Chernobyl

[Dmitry Markov]

Dowly, the film You've posted a link to is very close to "my" article.

[u crank]

Originally Posted by Dowly

Apart from making Dyatlov the "incompetent bad guy", I really hated how little they concentrated on the roof clearing operation.

I agree. The narrative about the 'biorobots' deserved a lot more attention. It was a significant and very dangerous part of the story. Quite a while ago I watched some pretty compeling documentaries about them. Cannot imagine doing that. On the other hand I thought showing the guys shooting dogs was unnessasary and a waste of time.

As far as Dyatlov being the bad guy....all film makers have to have a bad guy. It's part of their code.

[Catfish]

edit found a better description here: https://en.wikipedia.org/wiki/Chernobyl_disaster

as a summary, the reactor had bneen held in an in-betwen state for too long (hours, literally), which rendered several security measures impossible. Also some moderators (graphite cylinder bundles to subdue the reaction) had been retracted manually. Maybe this is not a problem but .. why did they do that.

Imho this describes best what lead to the catastrophe (excerpt from above link):

" [...] At 1:23:04 a.m., the test began. Four of the main circulating pumps (MCP) were active (of the eight total, six are normally active under regular operation). The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.

Unlike western light-water reactors, the RBMK had a positive void coefficient of reactivity at low power levels, meaning that when water began to boil and produce voids in the coolant, the nuclear chain reaction increased instead of decreasing. Given this characteristic, the No. 4 RBMK reactor operation was now at risk of spiraling into a positive feedback loop, in which the formation of steam voids would reduce the ability of the liquid water coolant to absorb neutrons, increasing the reactor's power output, causing yet more water to flash into steam, and yielding a further power increase. Throughout most of the experiment the automatic control system successfully counteracted this positive feedback, inserting control rods into the reactor core to limit the power rise. However, this system had control of only 12 rods, as nearly all the others had been manually retracted.
At 1:23:40, as recorded by the SKALA centralized control system, a SCRAM (emergency shutdown) of the reactor was initiated. The SCRAM was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment.

One view is that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data showing this. Some have suggested that the button was not manually pressed, that the SCRAM signal was automatically produced by the emergency protection system, but the SKALA registered a manual SCRAM signal. Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration. Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and under calm conditions.[48]:578[49]

When the EPS-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 0.4 metres per second (1.3 ft/s), so that the rods took 18 to 20 seconds to travel the full height of the core, about 7 metres (23 ft). A bigger problem was the design of the RBMK control rods, each of which had a graphite neutron moderator section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor. That is, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with 1.25 metres (4.1 ft) columns of water above and below it. Consequently, injecting a control rod downward into the reactor in a SCRAM initially displaced (neutron-absorbing) water in the lower portion of the reactor with (neutron-moderating) graphite. Thus, an emergency SCRAM initially increased the reaction rate in the lower part of the core as the graphite extensions of rods moving down in the reactor displaced water coolant. This behaviour was discovered when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike, but as the subsequent SCRAM of that reactor was successful, the subsequently disseminated information had been deemed of little importance. [...]"

We should not forget that those reactors were initially also built for high performance, and to produce plutonium for nuclear weapons.


What i do not undrestand.. in one of the videos posted (from the HBO film) the graphite moderators are popping up and down.. as if there was no electrical control for them? Where are the electric engines to retract or lower the moderators located? I take it there has to be at least one for every bundle of 12 rods?

[ikalugin]

Originally Posted by Catfish

Half-life and half truths..

