DPiepgrass

• DPiepgrass 20 Nov 2021 20:33 UTC

  1 point

  on: Eth­nic Ten­sion And Mean­ingless Arguments

  Jonathan Haidt and his team hypnotized people to have strong disgust reactions to the word “often”

  This link is broken (or the server is broken? “The connection was reset”), and I find the claim hard to believe.

• DPiepgrass 11 Nov 2021 0:45 UTC

  1 point

  in reply to: bogus’s comment on: How to talk ra­tio­nally about cults

  Heh. When chaos said “the 9/​11 of cults” I thought “wait, wasn’t 9/​11 the 9/​11 of cults?”

• DPiepgrass 10 Nov 2021 12:45 UTC

  13 points

  in reply to: Duncan_Sabien’s comment on: Com­mon knowl­edge facts about Lev­er­age Re­search 1.0

  The former curriculum director and head-of-workshops for the Center For Applied Rationality would not be welcome or safe at a CFAR event?

  What the **** is going on?

  It sounds to me like mission failure, but I suppose it could also just be eccentric people not knowing how to get along (which isn’t so much different?) ὡ5

• DPiepgrass 3 Nov 2021 17:24 UTC

  1 point

  in reply to: hold_my_fish’s comment on: The Du­pli­ca­tor: In­stant Clon­ing Would Make the World Econ­omy Explode

  I think Holden partly put the cart before the horse by talking about consequences of a duplicator without even trying to show that a duplicator is possible. Perhaps the previous article has his answer:

  [suppose] You think our economic and scientific progress will stagnate. Today’s civilizations will crumble, and many more civilizations will fall and rise. Sure, we’ll eventually get the ability to expand throughout the galaxy. But it will take 100,000 years. That’s 10x the amount of time that has passed since human civilization began in the Levant.

  The difference between your timeline and mine isn’t even a pixel, so it doesn’t show up on the chart. In the scheme of things, this “conservative” view and my view are the same.

  Except that Holden is building “the most important century” series, and discussing a person-cloning technology that could be 400 years away, or 99,900, doesn’t make the case for 21st-century-as-most-important.

  Also, the duplicator route is one that makes “the little people” like myself fairly irrelevant. My IQ is only like 125, and wouldn’t this duplicator be reserved for the top 0.1% or so? Maybe you need something like a 150 IQ to be worth duplicating, unless you are somebody like Beyoncé or Bill Gates with wealth and/​or a proven track record. Or perhaps Xi’s hand-picked successor takes over the world and only duplicates his most loyal followers and scientists. Even if this century is the most important, that matters little to me if I cannot participate in the revolution.

  In the current world, what I need to enable me to meaningfully influence the future is money I don’t have, but I am more likely to get that than new genius abilities. Now, I don’t oppose duplicators, but it seems like we’re very far from having one. Just bringing people out of poverty and wage-slavery (e.g. via UBI and/​or an Open Engineering program) would quickly enable a lot more smart people to do smart things rather than just obeying the boss’s orders.

• DPiepgrass 2 Nov 2021 16:27 UTC

  1 point

  in reply to: Randomized, Controlled’s comment on: Where do you live?

  Hm, somebody strong-upvoted this (and Calgary—I wonder who)

• DPiepgrass 2 Nov 2021 0:33 UTC

  1 point

  in reply to: bendini’s comment on: The ra­tio­nal­ist com­mu­nity’s lo­ca­tion problem

  Manchester sounds nice, but what can you tell me about UK immigration law? Is there a base of EAs, rationalists, or ACX fans already?

• DPiepgrass 1 Nov 2021 19:33 UTC

  1 point

  in reply to: Randomized, Controlled’s comment on: Where do you live?

  Yeah, and it is probably very tempting for people to click places they like :D. Also I got downvoted and probably most people will never see the survey.

  As far as I’ve seen, downvoting typically works the same way on other sites; LW is normal in this regard.

