Did Chernobyl divers prevent a multi megaton explosion?

[jim hardy]

You should watch Threads sometime.

and Peter Watkins' 1965 "The War Game" ..

 

[Astronuc]

Can we just call it 2 megatons per month? 24 megatons per year?
How long had Chernobyl run in that fuel cycle prior to the explosion ?

Anyhow -
That'd be my guess
as to why a pyhysicist told them to expect consequences similar to a few megatons ,

Reactivity transients (and explosions) take place over millisecond time frames. In PWRs, some tests have pulse widths of 10 to 30 ms, so the megatons would be greatly reduced to tons. During a pulse, the power can be many times the steady state level, usually less than a factor of 10. However, I have not seen numbers for the pulse height (peak power level) or pulse width for Chernobyl 4.

Mikhail V. MALKO, The Chernobyl Reactor: Design Features and Reasons for Accident
http://www.rri.kyoto-u.ac.jp/NSRG/reports/kr79/kr79pdf/Malko1.pdf
Malko provides an estimate of 1.0 TJ, "equivalent to the energy of explosion of approximately 200 tons of the trinitrotoluene." (top of page 22, and reference 18).

It is difficult to determine the configuration of the core since those records are not readily available. We know the following:

First Criticality: 25 November 1983
First Grid Connection: 21 December 1983
Commercial Operation: 25 March 1984
Permanent Shutdown: 25 April 1986 (Catastrophically destroyed)
Ref:
https://www.world-nuclear.org/reactor/default.aspx/CHERNOBYL-4
Chernobyl 4 was a RBMK-1000. It had 1661 fuel channels, and it was capable to replace 2 fuel channels per day, so it would take approximately 830 days to replace all the fuel in the reactor, which is more than two years, but Chernobyl 4 reactor was operating for 2.3 years at the time of the accident. Various papers mention that the reactor was shutting down for routine maintenance, but they do not mention the frequency associated with routine, e.g., an annual schedule, which means that it the unit was shutting down for the second outage.

Malko mentions comments "blue light corresponds to the temperature about 6,000 K," which is true, however Cerenkov radiation is blue, so very likely the "blue flash" was due to Cerenkov radiation, or ionization of the air/steam/hydrogen after the initial steam explosion. One can observe a blue flash in a pulsed reactor.

Another good reference on the accident (See Table 1 and Figure 11, and Figure 10)
https://mragheb.com/NPRE 402 ME 405 Nuclear Power Engineering/Chernobyl Accident.pdf

 

[artis]

interesting to see the gamma ray interference when the camera is rather close to the core and the water thickness is not high enough, in the other research type pool reactors the camera is further away, the blue light still can be seen but there seems to be no destructive interference with the camera photo-electronics , would this indicate that a thick layer of water not only moderates/absorbs neutrons but to a large extent shields gamma radiation?I have no detailed information on the subject but my intuition tells me that they rushed the test so it can be finished (hopefully) before the planned maintenance shutdown of the reactor where probably also some (most?) fuel assemblies would be changed because the dates from commercial operation until spring 1986 suggest 2 years of full power which would seem as many fuel assemblies were at the end of their useful life so full of fission products.
PS. @Astronuc , even though a bit off topic, I wonder when they changed a fuel assembly while reactor is online, as the fuel assembly comes out, did they close the water flow of that specific channel beforehand? Otherwise I fail to see how this would not result in hot water/steam spewing everywhere.
What still intrigues me is how each assembly rod had a seal that could be tightened or loosened by the remote machine in order for the channel to be sealed before being put back into operation. I assume CANDU'S have similar things since their also channel type.

 

[jim hardy]

However, I have not seen numbers for the pulse height (peak power level) or pulse width for Chernobyl 4.

surely there's no direct measurement.

This statement from your second reference seems plausible
pdf page 17, right below the picture fig 14.

 

[Astronuc]

PS. @Astronuc , even though a bit off topic, I wonder when they changed a fuel assembly while reactor is online, as the fuel assembly comes out, did they close the water flow of that specific channel beforehand? Otherwise I fail to see how this would not result in hot water/steam spewing everywhere.
What still intrigues me is how each assembly rod had a seal that could be tightened or loosened by the remote machine in order for the channel to be sealed before being put back into operation. I assume CANDU'S have similar things since their also channel type.

I'm more familiar with the CANDU system, but I understand in both cases, the refueling machine locks onto the cooling channel, then the cap is removed, the spent fuel is retrieved and fresh fuel inserted, then the cap is returned and locked. I don't know if there are valves to block/stop the coolant flow into the channel, but perhaps there is.

