Orbiter-Forum  

Go Back   Orbiter-Forum > Far Side of the Moon > Brighton Lounge
Register Blogs Orbinauts List Social Groups FAQ Projects Mark Forums Read

Brighton Lounge General off-topic discussions. Political or religious topics may only be posted in The Basement forum.

Reply
 
Thread Tools
Old 10-17-2019, 11:20 AM   #25591
Urwumpe
Not funny anymore
 
Urwumpe's Avatar

Default

Quote:
Originally Posted by steph View Post
 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.
Urwumpe is offline   Reply With Quote
Old 10-17-2019, 02:13 PM   #25592
Artlav
Aperiodic traveller
 
Artlav's Avatar

Default

Quote:
Originally Posted by Urwumpe View Post
 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?
Artlav is offline   Reply With Quote
Old 10-17-2019, 03:42 PM   #25593
Urwumpe
Not funny anymore
 
Urwumpe's Avatar

Default

Quote:
Originally Posted by Artlav View Post
 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/...987/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.
Urwumpe is offline   Reply With Quote
Old 10-17-2019, 11:05 PM   #25594
Thunder Chicken
Fine Threads since 2008
 
Thunder Chicken's Avatar
Default

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.
Thunder Chicken is offline   Reply With Quote
Old 10-18-2019, 08:09 AM   #25595
steph
Orbinaut
 
steph's Avatar
Default

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

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.

Last edited by steph; 10-18-2019 at 08:29 AM.
steph is offline   Reply With Quote
Old 10-18-2019, 09:22 AM   #25596
Urwumpe
Not funny anymore
 
Urwumpe's Avatar

Default

Quote:
Originally Posted by steph View Post
 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/public...ub913e_web.pdf


Quote:
Originally Posted by steph View Post
 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.
Urwumpe is offline   Reply With Quote
Old 10-18-2019, 09:37 PM   #25597
PhantomCruiser
Wanderer
 
PhantomCruiser's Avatar

Default

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.
PhantomCruiser is offline   Reply With Quote
Thanked by:
Old 10-19-2019, 03:18 PM   #25598
GLS
Addon Developer
 
GLS's Avatar
Default

Just in time for the Halloween: the SLS mask!
GLS is offline   Reply With Quote
Old 10-19-2019, 04:42 PM   #25599
MaverickSawyer
Acolyte of the Probe
 
MaverickSawyer's Avatar
Default

Looks like some Eldritch God has had enough of this world and has decided to go to Europa!
MaverickSawyer is offline   Reply With Quote
Old 10-20-2019, 01:40 AM   #25601
Thunder Chicken
Fine Threads since 2008
 
Thunder Chicken's Avatar
Default

Quote:
Originally Posted by Notebook View Post

I was pretty impressed with the presentation of the tech explanation in the HBO series. However, I don't believe Legasov was actually at the trial. I know there is footage of the trial but there are only snippets available.
Thunder Chicken is offline   Reply With Quote
Old 10-20-2019, 09:14 AM   #25602
Notebook
Donator
 
Notebook's Avatar


Default

Artistic licence?

I'm confused about one point(only one!)

In the HBO clip at 2:30 the negative temp co-efficient is brought up as reducing the nuclear reaction as temp increases, whereas in the Prof's explanation at 6:25 more heat accelerates nuclear reaction?
Notebook is offline   Reply With Quote
Old 10-20-2019, 04:12 PM   #25603
Urwumpe
Not funny anymore
 
Urwumpe's Avatar

Default

Quote:
Originally Posted by Thunder Chicken View Post
 I was pretty impressed with the presentation of the tech explanation in the HBO series. However, I don't believe Legasov was actually at the trial. I know there is footage of the trial but there are only snippets available.

Not sure, he is mentioned in every important investigation report, like INSAG-1 and INSAG-7.
Urwumpe is offline   Reply With Quote
Old 10-20-2019, 08:05 PM   #25604
Thunder Chicken
Fine Threads since 2008
 
Thunder Chicken's Avatar
Default

It's interesting (and rather frustrating) that there is so little primary source literature available to the West regarding this accident. In addition to the technical causes of the failure there is a human factor element as well. Knowing how all of the holes in the swiss cheese line up to enable such an accident is very, very important if one wishes to avoid something similar occuring again.


Reading Dyatlov's book (what parts of it are available in English) is very interesting as it casts a very different picture than how he was portrayed in the HBO series. It's a shame that they did that to him. Dyatlov notes (correctly, IMO) that the junior control room engineers very likely would have been tried and convicted had they survived as well. He was found guilty of doing things that were only found to be wrong in hindsight.
Thunder Chicken is offline   Reply With Quote
Old 10-20-2019, 09:05 PM   #25605
Urwumpe
Not funny anymore
 
Urwumpe's Avatar

Default

Quote:
Originally Posted by Thunder Chicken View Post
 Reading Dyatlov's book (what parts of it are available in English) is very interesting as it casts a very different picture than how he was portrayed in the HBO series. It's a shame that they did that to him. Dyatlov notes (correctly, IMO) that the junior control room engineers very likely would have been tried and convicted had they survived as well. He was found guilty of doing things that were only found to be wrong in hindsight.

As I wrote somewhere else already, that is his own perspective. And that is also not without faults.



He blamed the reactor design for causing the accident, but he does not speak much about the fact, that it had been his own commands, which configured the reactor outside the safe parameters.


It is very likely, that the accident would not have been possible if enough control rods had been left inserted into the core. Which means, this experiment would have had to be aborted or delayed until the Xenon poisoning is overcome.



In the second international report based on KGB documents published by Ukraine paints a different picture there, which is even more dramatic than the TV show: There was no safety culture at all in the USSR for operating the reactors, even known best practices had not been implemented, i.e. the reactor building roof was inflammible. The everyday operations had been unsafe. Shutting down the emergency cooling system for a pro-longed period of time was no issue, even during normal operations. And the emergency cooling system was, like inside first generation western BWRs not even capable of providing adequate cooling should the main circulation pumps fail.

Last edited by Urwumpe; 10-20-2019 at 09:13 PM.
Urwumpe is offline   Reply With Quote
Reply

  Orbiter-Forum > Far Side of the Moon > Brighton Lounge

Tags
avatars, images


Thread Tools

Posting Rules
BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
Forum Jump


All times are GMT. The time now is 05:59 AM.

Quick Links Need Help?


About Us | Rules & Guidelines | TOS Policy | Privacy Policy

Orbiter-Forum is hosted at Orbithangar.com
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2019, vBulletin Solutions Inc.
Copyright 2007 - 2017, Orbiter-Forum.com. All rights reserved.