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Urwumpe

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If a reactor is physically incapable of providing the energy, then steam is incapable of boiling violently and Chernobyl is incapable of exploding. You know, like a bomb that doesn't work.

But shouldn't the reactor also provide electrical power? How is it supposed to produce electrical power, if there is no thermal power?
 

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If a reactor is physically incapable of providing the energy, then steam is incapable of boiling violently and Chernobyl is incapable of exploding. You know, like a bomb that doesn't work.

I think what you meant to say is "if the reactor is physically incapable of providing the energy necessary to produce an explosion that ruptures the pressure vessel". Which is basically the exact same thing as Urwumpe was saying all along when talking about an explosion-resistant containment building. It's a shield around the reactor designed to contain its energy output when it goes critical.

The thing that probably confuses you in all of this is that the energy released at critical mass of any reactor is magnitudes higher than the energy it is designed to put out at regular operation. "Going critical" isn't refering to the reactor itself, as in it is suddenly outputing more power than it could handle. It refers to a state of the fissile material itself. Reaching critical mass means that the chain reaction in the fissile fuel has gone out of control to a point that it is now *physically impossible* to stop it. The material will quite literally blow itself apart and release a lot of energy doing so, and there's nothing anybody can do about it anymore.

This is not a state you can run a reactor in by design, as you can't control the reaction at that point. As such, you cannot design a fission reactor to operate at that energy output. All you can do is build another layer of protection around it that can contain that energy.
 

Urwumpe

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It's a shield around the reactor designed to contain its energy output when it goes critical.


Actually reactors do go critical, in their engineering lingo. It is achieved by reaching zero reactivity or in other words, when the nuclear chain reaction is self-sustaining and no special neutron source is needed to keep it running.



What you mean there is supercriticality, which is usually the cause of criticality events: When the reactivity becomes positive and power increases without any control input.



No containment building is yet designed specifically to handle criticality events. When there is a supercritical event like the secondary chernobyl explosion happening, even a modern european containment building will not last long. Those events are not impossible with current reactor designs, but more unlikely for some reactor types than others.


But: when a full volume steam explosion happens, you can be sure that the remaining reactor will sure not be in any controlled state and fighting a criticality event will be very hard to impossible.
 

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They are putting in solar farms at Chernobyl and IIRC also at Fukushima. There is a small 1 MW demonstration right by Unit 4.


Makes sense - large exclusion zones with uninhabitable real estate available and power lines capable of handling several GW of power to pipe it out.



This is also what is being done with station grounds after fossil plants are closed and torn down.
 

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Actually reactors do go critical, in their engineering lingo. It is achieved by reaching zero reactivity or in other words, when the nuclear chain reaction is self-sustaining and no special neutron source is needed to keep it running.



What you mean there is supercriticality, which is usually the cause of criticality events: When the reactivity becomes positive and power increases without any control input.

The big issue here is that of prompt versus delayed criticality.

To actually develop significant power, you have to be supercritical for at least some amount of time to bring your reaction rate up, but if you remain supercritical the power you develop will increase without limit.

The neutrons resulting from a given fission event can be divided up into prompt and delayed neutrons. Prompt neutrons are those released by the fission event itself. Delayed neutrons are released seconds or minutes later by neutron-emitting decay modes of the daughter nuclei. Your reaction has to always be subcritical with respect to prompt neutrons, because the time from the emission of a prompt neutron to the point where it causes another fission is so short that even the smallest margins of supercriticality that your control mechanisms have the precision to achieve will lead to your power doubling on a catastrophically short timescale.

However, since delayed neutrons are emitted seconds and more after the responsible fission event, if the reaction is prompt subcritical but the delayed neutrons make the total neutron flux high enough for the reaction to be supercritical, then the power doubling time can be long enough to allow human operators to keep the reaction rate under control (not to mention automated systems and, for reactors with positive void coefficients, the effect of temperature on the reaction rate).
 

Urwumpe

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Some delayed neutrons even arrive minutes later (half-life of 55s).

That is also one challenge for breeder reactors, they often operate with less delayed neutrons as classic reactor designs.
 

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Some delayed neutrons even arrive minutes later (half-life of 55s).

Thus "seconds and more" above.

I think there are even some isotopes involved with half-lives in the 10s of minutes to hours range, though the peak is in the seconds-to-minutes range, aren't there?
 

Urwumpe

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Thus "seconds and more" above.

I think there are even some isotopes involved with half-lives in the 10s of minutes to hours range, though the peak is in the seconds-to-minutes range, aren't there?


No, 55s half-life is the longest for a neutron source, but other isotopes important for the operation are in the hour range, for example Xenon-135
 

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Well, since they have the exclusion zone, they might as well operate one unshielded and remotely. How bad would the radiation levels get? I bet they wouldn't feel much in Kiev ,for example. Worst case, you're protected by earth's curvature after a few tens of kms,.

