Going Nuclear - The Science Of Nuclear Weapons - Part 2 - Chain Reactions

[@enthalpy]

Nuclear engineer and licensed senior reactor operator here. I operate a boiling water reactor. Thanks for helping to spread some education about nuclear power!

[@FiferSkipper]

Great username! I saw your comment and had to ask... With the reactors I am experienced with, US Navy, we did not control criticality (after start-up) with the control rods. The Navy has a strict requirement of a negative coefficient of reactivity. Meaning (for other readers) that if the core got hot, the moderator expands reducing the cross section of moderation and thus reducing the reactivity. I am curious how (not) similar it is in commercial applications. From what I understood the Navy's process was less efficient however, it allowed for the dynamic power-level changes required in warship-propulsion applications. Do commercial reactors control operational criticality with active control rod adjustments? Like Chernobyl?

[@233kosta]

@@FiferSkipper From my diggings, the RBMK (Chernobyl) type reactor (hereafter referred to as typical communist arrogance and callousness) is the only power reactor type in use which has a positive coefficient of reactivity as it is almost entirely graphite moderated. That's one of the main reasons for that disaster. The West operates a strict convention on using NCR reactors because of that very possibility. The main difference between civil and military reactors is scale (probably refinement level too, come to think of it). A ship or submarine simply does not need 800MW (over a GW thermal!) out of its reactor. Making them small keeps them agile, you could probably shut one down completely in seconds and not worry too much about keeping the coolant flowing. In a civilian reactor on the other hand, there is so much more decay heat that it takes hours to fully shut one down. The thermal inertia is also too great to rely on passive self-regulation. It makes active control an absolute necessity.

[@Bialy_1]

@@233kosta Yea you have no idea what are you about... There is a good explanation of the topic on "Illinois EnergyProf" chanel. The fact that the operator was complete moron that allready got some accidents on his resume(btw. in the NAVY) is not a foult of reactor design. "Although there was a standard operating order that a minimum of 30 control rods was necessary to retain reactor control, in the test only 6-8 control rods were actually used. Many of the control rods were withdrawn to compensate for the build up of xenon which acted as an absorber of neutrons and reduced power." US design with operator that stupid, not aware what is going on in his reactor and on top of that with zero regards toward safety procedures=>recepie for disaster(US reactors of that generation included). The only thing about chernobyl reactor that is not typical for that period of time is fact that it can work on crapy fuel as the pile of fuel is so big(so you dont need to lose time and energy on fuel enrichment).

[@HenriFaust]

@@Bialy_1 You've reminded me of an old joke about Richard Feynman: *If Richard Feynman applied for a job at Microsoft * Interviewer: Now comes the part of the interview where we ask a question to test your creative thinking ability. Don’t think too hard about it, just apply everyday common sense, and describe your reasoning process. Here’s the question: Why are manhole covers round? Feynman: They’re not. Some manhole covers are square. It’s true that there are SOME round ones, but I’ve seen square ones, and rectangular ones. Interviewer: But just considering the round ones, why are they round? Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology. Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones? Feynman: Yes. Round covers are used when the hole they are covering up is also round. It’s simplest to cover a round hole with a round cover. Interviewer: Can you think of a property of round covers that gives them an advantage over square ones? Feynman: We have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it. Also, the term “manhole” implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section. So a cylindrical pipe is the natural shape for manholes. The covers are simply the shape needed to cover up a cylinder. Interviewer: Do you believe there is a safety issue? I mean, couldn’t square covers fall into the hole and hurt someone? Feynman: Not likely. Square covers are sometimes used on prefabricated vaults where the access passage is also square. The cover is larger than the passage, and sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. Let’s assume a two-foot square opening and a ledge width of 1-1/2 inches. In order to get it to fall in, you would have to lift one side of the cover, then rotate it 30 degrees so that the cover would clear the ledge, and then tilt the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. Yes, it’s possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely. Applying common engineering sense, the shape of a manhole cover is entirely determined by the shape of the opening it is intended to cover. Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.) (Interviewer returns after 10 minutes) Interviewer: We are going to recommend you for immediate hiring into the marketing department.

[@drewgehringer7813]

@@Bialy_1 I mean good design practice now for nuclear plants is relying on passive cooling systems the operators can't circumvent (or can circumvent but not without immediately SCRAMing the reactor). The world needs idiot proofing because we're all idiots sometimes, even those with college education and decades of career experience; humans make poor decisions when stressed, humans can get overconfident when in a position of power in a strictly hierarchical system, humans cut corners to save money...