• DPiepgrass 31 Oct 2021 19:05 UTC

  1 point

  in reply to: thomblake’s comment on: Nega­tive and Pos­i­tive Selection

  Ahh, the The winner’s curse.

• DPiepgrass 31 Oct 2021 18:21 UTC

  6 points

  on: Where do you live?

  As a Canadian I can testify that Canada certainly seems to be welcoming to all skilled professionals, but from personal experience, their system for issuing temporary/​visitor visas stinks: they rejected my wife’s application to leave her son behind in the Philippines and visit me in Canada because she failed to “prove” that she would leave at the end of her visit. The thing is, it’s impossible to prove! A return flight is not considered evidence, I can’t buy a bond to promise she will leave, and leaving a child behind is considered evidence but, in my case, was insufficient. Notably, there is nothing on any government web site hinting at how hard it is to visit Canada, or how arbitrary and opaque the decisions are.

  As a preliminary check, I filled out a questionnaire and verified that a 35-year-old person with good English skill (7.5/​9 scores on IELTS), 2 years of college, 2 years experience in a skilled profession, married, one child, no job offer in Canada, but $30,000 CAD to bring to Canada, is eligible to apply for Express Entry if the wife scored likewise on the English test (oddly there is no provision for native English speakers). But this is just the very beginning; the Express Entry program doesn’t seem particularly “express” (or fast or cheap). You might be able to apply, but do you have enough “points” to actually immigrate? After a brief review I couldn’t figure out how the point system actually correlates to getting a “invitation to apply”. The most recent minimum number of points was 744 out of 1200, but I can’t find anyone saying whether (1) you’re guaranteed to get an “invitation to apply” if you pass the threshold, or (2) there is some kind of probabilistic formula for the “draw” system based on the points (as this page seems to imply). If you do get an “invitation to apply” I would guess you have some kind of temporary “right” to permanent residence if you immediately do the copious paperwork and pay the $1,325 application fee, but I don’t see anyone spelling out the probability of successful immigration conditional upon having an “invitation to apply”.

• DPiepgrass 31 Oct 2021 17:08 UTC

  3 points

  on: Where do you live?

  Calgary, AB, Canada (north of Idaho)

[Question] Where do you live?

• DPiepgrass 27 Oct 2021 23:25 UTC

  3 points

  on: Covid 10/​14: Less Long Cvoid

  Ivermectin studies looking more and more like outright fraud, as in the study never happened and they copied lines of data a lot levels of fraud.

  This link is not about ivermectin or repurposed drugs.

• DPiepgrass 27 Oct 2021 23:06 UTC

  1 point

  in reply to: IGotNerdSniped’s comment on: I Don’t Know How To Count That Low

  As a software developer, I completely expect this; it’s just an everyday consequence of limited-precision math. You’ll get the same kind of anomalies in almost any programming language, and other kinds of calculations can produce far worse inaccuracies than this (seems kind of odd to single out standard deviations which are relatively well-behaved, but maybe Excel used to have a worse algorithm). John Gustafson proposed using clusters of unums and then posits to mitigate the problem (preferably in hardware), but it’s not practical to eliminate it entirely.

• DPiepgrass 27 Oct 2021 22:54 UTC

  1 point

  on: Nanosys­tems are Poorly Abstracted

  I don’t see how the energy of a missile is fundamentally different from the energy of a motor.

• DPiepgrass 21 Oct 2021 17:44 UTC

  1 point

  on: How much should you up­date on a COVID test re­sult?

  They were given the information that the test has a sensitivity of 90% (10% false negative rate), a specificity of 91% (9% false positive rate), and that the base rate of cancer for the patient’s age and sex is 1%. Famously, nearly half of doctors incorrectly answered that the patient had a 90% probability of having cancer. [1] The actual probability is only 9%

  The probability surely isn’t 90%, but if the scenario presented to the doctors was anything other than “routine cancer screening that we do for everybody who comes in here”, the probability isn’t 9% either.

