Chernobyl and steam explosions

[sofia_rian]

Hello

I must do a report on Chernobyl: before, during and after.

I have all I need except for a clear and simple explanation of how steam explosions work in this regard.

All the online sites I've been on provide information that is too advanced, and as a non native English speaker, the language used on multiple sites (information in my native language is unavailable), is difficult.

Could someone provide me a simple and short and complete explanation of what caused the event?

Sofia

 

[Dr Dr news]

Consider a pressure cooker. As heat is added to the constant volume of the cooker, the liquid water is converted to vapor increasing the pressure which increases the boiling temperature which further increases the pressure until the pressure relief valve is opened, releasing some vapor and maintaining a constant pressure and temperature as long as there is some liquid. In the absence of a relief valve, the pressure and temperature continue to increase until the structure fails.

 

[russ_watters]

@sofia_rian

In the absence of a relief valve, the pressure and temperature continue to increase until the structure fails.

Note that a hundred years ago this sort of thing was a big problem. Now it's practically unheard of:

The ASME Boiler and Pressure Vessel Code (B&PVC) was conceived in 1911 out of a need to protect the safety of the public. This need became apparent shortly after the conception of the steam engine in the late 18th century. In the 19th century there were literally thousands of boiler explosions in the United States and Europe, some of which resulted in many deaths.

https://www.asme.org/engineering-topics/articles/boilers/the-history-of-asmes-boiler-and-pressure

 

[anorlunda]

I must do a report on Chernobyl: before, during and after.

I have all I need except for a clear and simple explanation of how steam explosions work in this regard.

Who is the audience for your report; engineers? the public? lawyers?

 

[scormus]

I try to give a slightly simplified account. It should be understandable to an engineer but the technicalities might be difficult for, say, a lawyer.

One or more steam explosions are believed to have occurred during the Chernobyl nuclear accident. In this context a steam explosion can occur when hot molten material, in this case overheated nuclear fuel at ca. 3000 K, comes into near-contact with water. The rapid transfer of heat from the melt to the water produces steam and a local pressure increase. Sometimes there will be a steam gap which partially insulates the water from the melt. However, if the steam gap is distubed the steam production may become so rapid that the surrounding water cannot "get out of the way". Then the pressure increase escalates into a large spike, initiating a shock wave. The shock travels through the region of mixed water and melt and as it does so it disturbs the steam gap, again forcing the melt and water into close contact and perpetuating the rapid steam production.

The situation is somewhat analogous to the detonation of an explosive mixture, but with sensible heat (i.e. high melt temperature) as the energy source instead of chemical energy. When that is delivered to the fluid, in this case the remaining unboiled water, it is converted into mechanical energy. In the case of Chernobyl it destroyed the entire power plant.

The steam explosion was not the initiating fault. The reason why the fuel became so overheated was due to a reactivity insertion when the reactor was being operated in an unstable state. Once the reactor power started to increase it increased more and more, maybe several hundred times in less than a second. A bitter irony about it all is that the operators were running an experiment to try and learn how to improve safety. Basically, they messed up big time.

Melt-water steam explosions do not have to be in nuclear accidents. They can occur naturally when volcanic lava flows into the sea, but without causing much damage.

Finally, I'd better mention that the interactions that determine the magnitude of the explosion, even whether there is an explosion at all, are very complicated and are not completely understood. So far studies show that an explosion is more likely if the space is constricted, if the melt iswell above its melting point temperature, and if the water is well below its boiling point. If all three, then it's fairly likely.

 

[.Scott]

Soon after the Chernobyl incident, a group from MIT was provided a briefing in Russia from engineers that were working at the reactor during and after the explosion. When they returned, they offered a briefing to anyone in the wider academic community interested. Only two people (both from Lowell Tech) took them up on this offer: Professor Fred Rojak and myself.

I just read through the wikipedia article on this. From my memory, there are couple of things that I can add. There were two major explosions and the wiki article describes them. The article also lists all of the main factors leading up to the steam explosion, the first and smaller of the two explosions. But I can understand how you could read through that and not really catch on to what was happening.

The key is that once cooling system water began to vaporize, water was ejected from the reactor core and that loss of H2O mass resulted in increasing reactor output - more heat and thus more vaporization. So once the reactor core started to loose coolant (water), the process was very self-aggravating.

The precise sequence of events that constituted that first explosion was not recorded by the instrumentation and is slightly different in the article than was explained to us in the briefing. Much of the the cooling system piping was described to us as having been severed along the top of the reactor and it was noted that the metal used in that cooling system was not intended to be subjected to rapid temperature swings - in that sense it was relatively "brittle". So the initial explosion was described as prompted primarily by this kind of cooling system failure that in turn resulted in the very rapid generation of steam within the reactor.

This is a bit different from the wiki article. The article describes the ejection of the cooling system (along with the control rods) to be concurrent with the explosion - rather than the cooling system failure leading the explosion.

The article refers to this self-aggravating condition as a "high positive void coefficient" - meaning a loss of coolant creates a net increase in reactor power. As a matter of designed-in safety, all US reactors have a negative void coefficient.

The second, more powerful explosion was almost certainly not related to steam - since there would have been little water vapor in direct contact with the reactor. During our discussions, it was noted that there would have been an abundance of Xenon in the reactor at that time and its rapid heating was not only likely to result in an explosion (by raising the temperature of the gas), but it would have also explained what was described to us as very fine fracturing that had occurred to the much of the core. At that stage, the Xenon was the only moderator remaining - especially in the lower portions of the reactor core.

 

[.Scott]

There is a document at this link: https://apps.dtic.mil/dtic/tr/fulltext/u2/a335076.pdf
It was written by a Soviet reactor operator who appears to be reconstructing the events at Chernobyl.
The portions most relevant to you question about the steam explosion are on pages 19 to 22.

Of particular interest is that the flow of coolant would have slowed during the seconds leading up to the explosion and stopped completely before the explosion.

The article also includes quite a bit of information about what was observed during the event.

The author has obvious first-hand engineering and operation knowledge of the RBMK reactor. But since his narrative seems to be a mix of his own knowledge and experience and the witness reports within his small industry, some caution is needed in using his article in citations.

 

[Rive]

I must do a report on Chernobyl: before, during and after.
...
Could someone provide me a simple and short and complete explanation of what caused the event?

Explanations of the event are available at various levels already, for readers with different level of knowledge. As @anorlunda already asked, for a helpful answer we need more details about your goals, level of understanding and your problems with the other explanations. Instead of giving just some n+1'th descriptions it would be more effective to patch up holes.