CANDUs have horizontal channels, whereas RBMK are vertical. In CANDU systems, to refuel a channel, a pair of fuelling machines latch onto each end of the channel. New fuel bundles are inserted at one end, old fuel is pushed along the channel, and spent fuel is discharged into the other machine. Typically, four or eight fresh bundles are added at each fuelling. The RBMK must refuel from one end, the old fuel must be retrieved, then the new fuel installed.

See page 4/5 - https://canteach.candu.org/Content Library/20043404.pdf

 

[jim hardy]

...so the megatons would be greatly reduced to tons.

Agreed . The energy released in the explosion is orders of magnitude less than a weapon generates.

The fission product inventory in the reactor core that's available for release by that comparatively small explosion however is disproportionately large because it's been building since reactor startup.

That was my point.
The physicist could have said "You're going to get maybe a ton's worth of explosion but megatons worth of fallout. "
Would they have stopped listening halfway through his statement ?

The scene as played made better drama.
I sure don't now what really was said

old jim

 

[Astronuc]

@jim hardy, I understand your point. Someone has estimated the contamination from Chernobyl,

The resulting radioactive release, Medvedev estimates, was equivalent to ten Hiroshimas. In fact, since the Hiroshima bomb was an airburst--no part of the fireball touching the ground--the Chernobyl release polluted the countryside much more than ten Hiroshimas would have done.

Ref:https://www.pbs.org/wgbh/pages/frontline/shows/reaction/readings/chernobyl.html
I'd like to see the calculation.

 

[artis]

Isn't it a bit misleading in general to compare fallouts from exploded very specific design nuclear reactors (specific design in a way that only worsens the damage, aka burning graphite etc) and nuclear bombs?

Every bomb ever has had the chance to fission for a very brief amount of time versus a reactor that has piled up large amounts of fission products , especially the RBMK given it had the largest sized core of all known commercial reactors and I guess also the most fuel in tons compared to other smaller cores.
Just asking.

As far as I am aware of all the countless nuclear bomb detonation places none is so heavily polluted that it requires a sealed off exclusion zone.

 

[Astronuc]

Isn't it a bit misleading in general to compare fallouts from exploded very specific design nuclear reactors (specific design in a way that only worsens the damage, aka burning graphite etc) and nuclear bombs?

Every bomb ever has had the chance to fission for a very brief amount of time versus a reactor that has piled up large amounts of fission products , especially the RBMK given it had the largest sized core of all known commercial reactors and I guess also the most fuel in tons compared to other smaller cores.
Just asking.

I believe it is quite natural to compare fallout from nuclear weapons tests, since there was substantial fallout and contamination from atmospheric tests.

As far as I am aware of all the countless nuclear bomb detonation places none is so heavily polluted that it requires a sealed off exclusion zone.

I believe the areas of the Marshall Islands, e.g., Bikini and Eniwetok atolls are off-limits due to contamination, and I believe other areas where weapons were tested are exclusion zones, e.g., Maralinga in South Australia.

 

[jim hardy]

Isn't it a bit misleading in general to compare fallouts from exploded very specific design nuclear reactors (specific design in a way that only worsens the damage, aka burning graphite etc) and nuclear bombs?

Maybe.
One tries to reach an audience by relating to something with which they are familiar.

My generation is old enough to remember cold war days when Civil Defense(in US) handed out educational material on fallout. So we learned that a weapon creates fallout that's dangerous over only so much area for so much time.

If i misled you (or anybody else) please forgive me. I picked a familiar-to-me idea assuming it would resonate.. i guess today's population by and large doesn't remember "Duck and Cover" drills in grade school.

It would be a mistake to present the explosive forces as similarly powerful
but not a mistake to present the respective fallouts as similar in effect.

How else would you reach an audience of non-scientific people? That's a question not an argument.

old jim

 

[artis]

No Jim, everything is fine the comparison was actually great,

 

[russ_watters]

I have a feeling that what @russ_watters is annoyed about is the hysteria around the movie, and not about the hysteria in the movie... What is indeed getting really annoying. It is getting really hard to do any search for facts, and it is just started to spin... I've already started using time-limited search: any hit from the past half year excluded.

The movie is a psychological/societal drama based on really well gathered background, but adapted to TV/movie. I see less and less reason to discuss the content of the movie (!) in science topics.

The problem is that [my understanding is] the movie is presented as a historical documentary, not a dramamentary* or even historical fiction, and it is being discussed in a technical section of PF, not the sci-fi section.