  Most people are tested for cancer because they have one or more symptoms consistent with cancer. So the base rate of 1% “for the patient’s age and sex” isn’t the correct prior, because most of the people in the base rate have no symptoms that would provoke a test. The correct prior would be adjusted for the patient’s symptoms. But how do we actually adjust the prior for symptoms? I don’t know. It sounds difficult.

  But I expect that usually a test has been used in the past in exactly this way: restricted to those with symptoms. So as long as someone is in charge of gathering data on this, we should already have an empirical prior for P(has cancer | positive cancer test result + symptoms), e.g. 30%, that doctors can use directly. This information should be available because, as time passes after a test, it should eventually become clear whether the patient really had cancer or not, and that later information (aggregated over numerous patients) gives us a pretty good estimate of P(has cancer | positive + symptoms) and P(has cancer | negative + symptoms) for new patients. (But I am not a doctor and can’t vouch for whether The System is bothering to calculate these things.)

  It’s good to see, then, that there are separate measures of sensitivity and specificity for symptomatic and asymptomatic patients.

  This post doesn’t tell me what I want to know, though, which is:

  • P(I am infected | positive test & symptoms)

  • P(I am infected | positive test & no symptoms)

  • P(I am infected | negative test & symptoms)

  • P(I am infected | negative test & no symptoms)

  So, if you got a negative result, you can lower your estimated odds that you have COVID to 0.4x what they were before. If you got a positive result, you should increase your estimated odds that you have COVID to 145x what they were before.

  My prior would just be a guess, and I don’t see how multiplying a guess by 145x is helpful. We really need a computation whose result is a probability.

• DPiepgrass 11 Oct 2021 21:51 UTC

  1 point

  in reply to: romeostevensit’s comment on: Why has nu­clear power been a flop?

  I like to point people to the DHS on dirty bombs, because you know the DHS is not in the business of downplaying national security risks.

  It is very difficult to design an RDD that would deliver radiation doses
  high enough to cause immediate health effects or fatalities in a large
  number of people. Therefore, experts generally agree that an RDD
  would most likely be used to:

  • Contaminate facilities or places where people live and work, disrupting lives and livelihoods.

  • Cause anxiety in those who think they are being, or have been, exposed

  That last point is obviously correct: people still act terrified of radiation and “exposure”. This year I got a solid dose of ~20 mSv for a single medical test. The way a lot of people talk about radiation, that’s a huge exposure. But if it’s administered by a doctor instead of a hypothetical dirty bomb, I expect people don’t worry too much about it.

  But let’s look at the terrorists we know about. How many terrorist groups do you know of that are less interested in killing people and more interested in causing anxiety and inconvenience?

  I mean, I guess if someone disrupts the power grid by blowing up some transformers, or puts a carcinogenic substance in our drinking water (hot dogs? kidding), we might call them terrorists, but that’s not a normal terrorist M.O., right?

  But also, if the “terrorist”’s goal is to just disrupt and not kill, there must be easier ways to do that than to steal used nuclear fuel. Used fuel is protected by security guards, containment buildings and containment casks, right? I’m not an expert on this, but couldn’t somebody just buy various carcinogens from a Home Depot and spray them into a reservoir?

• DPiepgrass 11 Oct 2021 21:22 UTC

  1 point

  on: Why has nu­clear power been a flop?

  Nuclear followed the learning curve up until about 1970, when it inverted and costs started rising

  This confused me because I thought that TMI in 1979 was when nuclear power really started dying off in the U.S., but Wikipedia agrees: Not a single reactor started construction in the U.S. after 1978! So I looked back on my favorite source on this, and its chart, starting in 1972, shows monotonically rising costs. Thanks for the correction. Rather than TMI happening somewhere in the middle of the coffin-construction, it really was the final nail.