I still don't get the insanity of continuing to operate the other reactors, tho. Like yeah, sure, the world's horrified and evacuating etc yet these people were still working next door. Did they even go into SCRAM when nr.4 blew itself to bits?
Bulgaria also had one on the Danube and there was always back&forth with Romania about just stopping the damn thing.
 
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Urwumpe

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Did they even go into SCRAM when nr.4 blew itself to bits?

Yes, the SCRAM (AZ-3 switch) actually blew it up. It was already out of control before, but unknown to the operators, the SCRAM in that situation increased the nuclear reaction rate for the first seconds at that low power level.

When the same happened during a SCRAM in 1977 at Leningrad, the reactor was operating already at a higher power level and the increase in reactivity was lower by higher coolant flow. Still it was noteworthy enough to write a report that the operators of Chernobyl never had the chance to see before the accident investigation.

The low power level of Chernobyl is important for understanding the accident - they had removed nearly all control rods, far below the safety limit of 20, to keep the reactor running at 200 MW despite strong Xenon poisoning.

Also, as workaround for some missing test conditions because of the accelerated preparation schedule, they reduced the inflow of coolant, which not only increased the coolant temperature and steam production, but also caused steam bubbles or voids in the lower core of the reactor, which, by design, increased reactivity since steam absorbs less neutrons as liquid water by volume.

Dyatlov documented this good in his own account of the accident, but he adamantly decided to not talk much about the circumstances that led to the steam voids in the lower reactor core.

The increased neutron production in the lower parts of the core quickly reduced the amount of Xenon poisoning there and made the power increase rapidly - large powerful steam bubbles formed in individual tubes, displaced water around them and caused water hammers, that made the 350 kg heavy concrete covers of the reactor tubes jump up and fall down again when the steam bubble collapsed. At this point, the reactor was already disintegrating and pressure tubes started to fail, deforming the graphite blocks around them and the neighboring control rod and instrumentation channels. Also power level increased rapidly and ran out of control, which made the operators decide to trigger a SCRAM of the reactor.

This meant, when they activated the SCRAM, many more graphite tips at the control rods acting as moderator entered the reactor at once (because they had been retracted), far more than considered safe by the engineers which designed those tips, and pushed a zone of very high reactivity downwards towards an already existing zone of high reactivity (by the steam voids).

Also, the control rods moved really slow because they had been lowered by gravity against the coolant flow (They just had been hanging on steel cables), having to displace water while moving the 7 m downwards. They fixed it in the 1990s for some reactors by putting special control rods into gas filled tubes that allowed to drop them in a much shorter time. This is also why the engineers tried to disconnect the clutches of the control rod drives when they stopped moving, they hoped they would fall by gravity into position, even if this means the reactor is damaged by the fall. They did not know and did not expect, that the control rods had been unable to move because the local steam bubbles are already forming deformed the core.

This all added up and caused a rapid, uncontrolled chain reaction and caused a massive steam explosion. Which lifted up the top cover of the reactor, opened the core and allowed the second larger and possibly nuclear explosion to happen. Its really easy to explain afterwards. Before, only very few people did know that a lower power level is more dangerous than a high power level for this reactor type.
 

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This all added up and caused a rapid, uncontrolled chain reaction and caused a massive steam explosion. Which lifted up the top cover of the reactor, opened the core and allowed the second larger and possibly nuclear explosion to happen.
Hm, i thought the simulations showed that it was the first explosion that was a prompt criticality, followed by a slower steam explosion?
 

Urwumpe

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Hm, i thought the simulations showed that it was the first explosion that was a prompt criticality, followed by a slower steam explosion?

There is also a newer theory by Dubasov, which is based on the isotope ratio of the accident, that suggests that the second was also a prompt criticality event.

One big flaw of the official hypothesis for the second explosion is, that the core had to be already open at that point, steam pipes ruptured open and fuel elements exposed and there is no enclosed volume that could permit the needed amount of steam enthalpy to accumulate.

There is also the theory of de Geer, which puts the final destruction of the core to the second explosion and makes the first small explosion a prompt criticality event, followed by a massive steam explosion. But that does not fit too well to the photographs of the destroyed reactor afterwards: The Elena structure was still mostly in one piece and not perforated by a jet of debris at hypersonic speeds, as calculated in that theory to explain the isotope ratio.

http://su.diva-portal.org/smash/get/diva2:1168987/FULLTEXT01

For that ratio to work out, something must have ejected fresh fission products in 3 km altitude seconds after the explosions. This is more likely if the graphite core was already exposed before this fresh nuclear explosion. The ratio would have been different, if the measured isotopes got transported by smoke and dust during the fire.
 

