Did the fuel rod caps jumping cause the Chernobyl disaster?

[PixelPicaso]

TL;DR Summary

I'm thinking steam pressure made the cubes jump.

Readings showed the reactor's temperature had climbed to 4,650 C, almost as hot as the surface of the sun. An engineer who had been on a catwalk above the reactor ran into the control room, shouting that the fuel rod caps were jumping in and out of their sockets. These caps weigh 350kg (772 lb) each.

Hi, please forgive me if this is a totally stupid question. I can find articles like the one above stating that the cubes jumped up and down. But I can't find what caused it. At first I thought it might have been steam pressure building up, there by making the cubes jump. But now I'm not so sure because no one mentions seeing steam escaping arround the cubes. Then my dad said sometimes you don't see the steam, but its there. Could heat building up cause the cubes to jump, or was it the steam that made them jump? I've looked all over, but can't find the answere. If someone would help me with this, I'd be very greatful! Thank you so much! Sorry if this is a dumb question. PixelPicaso

 

[trurle]

Summary: I'm thinking steam pressure made the cubes jump.

Readings showed the reactor's temperature had climbed to 4,650 C, almost as hot as the surface of the sun. An engineer who had been on a catwalk above the reactor ran into the control room, shouting that the fuel rod caps were jumping in and out of their sockets. These caps weigh 350kg (772 lb) each.

Hi, please forgive me if this is a totally stupid question. I can find articles like the one above stating that the cubes jumped up and down. But I can't find what caused it. At first I thought it might have been steam pressure building up, there by making the cubes jump. But now I'm not so sure because no one mentions seeing steam escaping arround the cubes. Then my dad said sometimes you don't see the steam, but its there. Could heat building up cause the cubes to jump, or was it the steam that made them jump? I've looked all over, but can't find the answere. If someone would help me with this, I'd be very greatful! Thank you so much! Sorry if this is a dumb question. PixelPicaso

More likely, it was the (overheating) rods been partially ejected from reactor active zone before steam explosion did hit the caps and cause them to jump. Seals were still generally holding yet at that time. Would steam hit the caps directly, the results would be likely worse (due larger volume/area of Chernobyl reactor) than SL-1 accident, in which one of shield plugs pinned operator to the ceiling.

 

[Rive]

Actually, it is a very good question. Nicely caught.

As far as I know those channels were closed on the top by multiple seals, while the caps themselves were not fixed, so even in case of steam escaping they would not 'jump' (just simply fly).

My humble bet is that in case of a channel rupture the released mechanical stress made them jump. Just as like when you make something on the top of a desk jump by hitting the bottom with a hammer.

 

[trurle]

I have re-read the
http://samlib.ru/p/panchenko_g/chernobylxskajapodlostx.shtmlCitation in russian:
Начнем с того, что "сборка 11" имеет массу всего-то 50 кг... Далее, чтобы не возникало лишних сомнений. Слабовато (мягко выражаясь), верится, что Чернобыльский реактор был разрушен "уникальным землетрясением", происходящем раз в 1 000 000 лет! Вот надо было ему произойти именно в 1:23:47, чтобы добить разогнанный неуправляемым разгоном реактор!? Про "прыгучесть "сборок 11"". С одной стороны - зрелищно и красиво, но с другой стороны - не соответствует физике. И вот почему. "Гремучая смесь" - перегретый пар и и продукты пароциркониевой реакции - могли появиться только в одном случае - при разгоне на мгновенных нейтронах. Но этот процесс - разгон - длится несколько секунд, и даже меньше. А что же живописал Медведев в своем "опусе"? Он живоописал (несколько раз прерываясь), то, как Перевозченко с отметки +50.0 (50 метров над уровнем земли), с балкона в центральном зале, бежал по винтовой лестнице и влетел метеором в помещение БЩУ-4 сразу после взрыва реактора. Я думаю, Читателю неоднократно приходилось спускаться по винтовой лестнице, чтобы убедиться - бежать по ним (да ещё и с олимпийской скоростью!) - вовсе не получится. И по времени не получится! Если разгон на мгновенных нейтронах начался в 1:23:43, а через три-четыре секунды уже взрыв реактора... Спутал автор "Чернобыльской тетрадки" первый энергоблок с четвертым, где винтовой лестницы в центральном зале не было по проекту.

Citation above argues what
1) Caps were 50kg in weight, not 350kg
2) The site of jumping caps was Block 1, not Block 4.

If true, then caps jumped simply due ground-transmitted shocks after destruction of Block 4. It explains nicely survival of witness of "jumping" without introducing controversial timescales.