  Perhaps anti-nuclear activism started all the way back in 1945? I wonder how things would’ve been different if Truman wasn’t so damn eager to drop atomic bombs on civilians (I understand he actually thought it would be a strategic advantage to let everyone know “we have The Bomb and we’re eager to use it”, as if nobody else would respond by building bombs of their own...)

  An incident in Taipei in which an apartment was accidentally built with rebar containing radioactive cobalt-60

  I’m curious what the book has to say about this, because I read that study after seeing it featured on a site called X-LNT as one of (only) four studies that ostensibly supported the radiation hormesis hypothesis (the idea that low-dose radiation is good for you). As you may know, there are far, far more than four studies on low-dose radiation exposure, so the lack of data listed on the site was a little suspicious.

  But when I actually went and looked at the cited studies, I saw that only one directly supported hormesis. The conclusion of the study on the Taipei incident said “The results suggest that prolonged low dose-rate radiation exposure appeared to increase risks of developing certain cancers in specific subgroups of this population in Taiwan”. The study itself did not seem high-quality (the p-hacking is plain as day) but the point is that some people seem to have deliberately misinterpreted it. IIUC, the researchers created a simple model to predict how many cancers they expected to see in the population, and the actual number of cancers among the irradiated group was lower than their prediction, but the researchers themselves didn’t think that this was evidence of radiation being good for you.

  Regardless, I’ve seen plenty of support in scientific literature for the idea that radiation is less than proportionally harmful at lower doses, although there seem to be a bunch of scientists that continue to support LNT as well. I am not sure how to tell how much scientific support each side of the debate has.

  By the way, who told me about X-LNT in support of his views? None other than Robert Hargraves of Thorcon, the same company that employs Devanney. I’m glad to hear that Devanney does not promote hormesis.

  My favorite 1990 book on nuclear power agrees with the general idea that small doses are less harmful:

  In view of all this evidence, both UNSCEAR¹² and NCRP¹⁹ estimate that risks at low dose and low dose rate are lower than those obtained from the straight line relationship by a factor of 2 to 10. For example, if 1 million mrem gives a cancer risk of 0.78, the risk from 1 mrem is not 0.78 chances in a million as stated previously, but only ¹⁄₂ to ¹⁄₁₀ of that (0.39 to 0.078 chances in a million). The 1980 BEIR Committee accepted the concept of reduced risk at low dose and used it in its estimates. The 1990 BEIR Committee acknowledges the effect but states that there is not enough information available to quantify it and, therefore, presents results ignoring it but with a footnote stating that these results should be reduced.

  But it should be stressed that if you accept the LNT hypothesis and therefore triple the book’s risk estimates, that would only change the conclusion from “nuclear power is fantastic and extremely safe” to “nuclear power is very good and quite safe”.

  The only deaths from the Fukushima disaster were caused by the unnecessary evacuation of 160,000 people, including seniors in nursing homes.

  While it seems to me plausibly unjustifiable to have evacuated nursing homes or ICUs, it’s important to make a distinction between evacuation and relocation. Evacuation is a much, much smaller step than relocation. It’s a way of saying “okay, please move while we evaluate the risks here”, and after Fukushima there would have been a certain amount of short-lived iodine isotopes that you don’t want in your body. But these isotopes are basically gone after a few weeks, and iodine pills might be sufficient to protect you and allow you to move back home.

  What made Fukushima really bad, I think, was the decision to do sudden and permanent relocation instead of just an evacuation, especially as they forced out elderly people who, even if they were to get a large radiation dose, probably wouldn’t have lived long enough to develop cancer from it. I don’t really understand how a thousand people can die of “stress” as the media reported (I don’t speak Japanese so how can I do more research on this?), but regardless of how their deaths happened, it seems clearly dangerous and wrong to suddenly relocate everyone, and not allow them to even visit their homes.

