Look at this video:

People face uranium directly. Does this mean the radioactivity of uranium is very weak? Because its half-life is very long? Personally, I would never dare to touch any radioactive element.

I also remember seeing people holding a big chunk of uranium in hand. See here

Natural uranium consists of $\approx 0.7\%$ $U^{235}_{92}$, where the rest is $U^{238}_{92}$. Fresh reactor fuel consists of $3.5\% -4.5 \%$ $U^{235}_{92}$. Both isotopes of Uranium have very low specific activity and their radioactivity will by no means, under normal conditions, cause a higher dose than $20mSv$, which is the annual limit dose for people working with radioactive materials (in the EU). Uranium is, however, chemically toxic (as are all heavy metals). Therefore, it should not be consumed or handled with bare hands. The low specific activity $\frac{Bq}{g}$ can be explained with the large half-life of the isotopes. This is best illustrated by the formula for calculating the specific activity $$A=\frac{N_{A}\log(2)}{T_{\frac{1}{2}}m}.$$ Therefore, large half-life $T_{\frac{1}{2}}$ results in very small activity $A$ per unit mass $m$.

It is a completely different, question if the uranium has been irradiated. In this case, you would start building fission products and Minor Actinides, some of which are highly radioactive. Handling them requires special equipment. As a rule of thumb,the larger the irradiation time (say in a reactor core) and the denser the neutron flux $\frac{n}{cm^2-s}$ the larger the radiotoxicity.

To summarize, fresh Uranium fuel and natural uranium have very small specific activity. Anyway, I don't recommend playing with such materials because they are chemically toxic and you never know if the material has been irradiated. In radioactivity as well as in Medicine it is all a question of dose.

Remark: I got some questions about the equivalent dose form Uranium. Here is a simple (highly conservative) estimate.

Suppose we had 1 kg of natural uranium. Natural uranium has specific activity of $\approx 15\frac{Bq}{kg}$. Suppose further it emits gammas at $Cs^{137}$ decay energy of $0.662MeV$. Assume also that you somehow absorb everything that is emitted by the uranium chunk. Plugging that into formulas gives $$1[kg] \times15\left[\frac{decays}{kg\cdot sec}\right]\times 0.667[MeV]\times 1.6\cdot10^{-13}\left[ \frac{J}{MeV}\right]\times 3600 \left[\frac{sec}{h} \right]= 6\cdot 10^{-9} \left[ \frac{Sv}{h} \right]$$ This estimated dose rate of $6\frac{nSv}{h}$ is much smaller than $0.4\frac{\mu Sv}{h}$, which is the upper limit of the background radiation dose rate. So for one year you would get way less than the typical background cumulative dose of $2mSv$ and the limit $20mSv$ will not be exceeded by any means. With the above assumptions you would need to absorb all the radiation from $100kg$ natural uranium for one year to exceed slightly the natural background dose. But it would be still less than $20mSv$

Of course uranium does not emit only $\gamma$ radiation and you can't absorb all of it, unless you ate it, something I advised against. Moreover, you would spend only a limited amount of time near the material. Therefore, the dose you would get form 1kg Uranium will be much less than what I calculated. You can play with other energies, radiation types and exposure times. I chose $\gamma$ because it has the highest penetration depth and travels freely in air. Whereas, $\beta$ and $\alpha$ travel only short distances in air and are typically stopped by the skin or the clothes. Therefore, $\gamma$ is a quite conservative estimate. If the uranium emitted only $\alpha$ radiation and you absorbed it all the result will become $27$ times bigger.

• Typo: s/bear hands/bare hands/ - although I guess bears shouldn't handle it either. – Mat Aug 8 '16 at 13:21
• Could you make clear how long one needs to be in the presence of how much uranium and still be below the 20mSv per year limit? I'm not doubting your answer - it's just that you seem to imply that a lump of uranium under your seat all the time would not irradiate you to a 20mSv a year extent. Is this what you mean to say? Also, even with mildly radioactive metals there is a risk of ingesting shards (as well as the chemical toxicity which you mention) and even low level emitters lingering inside the body are a problem. – WetSavannaAnimal Aug 8 '16 at 13:35
• @WetSavannaAnimalakaRodVance: There is no long range radiation, as far as I know. The decay chain is all alphas and betas, so the seat material, alone, is enough to shield it. What the gamma and neutron spectrum of a chunk of material looks like is a very different matter, though, for that you need to know where it came from and what else but Uranium and the decay products are in there. There is no way to know that without an analysis. – CuriousOne Aug 8 '16 at 14:05
• @WetSavannaAnimalakaRodVance I added some calculation for $\gamma$ dose and some very harsh conditions. It is still at about background level. – Alexander Cska Aug 8 '16 at 14:36
• It's worth noting that "depleted uranium" (essentially pure U238) is "popular" as a material for military bullets, given it's high density and the way it shatters on impact. – Hot Licks Aug 8 '16 at 18:05

As with all safety hazards the answer is "It depends!". A chunk of natural uranium that hasn't been enriched or exposed to the inside of a reactor is not strongly radioactive and you can handle it with few precautions. You can hold it in your hand safely, but I would treat it the same way as I would treat all heavy metals that have a certain level of chemical toxicity. You do have to be concerned about toxicity if you are exposed through your lungs or to compounds: https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@na+@rel+uranium,+radioactive

If you are thinking about machining the metal or about chemical processing, I would suggest serious precautions and controls, as with any other substance that has even the slightest hazardous potential.