" [...]The reactor had no "design flaws." The reactors of the 60s and 70s were optimized for performance. Safety was not a central issue (as with the Fukushima reactors, also a product of that time). The Russian designers were well aware of the weaknesses of the design, the operating rules considered this accordingly. But this only works with a good safety culture, and the latter was missing completely.
In the Leningrad 1 nuclear power plant 1974, the first year of operation with a "Chernobyl type reactor", several serious problems occurred. Among other things, a partial meltdown occurred with partial destruction of the reactor core. Three employees died. And in 1982 a meltdown also occurred in Block 2 of Chernobyl. In both cases, significant amounts of radioactive substances were released as the reactors had no containment. Lessons were barely learned; on the contrary, everything was kept top secret. [...] "

Some points:

The 1975 incident in Leningrad was on a powerplant operated by a different agency (not the one that operated other RBMKs), which indeed was very secretive.

During that period of operation several safety measures were introduced, such as the inclusion of bottom insertion rods into the AZ5 command, but those were not yet retrofitted to blocks 3 and 4 (they were on blocks 1 and 2 on Chernobyl for example).

[ikalugin]

Originally Posted by Catfish

edit found a better description here: https://en.wikipedia.org/wiki/Chernobyl_disaster

as a summary, the reactor had bneen held in an in-betwen state for too long (hours, literally), which rendered several security measures impossible. Also some moderators (graphite cylinder bundles to subdue the reaction) had been retracted manually. Maybe this is not a problem but .. why did they do that.

Imho this describes best what lead to the catastrophe (excerpt from above link):

" [...] At 1:23:04 a.m., the test began. Four of the main circulating pumps (MCP) were active (of the eight total, six are normally active under regular operation). The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.

Unlike western light-water reactors, the RBMK had a positive void coefficient of reactivity at low power levels, meaning that when water began to boil and produce voids in the coolant, the nuclear chain reaction increased instead of decreasing. Given this characteristic, the No. 4 RBMK reactor operation was now at risk of spiraling into a positive feedback loop, in which the formation of steam voids would reduce the ability of the liquid water coolant to absorb neutrons, increasing the reactor's power output, causing yet more water to flash into steam, and yielding a further power increase. Throughout most of the experiment the automatic control system successfully counteracted this positive feedback, inserting control rods into the reactor core to limit the power rise. However, this system had control of only 12 rods, as nearly all the others had been manually retracted.
At 1:23:40, as recorded by the SKALA centralized control system, a SCRAM (emergency shutdown) of the reactor was initiated. The SCRAM was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment.

One view is that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data showing this. Some have suggested that the button was not manually pressed, that the SCRAM signal was automatically produced by the emergency protection system, but the SKALA registered a manual SCRAM signal. Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration. Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and under calm conditions.[48]:578[49]

When the EPS-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 0.4 metres per second (1.3 ft/s), so that the rods took 18 to 20 seconds to travel the full height of the core, about 7 metres (23 ft). A bigger problem was the design of the RBMK control rods, each of which had a graphite neutron moderator section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor. That is, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with 1.25 metres (4.1 ft) columns of water above and below it. Consequently, injecting a control rod downward into the reactor in a SCRAM initially displaced (neutron-absorbing) water in the lower portion of the reactor with (neutron-moderating) graphite. Thus, an emergency SCRAM initially increased the reaction rate in the lower part of the core as the graphite extensions of rods moving down in the reactor displaced water coolant. This behaviour was discovered when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike, but as the subsequent SCRAM of that reactor was successful, the subsequently disseminated information had been deemed of little importance. [...]"

We should not forget that those reactors were initially also built for high performance, and to produce plutonium for nuclear weapons.


What i do not undrestand.. in one of the videos posted (from the HBO film) the graphite moderators are popping up and down.. as if there was no electrical control for them? Where are the electric engines to retract or lower the moderators located? I take it there has to be at least one for every bundle of 12 rods?

1) citing wiki is a crime in itself.

2) while graphite is indeed a moderator, the moderators job is to (broadly speaking) speed up the reaction, not to slow it down or shut it down.

3) as moderator is the de-facto standard internal filler material of the reactor (the channels for control and fuel rods etc are inside tubes placed inside of the graphite blocks) it is there are all times.

4) majority of channels do not have (moving) control rods and thus have simple covers on top of them.