[@enthalpy]

Side note: when we start up a commercial power reactor, we go slightly supercritical. For a commercial reactor we have a multiplication factor that gives us about a 15-20 minute doubling time. Very slow but also very safe and controllable. There are either reactor period trips or flux range trips to prevent a runaway and automatically shut down the reactor.

[@kotori87]

I'm a naval nuclear operator. We measure Start-Up Rate (inverse of period) in Decades Per Minute, often lots of them. Not much waiting for us :D

[@Verpal]

Decade per minute is a very technical and very real rate yet I cannot stop laughing.

[@smort123]

@@kotori87 What is a decade per minute?

[@MySkybreaker]

@@smort123 the way I understand it, they aren't using it as a year denotion. Say you have a 10hz signal. Upping it a decade would be 100hz. So if it went from 10hz to 100hz in 60 seconds, then that would be a decade per minute.

[@MySkybreaker]

@@kotori87 Is the general jist of it? Something going up by a factor of 10 is a decade?

[@kentd4762]

Thank you, Scott. Still so mind-boggling that so much energy can be released in mere fractions of a second; that a chain reaction can happen so fast. Just imagining megatons of energy being released in the blink of an eye, and covering so much area, is scarily amazing.

[@tanks608]

I just wanted to say that this is one of the most interesting series I have ever watched. It is really fascinating to hear a more than rudimentary explanation of nuclear history and theory that is clearly and effectively presented. Thank you so much and great job scott.

[@HisCarlnessI]

Also important note. When you were talking about explosive consequences for not moderating the reaction correctly, that's more often water flash boiling or hydrogen buildup/detonation. While the energy multiplies very fast, it's still not weapons grade Uranium, as you said. Thus meltdown literally means melt-down.

[@fridaycaliforniaa236]

Like in Chernobyl ?

[@Oneover_137]

@@fridaycaliforniaa236 no

[@FluxOrbit]

​@fridaycaliforniaa236 yes but really no. That *can* happen, but you're more looking at something like Three Mile Island, where things just heat up to a point where the core is damaged, but not completely destroyed. Far before that point, though modern reactors, and even old reactors, will be well aware of the problem before it can progress to any serious situation.

[@maggimann123]

Scott this is currently my most favorite ongoing youtube series in existence. Please just continue down this route and bring us many episodes and Spin-offs.

[@Pants4096]

I love descriptions of "instantaneous reactions" like nuclear detonations slowed down and discussed microsecond by microsecond!

[@theophrastusbombastus8019]

Dude, I hope this is a 50 parts series.

[@Elminator666]

My stepfather was involved with these weapons, I believe from the Manhattan Project until his retirement in 1990 I believe. I don't think he's particularly well known, but he is the editor of the Handbook of Nuclear Weapon Effects.

[@LtSpoon06]

When I worked at a research reactor, we would achieve prompt criticality all the time when we would replicate (on a much smaller scale) a power excursion by ejecting a control rod from the center of the core. The prompt negative temperature coefficient would automatically bring the power level back under control before any operator input. Pretty neat. Plus the blue flash of Cerenkov radiation is pretty cool too.

[@V1Tz]

Thanks for the upload, can’t wait for the third episode! I find this series so I interesting that the whole video felt only one minute long.

[@colinbarnette0]

Same!

[@MadsBorlund]

Thank you! This is the first good explanation of a "prompt critical" reactor I have come across.

[@grzegorzkapica7930]

Man, I hope they don't take your channel down for this. Great informative video. Thank you Scott.

[@04u2cY]

I have absolutely no idea what this guy talking about but the way he tells its is amazing in such detail and very smart.

[@gascan]

Will you be going into power reactor designs as well as bomb designs? I'd love to hear some discussions of the various designs currently in use (pressurized water reactors, boiling water reactors) proposed designs (thorium, fusion, etc)

[@DaytonaStation]

what is good about your talk is that you recall numbers time distances and for example cross sections and time that I cannot recall

[@Arikayx13]

Nukes are such a great example of knowledge vs practice. The basic design of a nuke is simple and even the more complex bits can be grasped by most people. But the practice of actually putting it all together in the real world is super complex.

[@BeCurieUs]

TELL ME ABOUT IT!

[@ElectricityTaster]

Blink twice if Kim has you captive. We'll send help.

[@jimbeck3230]

Later uranium type devices moved the rings, not the projectile. They were also surrounded by beryllium to reflect the neutrons. A small black powder charge propelled the rings up a tube into which the projectile was mounted. When the top ring of the stack neared the end it struck two neutron generators that initiated the main fission sequence and the weapon exploded. Moving the rings has an advantage that the generators can be placed further apart yielding a superior neutron spread and higher yield. Your analysis of the little boy, AKA thin man is correct. It was primitive compared to the ones designed in the fifties.