  And then there’s the bizarre debacle of all the giant drums filled with “radioactive” water that is pretty much safe to drink. My working hypothesis is that after making a bad relocation decision that killed a lot of people, the government can’t admit it made a mistake, so it must double down. The public only knows what the media says, and if the media in Japan is like American media, it misrepresents badly. Here’s Cohen’s 1980s survey:

  Here and elsewhere in communicating with the public, I try to represent the position of the great majority of radiation health scientists. [...] In 1982, I became concerned that I had no real proof that I was properly representing the scientific community. I, therefore, decided to conduct a poll by mail.

  The selection of the sample to be polled was done by generally approved random sampling techniques using membership lists from Health Physics Society and Radiation Research Society, the principal professional societies for radiation health scientists. Selections were restricted to those employed by universities, since they would be less likely to be influenced by questions of employment security and more likely to be in contact with research. Procedures were such that anonymity was guaranteed.

  Questionnaires were sent to 310 people, and 211 were returned, a reasonable response for a survey of this type. The questions and responses are given in Figure 1.

  TABLE 1

  1. In comparing the general public’s fear of radiation with actual dangers of radiation, I would say that the public’s fear is (check one):

  	2	grossly less than realistic (i.e., not enough fear).
  	9	substantially less than realistic.
  	8	approximately realistic.
  	18	slight greater than realistic.
  	104	substantially greater than realistic.
  	70	grossly greater than realistic (i.e., too much fear).

  2. The impressions created by television coverage of the dangers of radiation (check one)

  	59	grossly exaggerate the danger.
  	110	substantially exaggerate the danger.
  	26	slightly exaggerate the danger.
  	5	are approximately correct.
  	3	slightly underplay the danger.
  	2	substantially underplay the danger.
  	1	grossly underplay the danger.

  Not surprisingly, I think, the public does not know about all the efforts to make nuclear regulations more strict. In the 1970s someone listening to the media might think “nuclear power is a bit dangerous, but at least it’s affordable”, but after all the strict regulations are added, by 1990 they think “nuclear power is dangerous AND expensive!”

  This is quite different than what happened with airplanes, where we got multiple TV shows about airplane accidents, which always talk at the end about how regulations and procedures were changed to ensure This Will Never Happen Again.

  Even though flying is thousands of times safer than driving (per mile), people still feel it’s a little scary. Luckily, though, it’s not so scary that activists are picketing airports and demanding that the “passenger missiles” be prohibited from flying within 15 miles of a populated area.

• DPiepgrass 11 Oct 2021 17:08 UTC

  1 point

  in reply to: ChristianKl’s comment on: Trust and The Small World Fallacy

  Right. They have significant side effects but lie on the survey because...? Or maybe you’re saying they refuse to do the survey at all.

  But then, why didn’t they lie or refuse in the unnamed information sources you advocated?

  Of course I could tell a story where people who don’t have side effects forget about the survey and don’t bother to report their absence of side effects, but you’re going to like your story better, so that makes your story the true one. And also the size of the bias you’re assuming exists would have to be enormous, but whatever.

• DPiepgrass 11 Oct 2021 16:42 UTC

  1 point

  on: We need a ca­reer path for invention

  I think the most dire need in this area is something I call “open engineering”.

  If you create a physical invention then there is a difficult, but well-known career path: become CEO, raise money, do marketing, build the invention, sell it to customers.

  But what about those who want to make an intellectual invention? Yes, yes, copyright exists, so you can sell copies of (e.g.) a piece of software, but when was the last time you saw someone successfully selling a new general purpose programming language? That almost never happens, and there’s a very good reason why: because learning a new programming language requires time and effort, which for most people is not worthwhile unless there is an employer out there who wants to hire you to use that language. A language costs a lot of time even if it’s free! Good luck adding a monetary cost on top of that: you’re saying that after you’ve spent all this time learning the language, you can’t necessarily use it until you convince your employer to pay for it? Uh-huh...