5) using TV series as a source is a bad idea.

[Catfish]

Right, citing Wikipedia is sometimes a bad idea; the TV always

I just do not understand why it was built that way, but it is stated that it was an "effective" (read: cheap) method to generate a lot of energy while breeding plutonium for weapons of course.

So i have two questions:

1. Why are just of all the upper control rods tipped with nuclear graphite at the lower end, so the reaction is being fuelled just of all when you usually want to decrease the reaction.
The graphite fuels the reaction, and also the displaced water (by lowering the control rods) leads to less water and the remaining rest getting hotter, and fast.

2. I wondered whether the TV flick showed the central hall floor with the upper shield cover correctly (?) when those caps began to 'dance', popping up and down:
(20 seconds into the film)

As far as i learned the center area being shown here consists of individual removable steel-graphite plugs, located over the tops of the channels.

So is it possible like as shown that they were forced up by expanding water steam, hydrogen and maybe even the helium-nitrogen atmosphere usually contained in the reactor vessel (20 seconds into the film)?

I also wonder where the controls are to lower the control rods, i do not see any motors or linkage

[THE_MASK]

Sounds very similar to Fukishima . Everythings ok , dont panic .

[McBeck]

Originally Posted by ikalugin

1) citing wiki is a crime in itself.

2) while graphite is indeed a moderator, the moderators job is to (broadly speaking) speed up the reaction, not to slow it down or shut it down.

3) as moderator is the de-facto standard internal filler material of the reactor (the channels for control and fuel rods etc are inside tubes placed inside of the graphite blocks) it is there are all times.

4) majority of channels do not have (moving) control rods and thus have simple covers on top of them.

5) using TV series as a source is a bad idea.

Point 2) : Yes, you are correct that this is the effect, however the reason is critical here. The reason it speeds up the reaction is because it slows down the neutrons. If the neutrons are not slowed down, they will not cause a fission reaction. The water in this reactor served as absorbing neutrons, thus it remove the trigger for fission reaction and that water was then replaced with something which introduced a trigger for reaction.

[ikalugin]

Originally Posted by McBeck

Point 2) : Yes, you are correct that this is the effect, however the reason is critical here. The reason it speeds up the reaction is because it slows down the neutrons. If the neutrons are not slowed down, they will not cause a fission reaction. The water in this reactor served as absorbing neutrons, thus it remove the trigger for fission reaction and that water was then replaced with something which introduced a trigger for reaction.

Liquid water and steam are also moderators. This is what classical PWR and BWRs use in fact as the moderator.

[ikalugin]

 

Originally Posted by Catfish

Right, citing Wikipedia is sometimes a bad idea; the TV always
I just do not understand why it was built that way, but it is stated that it was an "effective" (read: cheap) method to generate a lot of energy while breeding plutonium for weapons of course.
So i have two questions:
1. Why are just of all the upper control rods tipped with nuclear graphite at the lower end, so the reaction is being fuelled just of all when you usually want to decrease the reaction.
The graphite fuels the reaction, and also the displaced water (by lowering the control rods) leads to less water and the remaining rest getting hotter, and fast.
2. I wondered whether the TV flick showed the central hall floor with the upper shield cover correctly (?) when those caps began to 'dance', popping up and down:
(20 seconds into the film)

As far as i learned the center area being shown here consists of individual removable steel-graphite plugs, located over the tops of the channels.
So is it possible like as shown that they were forced up by expanding water steam, hydrogen and maybe even the helium-nitrogen atmosphere usually contained in the reactor vessel (20 seconds into the film)?
I also wonder where the controls are to lower the control rods, i do not see any motors or linkage

The idea was not to breed Pu for the weapons, the idea was to take the well known Pu creating reactor and use it for civilian purposes because building a low density reactor (low pressure, does not require specially made pressure vessels, you can refuel it during operation etc) reactor using natural uranium (no need to enrich it, they didnt manage to get quite that far) seemed like a good idea.


