Could You Power a House with the Radioactivity from Bananas? - Nuclear Engineer Reacts to XKCD

Hinzugefügt: 2026-05-12

[00:00:00]Imagine you're a potassium 40 nucleus.

[00:00:01]Every second? Cuz that's Imagine you're [laughter] a nucleus. It's time for some more XKCD.

[00:00:09]Specifically, could you power a house with radioactivity from bananas? Not exactly what I had in mind with innovative nuclear power. Let's check out this craziness. This question comes from G, who asks, "I heard bananas are radioactive. If they're radioactive, then they radiate energy. How many bananas would you need to power a house?"

[00:00:29]Mhm. So, radioactive does not mean useful energy source. Every material with radioactive isotopes does indeed emit energy. That's true. But, what matters is power density. So, watts per kilogram, not just total energy. So, it'd be a bit like asking if spent fuel could power a city forever, or at least a while. But, power output is what matters operationally, and you're not getting that.

[00:00:58]Yes, bananas emit radiation, but not very much of it. They're very safe until you use them to power a house.

[00:01:04]>> [laughter] >> Bananas are radioactive because they contain potassium, some of which is the radioactive isotope potassium 40. Very small number. And, yeah, natural abundance is around.01% potassium 40. So, bananas are mostly stable matter with trace radioactivity.

[00:01:24]You see trace activation products in a reactor coolant system, but it's not a dominant source of energy associated with that system. The factoid about banana radioactivity was popularized by nuclear engineers trying to reassure people that small doses of radiation are normal and not >> [laughter] >> Yeah, and that is historically relevant, cuz banana equivalent dose is a little bit more intuitive than using units like microsieverts. unnecessarily dangerous.

[00:01:51]Of course, this kind of thing can backfire.

[00:01:52]>> [laughter] >> Thank Yes.

[00:01:54]So, people now think they're unusually radioactive when they're really not.

[00:01:57]Other sources of food like Brazil nuts are radioactive, too, but they're all completely ordinary biologically.

[00:02:04]Cuddling your spouse gives you a similar dose overnight as a banana. Thanks to their use as a radiation dose comparison, bananas now have a reputation as an especially radioactive food, but they're really not. The CRC Handbook of Radiation Measurement and Protection, the source of the original data behind the banana factoid, lists lots of other foods with more potassium 40 than bananas, including coconuts, peanuts, and sweet potatoes. Now, potassium content is going to vary, but it doesn't really matter cuz biological systems regulate potassium tightly. So, your body already has a lot of potassium, and a decent amount of potassium 40 relative to bananas, of course. So, the whole idea of using them to measure accumulated dose doesn't make sense because the body processes potassium. It's not going to like stay with you forever.

[00:02:48]A large cheese pizza might be three times more radioactive than a banana, and the potassium 40 present in your own Especially if it's a Ling Ling Zing pizza. I'm just kidding.

[00:02:56]human body emits more radiation than any of those. Yes, you are essentially a low-power self-contained radioactive source, which is again the whole cuddling your spouse, you're going to get some dose, a very small amount.

[00:03:09]Potassium 40 decays slowly with the nuclei of individual atoms sitting around for millions or billions of years before quantum randomness finally triggers their decay. Imagine you're a potassium 40 nucleus. Every second Cuz that's [laughter] Imagine you're a nucleus.

[00:03:25]Uh, this is this is awesome. You flip a coin and then roll 21 dice. If the coin lands on heads and the dice all come up sixes, you decay. Otherwise, you do nothing.

[00:03:34]So, the thing about radioactive decay is it is indeed governed by a constant probability per unit time and independent of history. Now, you can't really say when an individual potassium 40 nucleus is going to decay, but you can say when about half of them are going to decay away, and that's all due to it being a large-scale Monte Carlo distribution. Large-scale because, well, there's a lot of nuclei >> [laughter] >> in a source of a few grams or even milligrams of a substance. So, I appreciate the simplification, but you can't really say that about an individual nuclei, or you can't say for certain what's going to happen to an individual nuclei. You're just looking at the greater population. That is where those half-life formulas come in. But, just like anything else in nuclear engineering, like neutron interactions in a reactor, so probability of causing fission, of scattering, of absorption, but no fission, it's all going to be statistically dependent.

[00:04:38]>> But, there are gazillions of atoms of potassium 40 in a banana. Life is too short to sit around counting zeros and looking up the Latin prefix for big number. [laughter] >> [gasps] >> Uh, just use scientific notation. Which is enough that any given second, 10 or 15 of them make that heads plus all sixes roll, spit out a high-energy particle, and become stable calcium or argon. That's the right order of magnitude. So, mention 10 to 15 disintegrations per second. So, there's a unit for that in activity, that's um, becquerels. So, a becquerel is a very small unit of radioactivity, one disintegration per second. This is teeny, teeny, tiny in nuclear terms. A lot of, uh, radioisotopes used in medical facilities are on the, are given in gigabecquerels or a billion becquerels.

[00:05:26]>> [laughter] >> It's a very small unit of measure. And reactor fuels way, way higher than that.

[00:05:31]You don't even bother typically giving it in activity because it's ultimately going to be used within the reaction itself. You generally give units of activity in the waste products rather than the actual reactor fuel, but it's going to be a big number if you bother.

[00:05:46]>> That high-energy particle by the expiring potassium atom will promptly bump into other atoms leaving everything vibrating with a little extra heat energy.