What little I've read about it is not good: there are wildly false stories being told and due to the tone and statements from the producers claiming accuracy, people - including reviewers - are coming out claiming false things to be true, while championing the accuracy! In terms of informing the public, that's as bad as it could possibly be.

That's much worse than "The China Syndrome", which is an openly fictional movie.

*I just made that up; I don't know if there is a term for it.

 

[russ_watters]

Here's a critical review from no less a science stalwart than Forbes. There's two separate takes:
1. "Chernobyl" isn't about nuclear power (so it doesn't matter that it gets stuff wrong) -- but it is.
2. "Chernobyl" is pretty accurate -- but it isn't.

The first:

This is a point that the creator of “Chernobyl,” Craig Mazin, has stressed. “The lesson of Chernobyl isn’t that modern nuclear power is dangerous,” he tweeted. “The lesson is that lying, arrogance, and suppression of criticism are dangerous.”...

Personally, I’m not so sure. Having now watched all five episodes of “Chernobyl,” and seen the public’s reaction to it, I think it’s obvious that the mini-series terrified millions of people about the technology...

“I watched the first episode of Chernobyl,” tweeted Sarah Todd, a sports writer at the Philadelphia Inquirer. “Then I spent a couple of hours reading about nuclear power. Now I’m in a full blown panic and I need someone to explain to me how it is at all okay to live on the east coast when this is the situation.”

FYI, Philly is 25 miles from the Limerick nuclear plant and I live 6 miles from the plant. She's panicking over the plant while I'm happy it isn't a coal plant.

A couple of examples of the second point:

In interviews around the release of HBO’s “Chernobyl,” screenwriter and show creator Mazin insisted that his mini-series would stick to the facts. "I defer to the less dramatic version of things,” Mazin said, adding, “you don’t want to cross a line into the sensational."

In truth, “Chernobyl” runs across the line into sensational in the first episode and never looks back...

“Chernobyl” ominously depicts people gathered on a bridge watching the Chernobyl fire. At the end of the series, HBO claims, “it has been reported that none survived. It is now known as the "Bridge of Death.”

But the “Bridge of Death” is a sensational urban legend and there is no good evidence to support it...

The New Yorker repeated the claim that a woman’s baby “absorbed radiation” and died. The New Republic described radiation as “supernaturally persistent” and contagious (a “zombie logic, by which anyone who is poisoned becomes poisonous themselves”). The Economist, People, and others repeated the “bridge of death” urban legend.

The title topic of this thread is of course another example of an event that didn't happen (discussed in the article).

https://www.forbes.com/sites/michae...chernobyl-gets-nuclear-so-wrong/#52c2bc5a632f

 

[whitehare]

In reference to the OP's question of "Did Chernobyl divers prevent a multi megaton explosion?"and assuming the Soviets had a nuclear physicist worth his salt advising them at the time, would such a physicist have had any good reason at the time--perhaps allowing for imperfect or unknown information at the time-- to think that, in the worst case, such a megaton+ explosion was possible from the corium encountering a large pool of water under the reactor?

I'm not sure if it has any bearing on the answer, but from my reading these RBMK reactors were designed to both produce electricity and enrich fuel for nuclear weapons (unlike in the West, where there are reactors for power generation and different reactors for creating weapons fuel). That is according to Wikipedia, which says that for RBMK reactors, their primary use was:

Primary use	Generation of electricity and production of weapon grade plutonium

https://en.wikipedia.org/wiki/RBMK
So, maybe the Chernobyl fuel, used to produce weapon grade plutonium, was or became different enough and more dangerous than the typical western style nuclear power fuel that is perhaps being assumed here?

 

[artis]

No Chernobyl used the same uranium as western plants only with less enrichment as the reactor core physics and design was different that's it, any fuel develops byproducts as it is irradiated one of them is plutonium , the reason why an RBMK can be used for Pu production is not in physics but in the design of the reactor, as it can be reloaded while online, you can take individual fuel assemblies in and out of the core one at a time wile the rest of the core pipes are delivering steam and the chain reaction doesn't stop. That's it.
you could open a PWR and take it's fuel out for Pu production it just wouldn't be economical as the reactor design is such that it takes a lot of time to do that and the reactor must be shut off.