[@scottmanley]

6:50 I explain this.

[@jimbeck3230]

Scott Manley thanks for the reply. When I saw your drawing I assumed that was what you would describe and mentally skipped over the discussion. I am not sure exactly how the little boy was configured. In the W33 device even though the rings moved the “slug” was called the projectile. It was actually mounted on a cylinder of the same diameter. When assembling the device the rings were installed onto the cylinder. To prevent incidents they were locked in place. Then the slug was screwed into the cylinder. You can appreciate the fact that misalignment is impossible with this system. I am looking forward to viewing the remaining episodes of this series. I am curious as to what type of physics is your speciality? Thanks!

[@smarttarts]

I think I'm really going to enjoy this series :) I've been really interested in nuclear energy the last couple years, and if I ever went back for a Masters degree, it'd be in nuclear engineering. Thanks!

[@BeCurieUs]

It's what I did :D

[@fortyfukinseven]

I've always been fascinated by nuclear reactions, but due to the stigma, I'm hesitant to do much research. I appreciate you walking through it in a more educational, and historical manner.

[@catfish552]

If you want the history in more detail, and with good explanations of the science along the way, check out The Making of the Atomic Bomb by Richard Rhodes. It's a history of the Manhattan Project and really the entirety of nuclear physics in the first half of the 20th century. Great book, praised by both historians and the scientists that were actually there, and won a couple prizes, including a Pulitzer.

[@oskarwallin8715]

Woop! You should really consider starting a educational channel! Love listening to your videos like these

[@scottmanley]

This is my educational channel!

[@smokeydops]

this IS an educational channel

[@oskarwallin8715]

Scott Manley yeah but I meant a more focused! I really love your educational stuff and would love to see more! (Aswell as more like the interstellarquest)

[@DETHdressedInRED]

Oskar Wallin I totally agree! This could easily be it's own channel. Science with Scott Manley or something.

[@DETHdressedInRED]

Scott Manley you should totally think about making a second channel purely dedicated to the redundant explanations of the scientific properties of everything. (Yes I said it that way on purpose)

[@goofyiest]

very excellent description of delayed neutrons! And also a subtle description of what happened at Chernobyl.

[@Jamie-kg8ig]

Thank you Scott for this video series. It's especially interesting for me as later today I take the test to qualify for the Navy's nuclear program.

[@mattlawrence1983]

I've worked both military and civilian nuclear power. I am really enjoying this video series! Keep it up Mr. Manley ^_^ . Chernobyl was a prompt criticality event. The explosion however was a steam explosion not a nuclear one. The water inside the vessel was instantaneously vaporized and over-pressurized. It is a lot of energy density in Uranium.

[@BeCurieUs]

There is a recent paper on that we were talking about on twitter that says it was a nuclear explosion THAN a steam one. I doubt it, didn't do any actual kinetics and just sniffed some isotopes, but neat none the less

[@AbreedApart123]

Scott's got the bad men out there on Amazon ordering all the U-235 up now! Awesome series mate, wish young me had this thirst for understanding physics!

[@joelthefrog1]

"Supercritical Mass Of Fissile Material" Well, found my new bands name.

[@aligallaton3978]

I mean, it's a pretty nice name.

[@Weisior]

And it has to be a rock band!

[@ericmelto7810]

Or “reactor under the gym. “

[@intellectualiconoclasm3264]

@@ericmelto7810 That sounds more like an emo and/or porn sound-track band. 😉

[@ericmelto7810]

@@intellectualiconoclasm3264 that’s where they built the first nuclear reactor in Chicago. Under the high school gym.

[@BeCurieUs]

Fun fact, the gun style bomb was supper inefficient, consuming less than 2% of the fissile materials! A small contingent of folks thought this so wasteful as to consider not using this style of bomb, but of course, they lost that argument.

[@brianthomas9254]

But in 1945 no one would've known if it hadn't worked. It would've been a bombing raid with only one bomb. No harm, no foul.

[@BaronVonQuiply]

I love it! The only channel on KZbin to teach you how to build a rocket, launch it, and make sure there's a nuke onboa-... I've just been told I'm now on a list ='(

[@prof2yousmithe444]

I am following your series with a great deal of anticipation for the next! Thank you for pointing out the errors in the Little Boy schematics. Always drives me crazy lol! Have you thought of a discussion on the Krytrons or the PAL’s? I find that technology fascinating. Keep up the great work!