Hot fuel leads to a slower reaction all other things being the same. Broadly speaking the graphite is used to balance reactivity out with the water it displaces. The problem is the positive void coeff which was critical due to the low ammount of feed water going into the reactor, which meant that it was boiling from the bottom. Neutron fields in various heights of the reactor depending on the position of the control rods:


The issue with the ends effect was fixed by making the graphite tips longer so that they would produce the same effect through the entire height of the reactor.

No idea about the pop up effect, I think the motors are between the upper hull and the floor:

[Catfish]

^ Thank you, Ikalugin

So if i understand this right (much simplified):

The big mechanism (gantry crane) in the background is for extracting/exchanging the fuel rods.

Control rods consist of two sets, one bigger and longer set can be extracted/inserted from the upper side, the other smaller and shorter set from the lower side (control rods are different depending on task)

As you said the mechanisms/engines to move the control rods must be somewhere in or near the area of the upper and lower biological shield.

There are two fuel rods stacked above each other in each one pressure pipe.

So the pairs are connected/linked to each other so they can be axtracted together from above with the ganty crane?

The mechanism/engines for inserting extracting the fuel rods are in the area of the control rod engines? If the pairs are connected then the engines for fuel rod control should only be in the upper part?

[ikalugin]

Yes, the lower control rods are important, as the graph I showed shows the neutron fields spike at the bottom during the main control rod insertion. Unfortunately block 4 was not yet retrofitted to include bottom rods into the AZ5 command, as were other reactors, for example blocks 1 and 2 were refitted in such a way under the initiative of the operator to improve their safety. The other safety feature would be to use longer graphite tips, so that they remain in the reactor when the control rods are fully elevated.

The fuel rods are pulled up using the refueling machine, what happens is that the machine attaches a tube to the top of the channel, filling it with water and then uses an internal crane to pull the old fuel rod up and then place the new fuel rod down.

You can see the process on this 3d model in some detail.

[ikalugin]

This video, allegedly shows removal of radioactive pipes from Chernobyl exclusion zone for sale, protected by the Ukrainian FBI equivalent.
Allegedly (so said in video) the pipes radiate 50 Sv or broadly speaking equivalent to 5000 rem (which does seem pretty high, so maybe they missed the scale of the unit, ie milli or micro sieverts).



(they all talk in Russian btw)

[Catfish]

"The true story of Chernobyl". Report about how things unfolded around Chernobyl after the explosion.
It is in german, and no other subtitles abvailable; if anyone finds an english version ? Will also look for it myself.

Much worse than i expected even now. People in Germany were not informed properly, some rumours, schools closed, scholars sent back home, but no real information. I had no idea that so much people died, already that early.

And Russia? After two days the russian Politbureau was informed by Sweden about the 'incident', because of abnormally high radioacivity readings at the swedish Forsmark reactor, 1200 km from Chernobyl. It seems no one had told the Politbureau, all they heard from the scientists via KGB that the "situation is under control", "reactor will be online again soon". So no official coordinated action for a too long time.

The 'liquidators' shoveling radioactive debris from the roof in the film, were exposed to 10-12,000 Roentgen per hour, and they only managed to bring down the overall radiation by 35 percent.

[Dmitry Markov]

Catfish, as for authorities reaction inside USSR - actually destruction of reactor was confirmed by Government and Politbureau by 15-00 on 26th april - same day. Evacuation of Pripyat was nearly instant - next day 27-th april and it was very effective: during less than 24 hrs authorities gathered 1200 buses, 360 trucks, two diesel railway trains, recon of safe routes was made, and columns were moving according to reconned safe routes - by the 16-00 27th april evac was accomplished. So I cannot say it was a slower reaction than say Japanese reaction at Fukusima. What was wrong in my opinoin - a decision not to stop 1'st of May Demonstration in Kiev.