 

[artis]

@russ_watters I totally understand your point, people these days will freak out about everything, to give Craig Mazin some credit I must say he is a screenwriter not a physicist and there have been far dumber things presented on screen than Chernobyl , IMDB lists it as "drama, history" so obviously the viewers should at least get the point their not watching a documentary.
The thing that got us talking about this in the first place is the shows very high ratings and popularity which means that from an arts point of view it is rather good, I personally liked it too but maybe because I already know my facts so for me it was just another entertainment thing.
But in all honesty I think the ones to truly blame for the fact that Chernobyl ever became a word in the English language are the Soviet bureaucrats who cared more for promotions than anything else (a historical fact) and the designers of the reactor which somehow managed to screw up (knowingly) a few technically rather simple but very important aspects like the control rods and their insertion time.
Then of course there's Dyatlov whose best excuse until his death was "I did everything right"
And for Dyatlov's credit it's hard to work in a job if you are not told the whole truth and full specifications of the thing you are operating. The reactor designers never told the operators that the core has positive reactivity and the details of it, that is why in Ignalina NPP for example they found this fact out by surprise when shutting down one of the reactors for planned maintenance, it had happened in other plants too.
In Leningrad NPP back in 1975 this was probably the reason for a burst in one of the channels under a planned power increase. All these instances were reported but ignored by the higher authority and no changes were made only after 1986.

Pardon for getting off topic here but indeed this accident could have been totally avoidable and nuclear power today would have had a much better name, all of this because of lies, incompetence and the absurd secrecy surrounding everything in the USSR . At least Mazin has got this on point.PS. @russ_watters I totally agree I too would have nothing against living next to a reactor but a modern one operated by sane people like the one you are living next to, not an RBMK pre-upgrade version one operated by sociopaths and high school students (Toptunov was 25 and inexperienced)

 

[gmax137]

Here's an interesting "Viewpoint: Chernobyl and a very modern safety culture" from World Nuclear News.
http://www.world-nuclear-news.org/Articles/Viewpoint-Chernobyl-and-a-very-modern-safety-cultu

EDIT: Here's another piece worth reading:
http://www.world-nuclear-news.org/Articles/The-drama-and-the-facts-about-Chernobyl

 

[jim hardy]

FYI, Philly is 25 miles from the Limerick nuclear plant and I live 6 miles from the plant. She's panicking over the plant while I'm happy it isn't a coal plant.

PS. @russ_watters I totally agree I too would have nothing against living next to a reactor but a modern one operated by sane people like the one you are living next to, not an RBMK pre-upgrade version one operated by sociopaths and high school students (Toptunov was 25 and inexperienced)

Amen .. I worked in one for thirty years.

Agreed, it is important that workers understand of what these things are capable.
Humble attention to detail is a must. Else the small things of the Earth will confound the mighty.
Big machinery demands humility and it will punish the haughty cavalier attitude with a venegeance.

Remember that last scene in "Devil's Advocate" :


old jim

 

[artis]

I believe that from all the commercial reactor types RBMK, especially before the updates was the most "critical" (pun intended) reactor in terms of operator knowledge and ability to think and make right decisions in short time scales.
I talked with one engineer who worked his whole life in my local university's experimental reactor and we basically agreed that the operators should understand nuclear physics at least the basics like what drives what and which actions influence which outcomes.I wonder did the guys back at unit 4 of Chernobyl understood that for a reactor that operates on such a small reactivity margin, disabling turbine steam and decreasing water flow through the core means more(all?) water turning into steam and essentially the core loosing it's only left neutron absorber.
But then again nobody told them that the reactor also has positive reactivity at the low side of power,
essentially Dyatlov was carrying out the test as if he was operating a genIII or IV PWR plant but he wasn't.

The only thing I wonder is given the guys had some time and experience with their RBMK, do such things as positive reactivity somehow can be witnessed and felt by intuition(reading gauges and analyzing) during operation and shutdowns and startups, because the operators had noticed these things at other plants.
But I guess this is a topic for it's own thread.

 

[franklyamazed]

Lots of interesting discussion here. My view is that there was no certainty in April 1986, immediately after the accident they had to guess at the cause and guess at the best way to deal with each problem they faced. They must have just been in total shock to be in the center of such a catastrophic sequence of events. Many of their mitigating actions now seem misguided and have simply added to the tonnage of radioactive debris that is now going to need to be removed and safely processed.

Firstly, they did not know the core was 'empty' and I do not think at that stage they were aware of the corium, or where it was located, this happened during the 'Complex Expedition' in December 1986, and the discovery shocked them. In April they were assuming the core was still mainly one mass and that a meltdown was in progress below it, this was the basis for imagining that contact with the water could simultaneously cause a core implosion and neutron reflection, and led to a worst case scenario assumption of the possibility for a further Criticality Event!