[@trogers99]

Wow Scott, I can't believe your posting this anti advertiser friendly content again ;) (KZbin is ridiculous)

[@wierdalien1]

ofsamsonov93 bombs

[@unlikelysalmon786]

ofsamsonov93 Nuclear -> Hiroshima -> WWII -> Nazis Bleeped out because KZbin scans the comment section. Edit: fine.

[@espalorp3286]

What did you bleep out? Nozis? Nezis? Oh wait, you meant nazis. That's it.

[@Drrolfski]

It's rogue state-friendly though. Who needs advertisers if Kim Jong-Un will happily pay your bills :p

[@deltuhvee]

hello, i thought this was funny

[@Veptis]

It's funny how you can make videos talking about nuclear weapons and missiles so publicly. But we get a whole storm of discussion on a forum that discusses thermal cameras.

[@leonardkutyla2921]

Hello, Scott...thanks so much for your videos (they're fascinating and enlightening...). Please note that the court under the stands of Stagg Field at the University of Chicago was a Squash Court! (Racquetball had not yet been invented:)

[@zubmit700]

Very interesting. Can't wait for the next part!

[@Dai5tr0y3r]

I know this got demonetized, but this is an amazing series and would love to see more!!!

[@Turner_04]

KZbin demonetises first video, uploads second anyway :D

[@scottmanley]

Because I have a day job.

[@Turner_04]

Scott Manley Oh I know that I’m just happy you uploaded :)

[@TheHacknor]

It's not about the money, it's about sending a message

[@wierdalien1]

GreenManAiming what message are you sending to youtube?

[@jeremyowen1]

Scott is here for us, to help educate and entertain. He spent the time getting a solid education and career.

[@baz6128]

Very interesting. I can't for part 3. Thanks Scott.

[@PaulHigginbothamSr]

scott: when looking for the nuclear initiator, look for urchin. You will find it is beryllium 9, and polonium 210. These items for the trinity blast and hiroshima and nagasaki might have been difficult to obtain, but they did have a reactor at hanford, from which to aggregate the necessary amounts of these man made items. When you are getting plutonium in the 5 kilo-gram amount completely made by man, these other components in the amount necessary will be pretty easy to obtain. The urchin may have been diff in the first units, something with good production and a neutron emitter. Labs may have had this before trinity. I read many years ago it was a different element which I am reluctant to state here.

[@bryanFDNY]

Will you be talking about Fusion devices ? I find those personal very intriguing, the risk they took during their development and nonchalance with witch they conducted the tests are absolutely mind boggling

[@BlackWolf18C]

Or Layer-cake designs, where an initial fission triggers a round of fusion, which triggers more fission, triggering more fusion. Designs got absolutely ridiculous by the end of the Cold War.

[@die1mayer]

That's not ridiculous, it's efficient.Don't want to waste nuclear fuel.

[@BeCurieUs]

Talking about ridiculously complicated. I have been reading about weapons designs and its astounding, for instance, the fission trigger stage isn't hot enough to fuse the fuel we use for our fusion stage by a power of 10. They solved this in a way which is mind boggling and hopefully he goes over, I don't want to spoil it :D

[@oliverturner1649]

Woohoo! loving this series so far.

[@IstasPumaNevada]

Always leave 'em wanting more. :) Quite enjoying these videos.

[@rdfox76]

Scott, while I don't have any specific information on the Abner initiator, I've always been under the impression (from my, admittedly, solely open-source information) that it was similar to the polonium-beryllium "Urchin" initiators used in the early implosion designs. I won't go into more detail on their design here (though it can be found online if you know where to look), but I suspect that, having a working knowledge of nuclear physics, you can guess the generalities of how such a device would work from what I've said here. Interestingly, the preferred method of initiation since the early 50s has been to use a betatron built into the weapon case; it not only gives a stronger initial neutron flux and more precise positioning of the neutrons, it also gives much more precise timing of the initiation event, allowing for optimal compression of the pit before initiation.

[@scottmanley]

+rdfox76 Chavez’s you been reading the scripts for my future episodes?

[@rdfox76]

XD Nah, just the Nuclear Weapon Archive.

[@criticalpoint7672]

Wow, nice video, I've seen before videos explaining the nuclear process but not like this, I have learned new things.

[@BeechSportBill]

Where did they get the HUGE quantities of graphite? I’ve toured the “B” reactor at Richland Washington...

[@Wardoggy63]

Thank you DR. Manley. Great series

[@Karibanu]

This is a good level of explanation, I like it. You're sounding more scottish than usual today for some reason...

[@bend1483]

Very interesting scott, as per usual! looking forward to the next episode(s)!