 

[artis]

I think we need to be more cautious with citations from foreign languages (Russian) and unknown sites.
The author of the citation also argues that the man who saw the control rod caps jumping did not have the time to make it back to the control room because the reactor when accelerating on "fast neutrons" should have exploded much sooner, which is false because an RBMK reactor is a graphite moderated reactor so even if all coolant was lost to the last drop of it the graphite would still moderate neutrons which is entirely the reason why the blast happened.secondly , google "RBMK" and see the reactor hall pictures of which there are many , take notice of the caps , the rectangular blocks which make up the reactor and look like some sort of 1960's "minecraft" monster.
There are two kinds of caps , ones for fuel channels and ones for the control channels where the control rod drives and instrumentation were located. Even in the pictures one can clearly see the fuel rod caps are thick and massive while the control rod ones have two small openings and seem lighter which would be logical because the control rod caps have devices and "stuff" under them so they have to be thinner to allow more space. So maybe they indeed weigh 50 kg, but the fuel rod caps I believe weight the full 350kg each, which is mentioned in most of the sources I have ever come by.
The reason why those caps are so heavy is two fold, first of all they provide a means to walk over the reactor and being of dense metal they are the last shield against leaking radiation background from the channel tubes under power operation.Also the assertion that the reactor caps were indeed bouncing in unit 1 which was located the furthest from 4th block is to my mind false, if the shocks were so large as to bounce reactor cover plates weighting hundreds of kilos then also the 1st unit would have been damaged in the accident which was not the case.Finally it is not hard to imagine heavy metal caps flying in the air given that the total energy released when the core finally "went off" was equivalent to about 10 tons of TNT which is almost twice as strong as the strongest conventional bombs used in WW2

 

[trurle]

I think we need to be more cautious with citations from foreign languages (Russian) and unknown sites.
The author of the citation also argues that the man who saw the control rod caps jumping did not have the time to make it back to the control room because the reactor when accelerating on "fast neutrons" should have exploded much sooner, which is false because an RBMK reactor is a graphite moderated reactor so even if all coolant was lost to the last drop of it the graphite would still moderate neutrons which is entirely the reason why the blast happened...

You mistranslated Russian term "мгновенных нейтронах " as "fast neutrons" while it actually mean "prompt neutrons".

Even in the pictures one can clearly see the fuel rod caps are thick and massive while the control rod ones have two small openings and seem lighter which would be logical because the control rod caps have devices and "stuff" under them so they have to be thinner to allow more space. So maybe they indeed weigh 50 kg, but the fuel rod caps I believe weight the full 350kg each, which is mentioned in most of the sources I have ever come by.
The reason why those caps are so heavy is two fold, first of all they provide a means to walk over the reactor and being of dense metal they are the last shield against leaking radiation background from the channel tubes under power operation.

You played minecraft too much. Real humans are about 40cm wide, not minecraft`s 1m. Therefore, you perception got distorted.

Seriously speaking, russian forum describe caps as "Made of steel 20, 250x250x250mm". Size and material are consistent with the 48 rows of silvery-gray caps fitting across 12-meter wide active zone of RBMK as seen on photographs. If "cubes" are made of solid steel, their weight would be 120kg maximal, and likely weight is smaller due cavities. Actual photos of RBMK cover cubes shows thick (but definitely less than 150mm thick) top plates and something resembling legs below, therefore 350kg weight is false statement - "cubes" must be at least 70cm thick for 350kg weight which is clearly not the case.

By the way, Russian "Steel 20" is Iron-Manganese low alloy steel, with density nearly the density of iron. It is very similar to AISI 1020.

 

[artis]

I haven't played minecraft in fact at all, the reference was more of a joke than anything real.

If the correct translation is "prompt neutrons" than that would be accurate.

In fact I did a simple calculation of taking the amount of channels and weight of each cap and to be honest indeed it sounds more reasonable to think the cap weight might be about 50 kg than 350kg.
It is said that the whole upper structure weighs about 200 tons, meant by the reactor vessel end plate through which the channel pipes are welded through so if each cap weight was 350kg then the weight of caps alone would surpass the weight of the reactor upper structure and would amount to something like 500 tons which sounds off the scale.
I assume I could be wrong and the real weight of the caps could indeed be 50kg or so with the ones over rod insertion drive channels weighting less even than that probably.

I tried googling but couldn't find any relevant data and the english sites saying 350kg are probably wrong as they are misinformed.
I do apologize if my faith in some common information sources in english about RBMK reactors have let me down.But I still stand on the assumption I made in my earlier post that the caps jumping in power unit number 1 instead of the 4th block shortly before accident happened is false. As I said before if this were the case then the shockwaves could have also destroyed the other 3 reactors which mind you were still working full power at the time of accident in reactor number 4. As we know this was not the case. each reactor had hundreds of high pressure steam pipes welded to the vertical core channels and such swaying and shocks could have easily broke multiple welds etc but as far as we know this did not happen.
Neighboring reactor continued to work to almost 5AM in the morning 3 hours into the blast of unit 4 and was only shut down over safety concerns and the much increased radiation background around workers.