It would have been irresponsible to not assume the worst, considering they DID understand how bad everything already was. Don't forget the one thing they did have was horrendous radiation readings emanating from the site. It certainly could have caused worse site contamination, but considering how bad everything already was, the idea that they could lose the other reactors on site MUST have been an even greater nightmare. Imagine how stunned the Japanese were as they sequentially lost each reactor at Fukushima Daiichi? Experiencing the Loss of Coolant Accidents that EVERYTHING had been designed to prevent is definitely a WTF shock.

Let's face it, even retrospectively, we are still uncertain about the exact chain of events even 33 years on. It is actually a lot of clever guesswork, and the one thing that stands out about the accident is the lack of recorded instrumentation data, and a lesser person may suspect that this information received a political burial, unless it was radio-logically erased during the event, even so the Soviets usually preferred to rely upon more basic systems like paper plot recorders, and I do understand that most of Unit 4's instruments simply went off their scales and then failed.

I have found that one of the most interesting aspects of the accident is the 'Nuclear Jet Event' theory about the reactor's explosion and you can read some ideas about it ...

Chernobyl Nuclear Jet Discussion

This hypothesis has come from examining the forces involved in launching the 2000 ton upper biological shield through the roof of the reactor building and there is evidence of extreme downward forces through the lower biological shield too. When I first came across the suggestion I immediately thought that if that had happened then telltale fission byproducts would have been detected, and then I discovered that they had been.

The sheer scale of the event has largely been buried, the speed that the Soviets buried the evidence seems more than just to make it safe, they wanted it to 'disappear' as if it never happened...
Red Forest UAV Overflight


I hope you all enjoy the continuing discussion...

 

[dangderang2000]

Judging the TV series by what is written about it isn't fair, and if you haven't seen it you're missing out. It's actually a brilliant psychological drama, one of the best I've seen. It's filmed in a highly realistic way, when it would be all too easy to go over the top with special effects. The themes are mostly about human behavior, lies, ambition, oppression, bravery, sacrifice.

Reactor physics is not the main topic, but many of the key characters are physicists - both good and bad. Some aspects seem exaggerated, like the one OP brought up. But even though it's a drama, it's more scientifically sound than 90% of any popular science stuff on TV about physics - be it quantum computing, solar cells or nuclear power. It's 5 hours long after all.

I'm reminded of some of Kubrick's classics like Dr. Strangelove and 2001. The main message I felt, is that humans, including scientists and engineers, are not perfect nor rational. Combining this with the potency of nuclear fission creates a very dangerous mix. The lesson learned should be to remain humble and accept that we can't foresee every technical possibility or every human behavior. That's why it's so important to watch for scientists.

Towards the end, there's also a not so subtle reference to the fight about the truth around climate change. That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

 

[artis]

The show is good but don't be too overtaken by it, the truth is that in fact most scientists knew the flaws of the RBMK reactor as well as many of the engineers that worked on it even though they were not told about them, they found out by simply being smart and seeing the reactor in actual working conditions.

The truth as always is a bit more hidden and trivially rotten, fission by itself is not dangerous , in fact nothing truly is. But take any thing and combine with secrecy , neglect, systematic pressure and a utopian ideology driven bureaucracy and you get a bad bad result. Most people don't realize this but an awful lot of things had to be neglected and done wrong for a long time until the "lottery" was finally won in 1986.
From this point I agree with the show that lies and laziness can make many things go wrong.

In fact we can see potential technical problems or bad human behavior, many saw it in the Soviet Union , the problem is that once they saw what they saw and tried to do something about it they were stopped or simply silenced.
The same year 1986 few months before Chernobyl, the Challenger disaster happened and it was almost like an omen to Chernobyl, the designers and builders/contractors of the NASA space shuttle also knew their o-ring system had modes where it could fail (temperature etc) but never fully disclosed this to the managers at NASA which not being physics geniuses decided to go with the launch even though the parameters (air temp) was lower than deemed safe by the designers of the boosters but since it was their product they were not keen on reporting the potential problems that could arise in some very rare situations much like the designer of RBMK thought the operators would never put the reactor in a low power state in which the reactor was known (by some) to be dangerously unstable.

Quoting Bob Marley from his Redemption song :"Have no fear for atomic energy,
Cause none of them can stop the time"

I am not afraid of nuclear reactors or spaceflight but I am afraid of idiots and egomaniacs being in positions of power approving reactors or building and operating them.