[@TarisRedwing]

pretty cool background info for when I'm playing Factorio and building a reactor and painstakingly have to make 235 b/c 238 is garbo lol

[@DETHdressedInRED]

Mister Scott, I freaking love your videos. MAN I LOVE SCIENCE!

[@dominiquegobeil5831]

This video is gold, youtube give some more money to this awesome speaker!

[@stephenkneller9318]

Abner was most likely a simplified version of the Urchin initiator in the implosion type weapons. "4.1.6.1.4 Initiation Once insertion is completed, neutrons need to be introduced to begin the chain reaction. One route to doing this is to use a highly reliable "modulated" neutron initiator, an initiator that releases neutrons only when triggered. The sophisticated neutron pulse tubes used in modern weapons are one possibility. The Manhattan Project developed a simple beryllium/polonium 210 initiator named "Abner" that brought the two materials together when struck by the projectile." "4.1.8.1 Modulated Beryllium/Polonium Initiators ....The Urchin was a sphere consisting of a hollow beryllium shell, with a solid spherical beryllium pellet nested inside. The polonium was deposited in layer between the shell and the pellet. Both the shell and the pellet were coated with a thin metal film to prevent the polonium (or its alpha particles) from reaching the beryllium. The mixing was brought about by using the Munroe Effect (also called the shaped charge, or hollow charge, effect): shock waves collide, powerful high velocity jets are formed. This effect was created by cutting parallel wedge-shaped groves in the inner surface of the shell. When the implosion shock collapsed these grooves, sheet-like beryllium jets would erupt through the polonium layer, and cause violent turbulence that would quickly mix the polonium and beryllium together. By placing the small mass of polonium as a layer trapped between two relatively large masses of beryllium, the Urchin designers were hedging their bets. Even if the Monroe effect did not work as advertised, any mixing process or turbulence present would likely disrupt the carefully isolated polonium layer and cause it to mix. The whole initiator weighed about 7 grams. The outer shell was 2 cm wide and 0.6 cm thick, the solid inner sphere was 0.8 cm wide. 15 parallel wedge-shaped grooves, each 2.09 mm deep, were cut into the inner surface of the shell. Both the shell and the inner pellet were formed by hot pressing in a nickel carbonyl atmosphere, which deposited a nickel layer on the surfaces. The surfaces of the shell and central sphere were also coated with 0.1 mm of gold. Combined with the nickel layer, the gold film provided a barrier between the polonium and the beryllium. 50 curies polonium-210 (11 mg) was deposited on the grooves inside the shell and on the central sphere. This much polonium produces a thermal output of 0.1 watts, causing very noticeable warming in such a small object. Post war studies showed that no more than 10 curies still provided an acceptable initiation effect, allowing the manufacture of initiators that remained usable for up to a year." nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html

[@williamgorham7339]

The first graph of the little boy was backwards too lol 😆. I love this series and would love to see more videos on the going nuclear ☢️ playlist.

[@ryccoh]

This series is excellent. Can you talk about fusion and combining it with ion thrusters to finally explore the solar system.

[@dizzolve]

this is a great series here - thank you

[@MrSaliVader]

Can't wait for the third part!

[@bhaskarsingh1564]

Scot I love this series so much

[@thedreadnote]

Loving this series, so fascinating!

[@coorbin]

Aw yiss, my favorite series right now :D So interesting!!!

[@louiscaston9692]

Nicely done, Scott! Thanks!

[@M12Howitzer]

Another very interesting science video! Keep it up and thank you!

[@JoannaHammond]

To be honest, I can't see why you've been demonitised as all this information is out there and easy to get hold of if anyone is seriously interested in finding out. Building a basic nuclear device would not be that hard, getting hold of the materials to do so would be.

[@lobsterbark]

I would think it would be nearly impossible to get, build, and test the conventional explosive part without getting caught, let alone the nuclear materials.

[@WillArtie]

Awesome series!!! Want more now!!!

[@elboutcho]

Cool series this, would be awesome if you did a follow up on the power side of things and talked about Fusion reactors? :)

[@Richard_Cranium]

Hey Scott. Could you make a video about fusion and the materials that might be used in achieving sustainable fusion.

[@AaronB99999]

Another reason the Little Boy/Hiroshima bomb wasn’t tested was because they only had enough near-pure U235 for the bomb itself. The enrichment process was so time- and energy-intensive that it took a year or so to get enough for one bomb. The Nagasaki bomb used plutonium, which could be made in a nuclear reactor but required a more complex firing mechanism, hence the test in Nevada.