  But normally, employers don’t want to use an obscure language, even a free one, and indeed, I’ve heard various examples of crappy proprietary languages built in-house at companies that the employees of those companies like to groan and gripe about. There’s no theoretical reason that a proprietary language couldn’t be good, but in practice they’re not, because there is virtually no market for them, so they tend to be underfunded even in cases where they exist at all. People who learn brand new languages, at first, tend to be people who do it for fun, not for profit. Therefore if the inventor charges money for it, these early adopters probably won’t be interested.

  By producing an open-source language, the inventor forfeits any practical path to profitability, but it’s much easier to get early adopters. More than that, it provides a path to creating a crucial network effect: employers, very slowly at first, become interested in using the language if it has major benefits, which slowly encourages developers to learn the language, and as the number of developers who know the language increases, to too does the number of employers willing to use it, in a virtuous cycle.

  As evidence of this, I cite its opposite: Mathematica, a very cool, capable and versatile programming platform, which however is proprietary, and I think this is why it has never taken off among software developers. You buy it if you really need what it offers; otherwise you ignore it and use JavaScript (all the while griping about its stupid design decisions). Mathematica has something no other language has: an huge and unified library of mathematical functionality. It’s also fundamentally graphical in nature—another rare feature. Clearly this wasn’t cheap to produce, so they recoup their costs by charging money, which, however, may have reduced Mathematica’s popularity by orders of magnitude.

  No only that, but by going open-source the inventor gains something else that any EA like myself salivates over: the ability to improve the world in a real and significant way. If my language is free, anyone can use it, and that includes schoolchildren and developing-world residents who normally can’t afford to buy software. That’s a real win on the “warm fuzzies” side of things. People like me care more about improving the world than making money, so producing expensive proprietary software has no appeal except for the purpose of making money. I do need money, sure, absolutely, very much so. But I know that making a proprietary programming language or library is a slow path to riches, so instead I am building software for the oil and gas industry which, if not for the money, I would prefer not to support! (I’m in Calgary, Canada’s oil & gas hub, so that’s a very common kind of job here, so I took that job and spend some of my paycheck on clean energy advocacy.)

  Another key foundational component of software—arguably more important than programming languages—is libraries, units of code such as algorithms, data structures and protocols, that are designed to be re-used by an unlimited number of people and companies. Here too, the proprietary route is possible, but usually you have to charge very high prices to make it worthwhile, since most developers prefer to use an inferior open-source product if available, and you’ll probably have to spend a lot on marketing for people to even discover your product.

  Case in point, my company was looking for a Simplex algorithm for C# — a algorithm taught in many university courses, but not an easy algorithm to implement correctly and robustly. There were expensive commercial products available that supported it, but we’re a tiny company with no customers, so we decided to go the open source route. Guess what, though? There are no good Simplex implementations for C#! To get Simplex in C#, I extracted the Simplex algorithm from the massive Apache Math library for Java. This was not easy, because that algorithm is embedded in a complex object-oriented framework and the transitive closure of its dependencies was perhaps dozens of times larger than the core algorithm.

  I got the job done, so now we have a “free” Simplex algorithm in C# that just required a few days of engineering work, but have we released it as open source? No, we have not. Why? Because although Simplex is useful in a wide variety of industries and could significantly benefit the world, it could potentially also benefit a competitor. I think this phenomenon happens very widely in the software industry and is certainly an impediment to progress. Now, my company has good people who support open source, so they are willing to release the code after we have a few customers. But we’re not planning to support or improve the software, it’ll just be a code drop. Look at the wasteful duplication here: Simplex for C# already exists, but we didn’t want to pay for it, so we built our own. This is a waste of human labor that we accept because we have no better system. (Imagine if, instead, engineers spent more time creating new things instead.)