 

[jim hardy]

That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

Oh Pu-lease, don't restart that GW debate here.

 

[PeterDonis]

Towards the end, there's also a not so subtle reference to the fight about the truth around climate change. That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

This is off topic for this thread, and it is also a topic where PF has strict rules for discussion. Please be advised.

 

[artis]

A recent Sky short documentary about their own created tv show, but go to 22:45 and here I give them credit for being realistic with respect to the importance of the Chernobyl divers which is also the title of this thread.
disregard the short intro from the Chernobyl tv series and pay attention from the moment the woman voice narrator starts saying that they wanted to drain the water because there was a risk of an explosion which could have blown up the entire plant itself and so destroy or damage the neighboring 3 intact reactors. Then one of the divers start telling his recount of the events.

I think that this was the most probable scenario considered even back then, the idea that the core meltdown material won't create a H bomb like blast(which is impossible and scientists knew that even back then with limited info I think) but instead might create a blast strong enough to destroy the structures surrounding the blast area. Now this scenario was entirely real as the unit 3 reactor was located just besides a few walls from the 4rth unit, these walls had already been shocked and vibrated with the strong blasts from the original explosion so I think they took no chances and no risks.

Just as a sidenote , can someone with knowledge say how does the critical mass requirement changes for reactor grade fuel when it is fused together in a lava mass instead of when it is set apart in individual fuel pellets packed inside fuel rods which are then separated by some distance and each fuel assembly is separated from the next by an even larger distance when it is under normal operating core conditions,
I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced? This IIRC is also the reason why molten salt reactors needed less fuel mass for criticality right?

 

[anorlunda]

When the fuel is diluted with other materials, it makes it harder or impossible to get a critical mass. A whole mountain of uranium ore is far from critical. Yet, there is evidence of one location that had naturally occurring critical reactions in nature.
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

 

[jim hardy]

I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced?

While that's quite plausible it's as yet speculation for the reason @anorlunda mentioned - dilution of the fuel by whatever else got melted into the ball.

Dropping such a ball of hot molten metal and fuel into water would increase its reactivity all right,
but i believe the mechanism would be more by reflecting neutrons back into the mass from its periphery than by water migrating into that hot molten mass and remaining in liquid form to do its moderating.. .

Here's a picture of an experimental reactor that uses reflectors , but they're solid not liquid. And they're made from heavy atoms so as to reflect neutrons without reducing their energy.
Recall from high school physics that in elastic collisions, energy division between the bodies is proportional to their masses - that's why a Volkswagen will bounce off a Buick but will dominate a motorcycle.
So uranium atoms make a better reflector than water molecules .

so a return to criticality just long enough to make another big steam explosion is i believe not implausible.
Would it have happened ? I surely don't know.

old jim

 

[Rive]

so a return to criticality just long enough to make another big steam explosion is i believe not implausible.

In general, this kind of recriticality events are expected to be a kind of slow/pulsing reactions like in Tokaimura: as the water boils, the reactions subsides, then with the water cooling down it starts again.
Given that any kind of criticality there would have rendered that area inaccessible the chance that it actually happened is low.

 

[Astronuc]

I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced? This IIRC is also the reason why molten salt reactors needed less fuel mass for criticality right?

Not necessarily, and more unlikely, to the first question. No to the second question.

In the case of a molten core, the molten mass of corium includes neutron poisons, e.g., boron, or silver-indium-cadmium, and some fission products, although likely little or no Xe-135, and largely excludes the moderator (water). So it should not go critical even if immersed in water, yet that scenario must be considered, which is why some of the emergency cooling water contains high concentrations of boric acid.

In an MSR, criticality depends on the enrichment and neutron energy spectrum, i.e., whether the system relies on fast vs thermal (moderated) spectrum. Fast reactors require higher enrichments, or more Pu-239, -241 than U-235. In order to compare the masses of fuel required for criticality and power density in different systems, one must perform detailed calculations using a full neutron energy spectrum from thermal energies (~0.01eV) up to fast energies (10 MeV).

In the Flat Top slide in jim hardy's post, it refers to U-235 to Pu-239 cores, which take to mean better than 90% or essentially fully enriched. The more highly enriched U or Pu, the less mass it takes to become critical, and such a system is far different from a power reactor.

 

[artis]

@Astronuc I guess I was wrong as it has been some time since I read about MSR theory.
So a MSR reactor on average needs either a higher enrichment or a larger total fuel mass in order to reach criticality than a conventional solid state reactor?