[@olivier0092]

Hi Scott. I've asked Amy from Vintage Space to do a video on the Ekranoplan. But perhaps you could do a physics explanation? That would be awesome!

[@TheManFrayBentos]

There was only enough U235 to build one gun-barrel weapon, so it was decided that since the design was probably going to work, it would be dropped untested. This was a result of production and scheduling difficulties at Oak Ridge (fantastic place to see, if only by photograph).

[@nickhowatson4745]

we really need a "Nuclear Reactor Reacts to..." video series on KZbin. I bet the reactor laughs in Cherenkov Radiation

[@EnderMalcolm]

Coming back to this with a question. You talked about "Non-natural nuclear reactor" when describing the first critical pile. What's a natural nuclear reactor?

[@scottmanley]

There was one in Olli, Gabon en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

[@patryksoko4676]

When core is going over critical mass the key thing is to keep it in one piece. When you can't do that the core will be sever and explosion will be really small. This made me think. What if we have undestractable well made of material which is good in reflecting neutrons. Than we start throwing brick-shaped pieces of Uranium-238. They pile up, go over critical mass, start chain reaction, but nothing is keeping them together, so raising pressure bounce them away and stops reaction before it become anything serious. So does that mean we would have well with jumping uranium bricks?

[@dimmerdoon]

I love these information videos Scott really cool to have to all explained out with out going through pages of wiki. One question though if going prompt critical causes it to want to explode then what exactly happens to make it go into a nuclear meltdown like at Trinoble?

[@kazsmaz]

Love this series

[@PiezPiedPy]

NSA are now giving away Free NSA T-Shirts for all those new to the list

[@amperzand9162]

Nice! I want one.

[@oldfrend]

does it come with a complementary body cavity search?

[@STUCASHX]

It's built in to the shirt...

[@michaelfurgessons2896]

Opin-Eon Oh how considerate of them!

[@nigeldepledge3790]

The main reason they didn't test fire a Little Boy type device was that they had only been able to produce enough enriched uranium for one supercritical core. The Manhatten Project produced enough plutonium, however, for three cores. One of these was used in the Trinity test. The second was used in Fat Man. The third was used for experiments and was not built into a weapon.

[@Edax_Royeaux]

So I have a question about reactors. I watched a documentary about them, and they claimed that Sodium reactors were extremely safe and were the future. However, I can't help but notice that we had a Sodium reactor meltdown in Simi Valley, California, not far from downtown LA. Why did this meltdown happen and are Sodium reactors actually safe?

[@BeCurieUs]

Sodium reactors have reach a super high level of development at this point. Many can be said to be passively safe, shutting themselves down even in the event of a full system blackout! There is a great video about that here watch?v=Sp1Xja6HlIU&t Don't judge the merits of sodium stuff on the Sodium Reactor Experiment, it was literally a learning experience, there we 0 on hand knowledge of using those types of materials (both sodium and the cladding used). The use of sodium bothers some, but the lower pressure operation make is likely a horizontal trade off in terms of overall risks. The meltdown happened because of some blocked coolant passages because of different contaminates from some spilled tetralin into the sodium coolant. This caused all sorts of metallurgical changes and decomposition that plugged up the pipeworks. Tetralin decomposes into naphthalene, which in turn "reacts with sodium to form a very reactive, pyrophoric naphthyl- sodium compound." Doc._No._2_SRE_Fuel_Element_Damage_Final_Report_1961_NAA-SR-4488_(suppl).pdf It was design didn't have enough care if coolant channels failed and faulty monitoring procedures allowed for coolant contamination doomed this experiment. In a way, it was a good experiment, it did exactly what it was supposed to, expose flaws in a controlled way.

[@Edax_Royeaux]

Well, the fact that they kept running the sodium reactor for weeks after the meltdown, and they covered up the incident including all that radiation released into Simi Valley, combined with how the nuclear industry reacted to "The China Syndrome" and the subsequent Three Mile Island incident with the blatant lies, makes it really hard to separate the fact from fiction. Just cause the industry expert says it's safe means nothing, figuring out why it's safe seems the logical course of action. I do know that Sodium Reactors were rejected from the military, especially on subs because a breach in the reactor means molten sodium spilling everywhere, which sounds decidedly unsafe.

