test they made actually wanted to improve security, to deal with the consequences of a power shortage.

I wonder how could a present day nuclear power plant deal with such a situation?
It is good that my country has PWR type reactor with negative void coefficient.
There are a lot of emergency cooling systems that can work on battery power, or passive cooling
https://en.wikipedia.org/wiki/Boiling_water_reactor_safety_systems
They generally need either battery power or a cold water storage. Both can run out, but not until long after the turbines would spin down in an RBMK.
 

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