There is a lot of Uranium in your Backyard

Super fact 49 : The top one-meter (3.3 feet) of a typical 10 meters (33 feet) by 40 meters (131 feet) garden contains 2 kilograms (4.4 pounds) of Uranium. For comparison, the Hiroshima bomb contained 64 kilograms (121 pounds) of Uranium. Certain rocks such as Granite and Shale contain much more Uranium than soil. Uranium also exists in the atmosphere and there is 4.5 billion tons of Uranium in the ocean.

The numbers above come from the IAEA (International Atomic Energy Agency) and Stanford University . I should mention that the numbers vary depending on Geography, type of soil, etc. For example, there is much less Uranium in the soil in Florida compared to the soil in the Midwest.

This may come as a surprise to many people. Isn’t Uranium radioactive? How come we are still alive? That’s why I call this a super fact. The answer is that even though Uranium is used in nuclear bombs and nuclear reactors, it is by itself not very radioactive. You can hold natural uranium in your hand without much risk. The radioactivity from, for example, nuclear explosions come mainly from the fission process and the radioactivity from nuclear reactor waste is mainly from other isotopes created by the fission process in the reactor rather than the uranium itself.

An enormous nuclear bomb explosion in the dessert featuring a huge mushroom cloud | There is a lot of Uranium in your Backyard
If Uranium is not very radioactive, how come a nuclear bomb spread so much radioactivity. The answer is that the radioactivity comes from the fission process and the resulting new isotopes, not the uranium.

What Are Isotopes?

Before I explain some facts about the radioactivity and decay rate of Uranium, I should explain what an isotope is. Atoms consist of a nucleus and electrons surrounding the nucleus. In the nucleus there are protons and neutrons (and some other stuff). Neutral atoms have an equal amount of electrons and protons, which determines what kind of element it is. Hydrogen has one electron and one proton. Helium has two electrons and two protons. Oxygen has eight electrons and eight protons, etc. The number of protons/electrons is called the atomic number of the element.

The number of protons plus the number of neutrons is called the mass number. Atoms of the same element but different number of neutrons are called isotopes. Uranium-235 or U-235 has 92 protons and 235 – 92 = 143 neutrons. The number if protons/electrons determine the chemical properties of the element. The number of neutrons determines nuclear properties such as the stability of the nucleus, radioactivity, etc., as well as the weight. Therefore U-238 and U-235 are identical chemically and look and feel the same, but U-235 is more radioactive, and you can use U-235 for fission but not U-238.

Bohr model representation of the uranium atom, number 92 and symbol U. Conceptual vector illustration of uranium-238 isotope atom, mass number 238 and electron configuration 2, 8, 18, 32, 21, 9, 2
This is a simplified Bohr model of the Uranium atom. There are 92 little blue balls circling a nucleus in the middle of the atom. Those are electrons. In the nucleus there are 92 protons. Those are the red balls with plus signs. In addition, there is a yellowish smudge around the protons in the nucleus. Those are the neutrons. Depending on the isotope, there are 143 neutrons for U-235, 146 neutrons for U-238 and 142 neutrons for U-234. Shutterstock asset id: 1999370450 by Patricia F. Carvalho

The decay rate of Uranium

There are three main Uranium isotopes. Uranium-234, Uranium-235, and Uranium-238. Uranium-238 is the most common. 99.28% of natural Uranium is Uranium-238, 0.72% is Uranium-235 and 0.0057% is Uranium-234. Uranium-235 is the isotope we use for nuclear weapons.

The different isotopes have different decay rates and different levels of radioactivity. The half life of a radioactive isotope is the time it takes for an isotope to decay so that only half of it is left. The half-life of Uranium-238 is four and half billion years. That means that it will be around for a very long time, but since its decay rate is so slow, it is not very radioactive. The half-life of Uranium-235 is 710 million years, again it will be around for a very long time, but again, since its decay rate is so slow, it is not very radioactive. The half-life Uranium-234 is 247,000 years, a little bit faster but it still has a pretty slow decay rate.

This should be compared to Cesium-137, which has a half-life of roughly 30 years. In other words, it decays 150 million times faster than Uranium-238 and 23.7 million times faster than Uranium-235. Since Cesium-137 decays so much faster than the Uranium isotopes it means that each atom of Cesium-137 will send out radioactive particles much more often than a Uranium atom will, making it much more radioactive.

If you want to read about when I was walking around a whole day with a Cesium-137 sample in the back pocket of my jeans, click here. Radon-222, an extremely radioactive isotope of radon, which seeps into our basements from the inside of earth. It has a half-life of 3.82 days giving it a decay rate that is 430 billion times faster than Uranium-238 and 68 billion times faster than Uranium-235.