[@BeCurieUs]

Ya, transparency back in the day wasn't great, mostly because they played inside baseball and anything that was in their predefined rule book of safe was just taken to be so and they negged anyone saying different. Even so, particularly in the TMI incident, there have been 2 independent scientific studies that said they anticipate zero deaths as a result of the relatively small releases. Similarly here, a study "Feasibility of developing exposure estimates for use in epidemiological studies of radioactive emissions from the Santa Susana Field Laboratory" concluded the expected doses to people is likely so close to background as to be unattainable to any real increase in mortality. He talked about how hard to is to reconstruct this stuff given the cover-ups and such, but using what we know about how much might have been released and wind data, we can at least get a simulation set going and find a median, and that value was basically 0. And to be fair, they didn't exactly know they had a meltdown, only after the investigation did they understand that in context to the gas releases that were happening. They likely should have suspected, but lack of experience in this particular design was mostly to blame. I agree that is hard to separate fact from fiction, though. But sodium reactors might have a place in the future, or perhaps it will be gas reactors, or pebble bed, or molten salt, hard to say at this point. They all have advantages and weaknesses. I used to really hate on the sodium reactor cause of history like this, but after investigating how the engineering has advanced, I would submit they can be as safe as we demand things to be these days, and so too for most of the new advanced designs going around.

[@daniellassander]

More please i love this sort of information!

[@DJW1959Aus]

This criticality sensitivity can be addressed if the fuel and fuel carrier are liquid such as molten salt. A molten fuel would expand with heat, this gives the possibility of self-moderation.

[@mohamedzh8555]

Will you consider making videos about fusion reaction ?! it is a hot topic as a new source of energy !

[@robertlehman3660]

The uranium design was also not tested because the sheer amount time it took to acquire fissile uranium. They were literally separating the U-235 from the U-238 by mere atoms at a time. It took years to enrich enough for just the little boy weapon. That's why the reactors were needed for plutonium production.

[@peterpiso7384]

Great video love to learn about the Evolution of the atomic bomb... Thanks

[@hansulrichjohner2694]

There is a really wonderful book about nuclear weapons: "The Making of the Atomic Bomb" by Richard Rhodes, a fantastic book, he even won the Pulitzer price. A second book describes the development of the hydrogen bomb: "Dark Sun". The neutron generator is described in the first book as beeng a foil of alpha emitter on one 235U body and a foil of beryllium on the other. The yield of an A-bomb is statistical in nature as it depends on the number of neutrons in the first generation, thus the n-source was added to assure a sound first generation, which is not given by the 235U-neutrons.

[@VicariousReality7]

8:00 What is the reason for using carbide then? That is significantly less dense than pure tungsten?

[@vampyricon7026]

Thanks for the video! If you're going to cover nuclear stuff besides weapons, can I request a video about liquid thorium fluoride reactors?

[@Renzsu]

Thanks for this great tutorial! Fun weekend project :)

[@maxmusterspace6037]

I really enjoy this series. Thank you! =)

[@halmaibalazs3361]

Love this series!

[@wbwarren57]

Thank you. This is extremely good.

[@recklessroges]

7:46 Isn't it the *inertia* rather than the momentum, that keeps the two masses together?

[@adamdapatsfan]

Can't wait to see what you've dug up on Gadget and the like!

[@warren286]

Neutron multiplication factor is also known as criticality, >1 super critical, 0 critical,

[@bilbo_gamers6417]

How snug does the fit have to be between the uranium core and sleeve in the bomb?

[@Youre_Right]

I’m from Oak Ridge, TN home of the plants that enriched the uranium for the Manhattan Project.

[@cyberdeckcipher]

Hey Scott, I've heard about an liguid salt nuclear reactor.. But how this work and what is the diference between this one and the grafite blocks reactor?

[@puncheex2]

In most present reactors the moderator is graphite, heavy water or plain water. Helium was used experimentally (the Ft St Vrain reactor in Colorado, since deceased). The liquid salt reactor uses graphite (usual design). The differences is not so much in the moderator but in the fuel and cooling. Most current reactors use water or HW for both; the LFTR uses liquid molten salt for both fuel and cooling.

[@russlehman2070]

@@puncheex2 IIRC, the Fort St. Vrain reactor used helium as the coolant, and graphite as the moderator.

[@puncheex2]

@@russlehman2070 So it was; I wasn't clear about that. Thanks.

[@copperhamster]

I think in... Richard Rhodes "Making of the atomic bomb' he said that the neutron source in Little Boy was something very active. Polonium or something like that. I may be wrong I read that 3-4 years ago.

[@puncheex2]

Polonium and beryllium. The polonium alphas knocked neutrons out of the beryllium

[@copperhamster]

Sounds right.

[@Ediranii]

Am I the only one who noticed the Coca Cola bottle with his Twitch name??? :O holy crud dude, you're either the luckiest person alive or really good at prop making :D

[@LvInSaNevL]

Where'd you get that big Yeti? And is it actually working?