The radiation is mostly low energy alpha and beta radiation which can't get through the skin to damage living cells... unless it's in the body, already, either through inhalation or by chemical absorption... same precautions as for the chemical poisoning problem.

But here is the real problem: how do you know that what you are dealing with is a fresh piece of uranium that has just come out of the ground and that hasn't been exposed to neutrons? How do you know that it does not contain other radioactive contaminants which would have long decayed in a natural geological environment but which can be present in any amount if the material went through a processing facility that handles hot materials? Do you trust the friendly uranium dealer from around the corner who sold it to you? Really? What does he care about your health? I wouldn't trust that unless the material was properly tested by someone who can be trusted and I have an independent way of verifying that trust, i.e. at the very least I want to have a calibrated gamma/neutron monitor myself and a system to mark all materials that go through my possession in a reliable way. That's priceless.

• Natural uranium does not emit $\beta$ radiation. Beta radiation is characteristic of the fission products, which are not present in the natural uranium. U235 emits 4.4 MeV $\alpha$ and 0.1keV $\gamma$ due to spontaneous fission. Whereas, U238 emits only 4.2MeV $\alpha$. Anyway the chance for spontaneous fission is very low. – Alexander Cska Aug 9 '16 at 12:52
• A chunk of natural uranium that hasn't been enriched or exposed to the inside of a reactor is not strongly radioactive. Both U235 and U238 are very weakly radioactive, mostly \alpha emitters and the \gamma will come from the very rare spontaneous fission. Therefore, even enriched uranium will not produce dose higher than the natural background. exposed to the inside of a reactor that is called fuel irradiation and can happen also with other means. – Alexander Cska Aug 13 '16 at 15:01

You wouldn't dare touching any radioactive element? So, you wouldn't eat, say, a banana? You are radioactive, as is pretty much everything you eat, and the ground where you live, and the air you breathe. Radioactivity is everywhere.

Most of the radioactivity in humans is from potassium-40, and a bit from radioactive carbon. Potassium-40 is more radioactive than U-238.

Of course, this is mostly a jab at your "I wouldn't touch anything radioactive". The popular understanding of radioactivity is dangerously bad, which is why people are afraid of nuclear fuel more than, say, the waste out of a coal power plant or of their own wood-burning furnace.

The main risks you have when handling something like a pellet of U-238 is:

• It can be ingested. Uranium is one of the more dangerous here, because it easily produces shavings that can move in the air and burn quite easily. U-238 decay mostly emits alpha radiation which is relatively harmless to humans, as long as you keep it outside. Needless to say, it becomes a lot more of a problem when it sticks to your lungs and gets into your blood (though that already poses extra problems due to it being a heavy metal - it's highly toxic regardless of its radioactivity).
• It's very concentrated - you're holding a big slab of radioactive material. The potassium in a banana is highly radioactive, but there's so little of it that it doesn't pose a real hazard.

As long as you keep your gloves on and isolate the air (as in the video), you'll be fine, especially if it's something you dug from the ground - danger from radioactive source is inversely proportional to lifetime of that source; uranium must necessarily have very little radioactivity, since it's existed as long as the Earth and there's still plenty to go around.

Don't mess around with those radiotherapy sources, though (warning: very much not pretty with a lot of "how could they be so stupid"). If you decide to read about that incident, note that even with the vastly more dangerous radioactivity source, the serious health issues (including amputation and death, sadly) were a result of a long exposure (many hours) and/or ingestion.

Needless to say, this shouldn't be taken as an advice to go ahead and play around with highly radioactive stuff. It is dangerous, just like, say, mercury is dangerous. It can kill you. All facilities dealing with highly radioactive matter have strict measures to prevent accident and measure exposure, and the gloves you see in the video aren't your typical household cleaning gloves. Different radioactive materials can have vastly different dangers, depending on their half-time and the emission characteristics.

There are two sides to this question.

Naively, the answer would be "bah, not much" because it is not terribly active and neither alpha, nor beta radiation is really dangerous. The former (which occurs early in the decay chain) is absorbed even by a few centimeters of air, and the latter (which appears later in the decay chain) is unable to penetrate the callus layer of your skin. The callus is dead tissue either way, so radiation doesn't really do anything to it.

However, uranium is directly toxic (nephro- and hepatotoxic, and causing neurological effects) and finally decays to an accumulating neurotoxic element (lead). The toxity is generally much more severe than the radioactivity. Uranium dust can very well be inhaled if no precautions are taken (not uncommon in fertilizer production).

But what's worst, your body happily absorbs uranium as "calcium" and puts it in your bone matrix.
Now, you will remember I just said alpha and beta radiators are pretty harmless. Alpha and beta radiators inside your body and especially near highly active tissue (such as certain organs, but also... bone marrow) are extremely harmful.

Further, if you look in the decay chain, you will notice quite a few elements appearing, some of which (radon) are gases which you can neither smell nor see but nevertheless inhale and absorb. Polonium... remember what substance it was the KGB used to murder Alexander Litvinenko?

Therefore, from a biological point of view, the answer must be: "very". You can certainly handle uranium safely with simple rubber gloves and behind a suction (or wearing a breath mask), but otherwise playing with it is not such a terribly good idea.

• Yikes! I didn't know about the "masquerading as calcium" bit. That's truly scary. – WetSavannaAnimal Apr 27 '17 at 11:08

## protected by Qmechanic♦Aug 8 '16 at 21:02

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