What makes it possible to make a nuclear bomb from Uranium-235 is not because it is very radioactive. It is not. It is because it has properties that make it perfect for bomb making. Each nucleus emits more than one neutron, in fact more than two on average, and the neutrons colliding with other Uranium-235 nucleuses can be made to travel at the correct speed to cause fission. In other words, it is fissile. It is a goldilocks situation. It is just right. Below is an illustration showing a chain reaction. Observe, the picture indicates that Uranium has 95 protons. This is wrong. Uranium has 92 protons. When I have the time, I will fix this picture.

Illustration of nuclear chain reaction. Uranium-235 fission | There is a lot of Uranium in your Backyard
This is an illustration of a chain reaction with fission of a Uranium-235 isotope. Notice the atomic number (number of protons) is incorrectly stated as 95 in the picture. It is 92.  When I have time, I will fix that. Shutterstock Asset id: 73714504 by Mpanchenko.

Other Nuclear Related Posts

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Author: thomasstigwikman

My name is Thomas Wikman. I am a software/robotics engineer with a background in physics. I am currently retired. I took early retirement. I am a dog lover, and especially a Leonberger lover, a home brewer, craft beer enthusiast, I’m learning French, and I am an avid reader. I live in Dallas, Texas, but I am originally from Sweden. I am married to Claudia, and we have three children. I have two blogs. The first feature the crazy adventures of our Leonberger Le Bronco von der Löwenhöhle as well as information on Leonbergers. The second blog, superfactful, feature information and facts I think are very interesting. With this blog I would like to create a list of facts that are accepted as true among the experts of the field and yet disputed amongst the public or highly surprising. These facts are special and in lieu of a better word I call them super-facts.

44 thoughts on “There is a lot of Uranium in your Backyard”

  2. This was absolutely mind-blowing and brilliantly explained! 💥 It’s so fascinating to realize how much uranium exists naturally around us and yet poses such little threat in its raw state. The comparison between the half-lives of Uranium and more dangerous isotopes like Cesium-137 and Radon-222 really puts things into perspective.

    I especially appreciated the clear explanation about isotopes and the emphasis on how it’s not uranium’s radioactivity, but its fissile nature that makes it so critical (and dangerous) in nuclear reactions. That “goldilocks” analogy was such a simple yet powerful way to describe U-235’s unique properties.

    Thank you for making nuclear science accessible and genuinely interesting! If you enjoy curious facts, tiny doses of mom life, and slices of everyday wonder, feel free to visit my twin mom blog Twin Chaos & Toddler Giggles at twinmom73.wordpress.com or connect with me on Instagram @twintales2025. Let’s keep learning and sharing stories together! 🌍✨

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  3. Interesting facts to know about. The numbers and math parts are way over my head in understanding, but that Uranium being in the soil got my attention. Back when I was a kid, my parents had a cardboard box (kind of small) up in the closet. It was labeled Uranium dirt. I never knew what it was or what it was for, heh heh I never asked, because I was snooping. Later on I think I was reading about Uranium dirt and it was used to ease pain of arthritis by sticking your hands into the dirt/sand. I googled this and it’s true that back in the 50s it was a popular and regular thing to do. 🙂

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    1. Yes Uranium isn’t very common but a lot more common than people think and we have it all around us. I was thinking up about using the title “You have eaten Uranium” since most of us have eaten dirt at some point, but I thought that was too click-baitish. I remember a guide at a natural history museum passing out uranium ore to the kids. It is radioactive and not healthy to be around for too long, or eat, but its radioactivity is highly overestimated. It is not that bad. Back in the 1950’s they had a very poor understanding of what radiation could do.

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    1. There is a lot of Uranium all around us but the good news is that it is only mildly radioactive. The special feature with Uranium or one isotope of Uranium (235) is that it is fissile, which I guess is scary depending on who succeeds in creating fission.

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        1. Yes you are right and we know it is not for nuclear power stations. For CANDU reactors you can use the completely un-enriched 0.7% of U-235, for BWR and PWR reactors you need 2%, and for a bomb you need at least 82%, typically 90%, and Iran has enriched to at least 60%. That is not for nuclear reactors. That’s for bombs. A very scary situation.

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    1. My guess is he must have been around concentrated uranium / uranium ore for a long time.

      I can add that even though the radiation is low (I held uranium ore in my hands for a while with no effect) uranium is also poisonous so if you ingest it or breath uranium dust you can be poisoned.

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  15. A timely and informative post! While I had read about the fissile nature of U-235, it was an eye opener for me when you compared it with Cs-137 and Rn-222. A great post indeed, Thomas 👏👏💐

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