  Another example from this year is a library called SyncLib that I built for a company before being laid off due to the Covid downturn. SyncLib is a unique serialization library that I have demonstrated can easily cut your serialization code size in half while making it less error-prone, supporting multiple data formats, and maybe even improving performance. It’s useful in any industry and great for hobbyists too.

  They refused to give me any of the source code of SyncLib, including even an interface file that would remind me how it worked. Why? Was it because competitors might benefit? No. It was because (under their own open-source policy) they would have someone review, publish and maintain the code, and they didn’t want to spend money on that just for some forgotten ex-employee.

  So in order to build an open-source version I had to do it all from scratch, again. The new version is much more ambitious, and of course, free. But how can I afford to build an open-source product? Simple: I have a deal with my employer in which they pay me 20% less money in exchange for 20% open-source time (because I get burned out if I have to do it all on weekends).

  This, Jason, is what I’d like you to advocate: there should be funding for open engineering. I don’t care if the pay is lousy, either. Low pay — and I mean low, as in minimum wage or thereabouts — has a huge advantage for anyone who wants to improve the world. The advantage is that whoever supplies the funds can be less picky about whose applications for funding they can accept. Paying $12/​hour, they can fund three times as many projects as they could at $36/​hr. Hopefully that means I don’t have to prove to some conventionally-minded grant reviewer that my radical idea is a good one before I can get any funding for it.

  Support Open Engineering! And by the way, it’s not just for software. No doubt there are people out there who would like to write free textbooks, or help design open RISC-V microprocessors, or blueprints for gadgets you can mostly print at home on your 3D printer, etc.

  What links here?

  • DPiepgrass's comment on The Du­pli­ca­tor: In­stant Clon­ing Would Make the World Econ­omy Explode by HoldenKarnofsky (3 Nov 2021 17:24 UTC; 1 point)

• DPiepgrass 11 Oct 2021 6:37 UTC

  1 point

  on: How count­ing neu­trons ex­plains nu­clear waste

  That was a great article. May I ask where you got your nuclear expertise from?

  That is, each fission event creates only one new fission event on average, giving an overall fission rate that is stable.

  Ahh yes, but that’s a neat trick, isn’t it? How do you arrange for there to be exactly one new fission event on average? 1.000 is a very precise number and one wonders how engineers can achieve such stability.

  My impression is that in general, the coefficient of reactivity (i.e. the tendency for atoms to fission) tends to fall as the temperature rises, due to basic physical phenomena such as thermal expansion and doppler broadening. As a result, if a reactor has approximately the correct amount of “reactivity” (i.e. U-235 or plutonium fuel plus moderator effects minus poisoning effects), the coefficient of reactivity will remain steady around 1.000, and so will the reactor’s temperature. This is because if it gets too hot the reactivity will naturally fall (thus reducing temperature, provided pumps are carrying heat away) and if it gets too cool the reactivity will naturally increase (thus increasing the temperature). This tendency toward stability is known as a “negative temperature coefficient of reactivity” and is a standard feature of all reactors.

  However, controlling reactivity isn’t exactly easy in standard Light-Water Reactors (LWRs), e.g. because unstable Xenon produced by fission has a strong “poisoning” (reducing) effect on the nuclear reaction, causing a tendency for the reactivity to rise and fall over several hours, and so, control rods are required to counteract this effect and increase or decrease reactivity as necessary. Also, correct me if I’m wrong, but in a LWR I don’t think the nuclear fission reaction can be stopped without inserting control rods, because nothing short of a meltdown can cause the temperature inside the reactor chamber to get hot enough for the “negative temperature coefficient of reactivity” to stop the fission itself. Plus, adding fuel is a bit of a pain, so they put somewhat more fuel in the reactor than strictly necessary, in order to refuel less often, but this increases the potential maximum reactivity, which is a potential hazard.