[00:05:53]Bump, [snorts] such a great technical term. But seriously, this is a pretty good explanation here. So, these beta particles, these free electrons, are depositing energy via ionization. That energy thermalizes and creates heat. Not unlike how reactor fuel converts energy into heat. You're just dealing with big chunks of energy, fission fragments. So, instead of electrons or gammas, you're talking big chunks of nuclei. And to give you a sense of scale, comparing an electron versus say a neutron or a proton, much less than an entire nuclei nucleus, you're comparing a golf ball and a wrecking ball in terms of size and mass. So, here you got very small energy release on the order of maybe a few electron volts. Whereas fission, you got millions of electron volts. Scale, electron volts the unit of energy for things on a microscopic scale. In principle, you can use this heat energy to do work. That's how the Mars rovers Curiosity and Perseverance are powered.

[00:06:54]In practice, the Mars rovers use a chunk of plutonium 238 big enough to have quadrillions of decays per second release Yep. You see, that's why you don't even bother with the decays per second. You're going to use You'd use a macroscopic unit like watts, yeah?

[00:07:07]Quadrillion, sure, why not?

[00:07:09]>> [laughter] >> So, plutonium 238 has high decay rate and high power density, makes it great for RTGs. That's radioisotope radioisotope thermoelectric generator.

[00:07:21]So, it uses radioactive decay as an energy source. Not the same thing at all as using a nuclear power plant cuz they're using fission as a heat source, but plutonium 238 is good for its high decay rate and high power density, way more than potassium 40. Potassium 40 is long-lived but very low power density. I know this is very different from plutonium 239 which is used in reactors and weapons because it is fissile.

[00:07:50]Plutonium 238 is not. It will not fission readily with thermal neutrons.

[00:07:55]You need to be operating in a fast neutron spectrum. Even then the cross section for plutonium 238 is relatively low compared to some other choices you have for for breeding such as thorium 232. Losing a lot of power. By comparison the 15 decays per second from our banana work out to a couple of picowatts of power.

[00:08:14]Picowatt 10 to the minus 12. So, effectively low power even though the total energy over the course of billions of years is large. I mean, it's just not useful. Too slow. Roughly the power consumption of a single human cell. Even if you captured that radioactive decay energy with perfect efficiency, powering a house would require about 300 quadrillion bananas which >> [laughter] [gasps] >> Uh, so the limiting factor is the decay rate rather than your total mass. Cuz even with perfect efficiency, you cannot accelerate decay. Bit like trying to run a reactor with fuel that refuses to fission fast enough. It just will not work.

[00:08:54]>> To form a heat large enough to bury most of the skyscrapers in the New York City metro area. Radioactive Yeah, trying to compensate for extremely low specific power is never a good idea. Same reason why uranium is valuable because of its extremely high power density. Why nuclear power is so effective. Cuz uranium is valuable and biomass [clears throat] is bulky. You'd be better off extracting a chemical using it as a biomass source. Decay of potassium 40 in bananas is a terrible source of energy. But that's okay.

[00:09:22]Because you know what's a great energy source? The banana itself. A banana >> [laughter] >> Exactly. I was going to say, you're better off just eating the stuff.

[00:09:30]Yeah, right there. About 100 kilocalories or 400 kilojoules. Way more usable. That's many orders of magnitude better.

[00:09:38]>> A banana contains about 100 calories of food energy and if you incinerate whole Note that anything associated with food, when you see calories on a food label, it's really kilocalories. I don't know why. I've only seen that with nutrition labels in English, too. It's I don't know why they do that. I guess people are just more comfortable dealing with smaller numbers and thinking in terms of a typical diet of somewhere between 2,000 to 3,000 calories a day rather than between two and three million calories a day, even though that's really if you if you look at the definition of a calorie, that's really what you're at where you're at.

[00:10:13]>> [laughter] [snorts] >> We're just saying kilocalories. With bananas as fuel, it would only take about 10 bunches per day to keep your house running. And this really, if you look at stored energy on this scale of a very slow decaying radioisotope, just use the chemical reaction. Chemical reactions release their energy very quickly relative to super slow radioactive decay. Now, keep in mind this is true specifically to bananas.

[00:10:40]Obviously, you're better off with plutonium 238 decay rather than trying to do a chemical reaction with a hunk of plutonium metal. And that being your energy source, but yeah, bananas burn your fuel rather than letting it sit and rot.

[00:10:57]Unfortunately for New York City, which we buried in bananas a moment ago while trying to make the radiation idea work, sorry. Radioactivity versus chemical energy It's always New York City. New York City gets hit a lot with these sorts of things. Isn't it either or thing? If you piled up a lot of bananas, they would start to release that chemical energy one way or another. The banana pile would start Okay, speaking of rotting bananas. to rot and the heat from the decomposing bananas would immediately overwhelm the heat from radioactivity. Decom Yep, and in this case, the biological decay is way faster than the radioactive decay. position by anaerobic bacteria deep in the pile would also produce various gases including highly flammable methane.

[00:11:32]>> Okay. All right.

[00:11:34]>> [sighs and gasps] >> Biogas [snorts] production.

[00:11:36]>> Then, >> [music] >> as the gas bubbled up to the surface of the burning hot banana swamp, it could ignite. Gas buildup from food waste is a major industrial explosion hazard.

[00:11:43]>> There's a reason why There's a surprising amount of engineering built into landfills to manage this, but if you just plop stuff here in a disorganized manner, yeah, you're going to have a bad time for whole messy reasons. Third, so [music] don't worry about the radioactivity in bananas. It's the rest of the banana that's the real threat.

[00:12:00]>> [laughter] >> This just shows that radioactivity is not inherently an energy solution. You need power density, energy release rate, and controllability. And here, well, you don't have any of those things.

[00:12:14]So, the moral of the story is a system that cannot control its energy release rate is not a power source. It is just a reservoir, and in this case, a particularly messy one. Thanks so much for the recommendation, and thanks so much for watching.

[00:12:28]I'll see you next time.