[@vancouveropenbsd985]

@6:08 in the military, that's called "private proof".

[@ModanoTheBest]

Of course this goes live while I'm playing Fallout 4. lol

[@coorbin]

Me too! "It's all over but the cryyyyyyingggg...."

[@F_Tim1961]

1. Tungsten Carbide ? I don't believe WC was used because the density is only 15.6 g/cc whereas up to 18 g/cc is available with HeavyMet alloys of Tungsten (which can be machined at room temperature) . In addition WC is rather more brittle and if you are not going to test the device why would you not go with a material with high impact resistance ? 2. The correct core topology is a male cylindrical target , a hollow annulus project and three toroidal outer cores held concentrically around the target. This enabled a 2.8 or 2.9 factor on critical mass to be achieved. The trick is that there is an air gap between the male target and the encircling toroids. This would reduce the number of neutrons per cm2 of internal bore of the toroids, in terms of receipt of spontaneous neutrons from the target. (It works the other way from toroid to target of course). 3. I have a strong suspicion that there was something novel about the securing bolt that holds the target. Specifically it might have been boron steel with a sheer pin on the contact face and the Big Nut at the far end so it could be punched out of the core at assembly time (and through the nose of the bomb case or into a recess) and in fact the wafers that make up the target are held in place by say 6mm diameter high tensile bolts or similar. The three outer toroids were flown out to Tinian, one per each of three C54 a/craft , I would guess because there was only enough U235 metal to make up the target and projectile in say June 45 and the last three components had to be site fitted, which would have been a nervous job. My guess would be that just two outer toroids would have been required to get at least 6kT out of the bomb. Little boy HAD to be dropped first because its yield was unknown and for psychological effect Fat Man had to have a bigger explosion or at least be equal to Little Boy in destructive output. Otherwise the Japanese army and emperor might try to call the bluff of the US. Effectively the did that anyway. They gambled 100 000 lives and a whole city that the US would not follow up quickly with a second bomb. 4. The Abner initiator was likely flat given how it was going to be crushed but beyond that I'll say no more. Another poster has provided data on initiators below. All the above information is already in the Public Domain (except for my guess that there might be something fishy, as in neutron absorbing, about the centre bolt of the assembly.) By the way, there is such thing as an absolute critical mass. There is a critical mass for a sold sphere, there is a critical mass for a solid cylinder of a given aspect ratio.. and so on. It is all about the surface area (neutron loss ) to mass ratio.

[@nickhowatson4745]

they did indeed utilize tungsten carbide as they were unaware of beryllium's superior nuclear properties at the time.

[@F_Tim1961]

@@nickhowatson4745 I'm not so sure of hat statement. Be produces a lot of neutrons when alpha bombarded so it is used along with Po in the initiator. The Tungsten is a reflector not a generator. if you have massive cylindrical lamina of WC then you have to have the hole through the middle off it. It's Near impossible to bore later as WC is so hard so you have to make this in place as the WC is created. This is a big ask. Hence my previous statements. Tim Fidler

[@theldraspneumonoultramicro405]

would hitting plutonium in a critical state with a sledge hammer made out of plutonium that is allso in a criticla state be a good idea?

[@scottmanley]

+Theldras Pneumonoultramicroscopicsilicovolcanoconiosis not good

[@armchairphysicist9067]

Please, no. Just no

[@WillArtie]

LOL! "be a good idea".. I like the terminology :)

[@mduckernz]

There was an accident similar to this I seem to remember, they dropped a sub critical mass of plutonium onto another one, caused by a screwdriver that was holding it up slipping (reckless AF!). All present died I believe.

[@BaronVonQuiply]

It was the neutron reflector, one dropped and one held up by a flathead screwdriver in the second incident. Famously known as the The Demon Core, I had thought it was used in Castle Bravo where it generated a much higher yield than expected due to the Lithium generated from previous testing but a quick check with Wiki tells me it was melted down instead.

[@randomtidbits7695]

As for the Abner Modulated Neutron Trigger, it was my understanding it was functionally equivalent of the Urchin device, it just contained less polonium.

[@scottmanley]

+Daniel Crowe a citation for this would be nice

[@randomtidbits7695]

A bit more reliable source would be nice to have. Unfortunately mine died a few years back but he was a graduate student recruited into the Dayton project and later worked at Oak Ridge before taking up teaching. I really wish (2007-2008 era) video cameras were a bit more ubiquitous. If I can find some of my old lab notebooks I'll see what I can salvage, he was full of some true gems - lines of research that ended up ultimately being forgotten to history.