  In contrast, the proposed Molten Salt Reactors (MSRs) have

  1. a strongly negative temperature coefficient of reactivity,

  2. a wide thermal margin, so that if they overheat, the rate of fission drops almost to zero without any manual or automatic intervention

  3. no Xenon poisoning, because Xenon simply bubbles out of the salt as soon as it is produced, which makes reactivity easy to control; thus there is no need for control rods, though shutdown rods can be used to shut down, if desired...

  4. an easy way to add fuel (usually? I can’t remember how Moltex manages this), and

  5. a reliable system for passive shutdown, such as a drain tank.

  Thus MSRs take maximum advantage of the laws of physics to remove hazards: more safety, less complexity.

  (The most it can do is get so hot that it melts.)

  Correction: the most nuclear fuel can do is get so hot that it causes a steam explosion. That’s what happened at Chernobyl.

  I gather that there were several contributing factors to the Chernobyl disaster, but among these were two primary flaws that made the accident so serious. These flaws (1) never existed in any reactors outside the Soviet Union and (2) were both intentional. The first flaw was the lack of a containment building; the second was a reactor design with a substantially positive void coefficient.

  Obviously, containment buildings cost money and you can save money by not having one. Mystery solved!

  But what is a “positive void coefficient” and why did Chernobyl have one? This phrase is short for “positive void coefficient of reactivity”, which means that if an empty space (void) appears in the reactor chamber, the reactivity (rate of fission) increases, which increases the amount of heat being created. If this heat causes the void itself to get bigger, then the heat causes more heat, in a dangerous feedback loop. The interesting thing about this design flaw—I think—is that it was intentional. I put two and two together after watching lots of videos on MSRs and then reading Bernard Cohen’s book (Chapter 7); I inferred that the positive void coefficient is a cost-saving measure.

  It’s related to a curious property of neutrons. You might expect that a neutron needs to hit a nucleus hard and fast in order to fuse with it, but it turns out that the opposite is true: nuclei can absorb slow-moving neutrons more easily than fast neutrons. But when atoms split, the released neutrons are moving very fast, so they are not easily absorbed by other uranium atoms, which makes it hard to keep a chain reaction going. Nuclear engineers use materials called “moderators” to solve this problem. A moderator is any material that slows down neutrons (but rarely absorbs them).

  In Western light water reactors, the light water itself is used as a moderator (“light water” means “normal water”, i.e. not enriched, which would make it “heavy”). But light water has two different effects: it can slow down neutrons (increasing reactivity) but it can also absorbs neutrons (decreasing reactivity). Normally, the moderating effect is the dominant one, so if the water boils (causing steam bubbles, which are “voids”) the amount of moderator decreases, which decreases the fission rate. But water’s “poisoning” effect of absorbing neutrons is big enough that light water is incompatible with natural uranium.

  Natural uranium has only 0.7% U-235 content, so it’s challenging to get a fission chain reaction out of it. For that reason, most western reactors use enriched uranium instead. But enriching uranium costs a lot of money, so, you know, why bother?

  There are two ways to run a reactor with natural uranium: either you can use heavy water moderator (expensive), or graphite moderator (cheap). Well, you can guess which moderator the USSR chose for Chernobyl!

  Since graphite was used as the moderator, the light water in the reactor was only there for the purpose of cooling, not moderating. When the reactor overheated in 1986, the coolant water boiled, which created steam bubbles (voids) in the reactor. So now, the reactor is losing a neutron poison that slows down the chain reaction, and simultaneously losing the ability to cool down the reactor. Meanwhile, it is not losing enough moderator to make a difference. Therefore, the nuclear reaction speeds up, which soon leads to the steam explosion that destroyed the reactor. Oh, and plus, their control rods were slow and may have had a design flaw that increased reactivity during insertion. Oops!

  So there you have it. Chernobyl happened because the Soviets were cheapskates, and a similar event cannot happen in the modern world because no regulatory agency today would allow the combination of natural uranium with graphite moderator and light water coolant. Even in the 1970s this design was already off-limits in the West.