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There was very likely hydrogen formed by thermal degradation of steam in the presence of the hot graphite and catalyzed by any zirconium cladding.



Someone mentioned that a simulation was done that suggested a prompt criticality event; do you have a source? I would like to see that.
 

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It sort of fits the witness testimony of "loud noise and explosion, follow by a flash of blue light and a second explosion" . The building withstood the blast since the lid had already blown and most of the energy went up. Then again, that 3km plume height might have been real and explained even solely by convection. That thing was burning hot as a furnace, if not more.

Other than that, yes, the reactor had design flaws, and they didn't know about the graphite tips. But that doesn't make their handling excusable. Disabling safety systems, withdrawing all the control rods manually, even those that should never have left the reactor... What were they thinking? Even if the reactor had xenon poisoning etc, they didn't know exactly what the hell was going on and they ignored all the alarms about unstable neutron flux etc. If it had gone out of that state, they were keeping it primed for a surge to max power. That during a low power test that would have seen the cooling pumps shut down :facepalm:

Makes me think if the test couldn't have been done in other ways. Perhaps they or someone higher than them knew that the turbine doesn't have enough energy to power the pumps while winding down. Couldn't they start the diesel generators then disconnect the main turbine just to see how much it generates while winding down, thus giving them a rough calculation of whether it could power the pumps?

I can't seem to find anything about the other reactors. Were they running? Did they shut down? Did they had separate control teams, or was it just that the night shift that we know of was running the whole powerplant? I can't imagine those people coming to work in the morning as usual.

Edit: It does seem plausible that the first one was a prompt criticality, if you look at the setup. No control rods, or at least just the graphite tips, and all the water probably flashed to steam. The fission was basically going unimpeded in all the columns with nothing to stop it.
 
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Urwumpe

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Makes me think if the test couldn't have been done in other ways. Perhaps they or someone higher than them knew that the turbine doesn't have enough energy to power the pumps while winding down. Couldn't they start the diesel generators then disconnect the main turbine just to see how much it generates while winding down, thus giving them a rough calculation of whether it could power the pumps?

Well, they violated the test setup in first place. Ordered was 700MW - 800 MW power for the test (20% rated power) and power had to be reduced by a carefully planned schedule. Also the reactor was supposed to be off-grid. But now:


  1. First of all, Dyatlov ordered the test to be executed at mere 500 MW or 20% rated power to conserve coolant (demineralized water)
  2. Next, the reactor had to be kept online and at high power for long than planned because the electricity grid demanded it.
  3. During this delay, a shift change also happened and the less experienced night shift had to go on with the test. For example, Toptunov, who was now in charge of the control rods for this difficult test, had been working just for three months as senior engineer in this function.
  4. For catching up with the test schedule, they reduced power too fast, additionally, an operator error nearly stalled the whole reactor in that phase, dropping to only 1% of the rated power. It is unknown why this happened and many people liked to blame the inexperienced Toptunov there, but its more likely to be caused by the control system design and the Xenon concentration in the reactor after running at full power.
  5. Everything below 20% power usually meant that the reactor had to be shutdown and restarted later.
  6. By pulling out almost all control rods, they managed to return power to 5% with an increasingly Xenon poisoned reactor.
  7. Dyatlov still orders the test at a far too low power level for the test and the safe operation of the reactor.
  8. The core decides to take a look at the world outside
https://www-pub.iaea.org/MTCD/publications/PDF/Pub913e_web.pdf


I can't seem to find anything about the other reactors. Were they running? Did they shut down? Did they had separate control teams, or was it just that the night shift that we know of was running the whole powerplant? I can't imagine those people coming to work in the morning as usual.

Not much is known about the state of the first two blocks. Each reactor had its own control team, as usual.

3 was running until either:


  • Dyatlov ordered the shutdown late after the accident, but was overruled by Formin at 5:00
  • The crew decided themselves to shutdown the reactor at 5:00
Dyatlov had been the senior engineer responsible for both units 3 and 4 and had the authority to order the shutdown.
 

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If I remember right, I recall reading that this test had been previously brought up at other power plants, but the plant supervision told them to pound sand. Chernobyl was just the first place that agreed to do it?

Even if this were a "good" test, there were far too many holes in the Swiss cheese lining up. When we have a similar situation at work, we are (supposedly) allowed to "stop when unsure". But we're also under the eye of the NRC for a chilled work environment. So practice doesn't always meet protocol.
 

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Just in time for the Halloween: the SLS mask! :rofl:
14_Rogers_S_SLSBoosterSep2_SC15_big.jpg
 

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Looks like some Eldritch God has had enough of this world and has decided to go to Europa!
 

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Re the Chernobyl tv series, this is the tech explanation:

35 seconds in.

 
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Thunder Chicken

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