Why do non-radioactive materials become radioactive?

Factual Questions
1

Question inspired by IMHO thread about the irradiated Russian soldiers (although I’ve wondered about this before).

When “ordinary” non-radioactive materials get exposed to radiation, why do they become radioactive? The irradiated Russian thread mentions “The Claw”, a piece of construction equipment that was used in the clean-up at Chernobyl. It’s just a big grappling hook. What might it be made of? Basically just iron or steel?

So they used it to wrangle pieces of the mangled reactor around. And now it is the most dangerously radioactive piece of junk around – so they brilliantly just took it out deep into the forest and dumped it there, where it sits to this day.

So what happens to a piece of iron, or steel, or whatever, when it gets irradiated, such that it becomes radioactive itself?

(No, Discobot, this topic is not similar to “When did non-stick rice become high-class”. It’s also not similar to “When did polygamy become non-Christian”. I think.)

ETA: Link to the relevant post in the Irradiated Russians thread.

2

The wiki:

3

If you expose a big hunk of steel that naturally has some stable cobalt 59 in it to neutron radiation, then the atoms can absorb one of those neutrons and turn into radioactive cobalt 60.

4

Thanks. I’m going to read that now.

ETA: Okay, I read that. It’s brief and vague. It mentions that Cobalt-60 has a half-life of just over 5 years, so it wouldn’t last all that long. They mentioned that the irradiation process for food uses low-enough energy to not make the food radioactive. Can just about any substance absorb free neutrons and become radioactive?

5

It’s only been 35 years, so even if you lose half your radioactive material every 5, you still could have plenty left to be dangerous after 7 “halvings”

And it is a lot less radioactive than it used to be.

6

The only form of radiation that causes elements to transmute into radioactive (unstable) isotopes in large quantity is neutron radiation; specifically, the ‘fast’ neutrons resulting from supercritical nuclear fusion. This is obviously a problem in nuclear reactors and facilities for processing nuclear materials where metals can convert over exposure, both by becoming radioactive or just damage to the metallic structure (neutron embrittlement) causing them to lose structural integrity. This isn’t really much of an issue with iron; although there are several isotopes of iron that are unstable most either have very long half-lives (and thus don’t produce much of a flux) or are extremely rare in nature, occurring only significant abundance in the bizarre nuclear chemistry in supernovae and neutron star collisions. Only 59Fe occurs regularly enough to be detectable by normal instruments and has a half-life of about ~45 days, but even it isn’t abundant enough to be of concern without very high neutron flux rates.

However, in steel iron is often alloyed with other elements that can be readily transmuted into unstable isotopes. I’m not going to crack open a reference book at this time of night but manganese, commonly used in corrosion resistant tool steels and stainless steel. The stable isotope, 55Mn, has a high neutron capture cross-section (about 12 barns) which transmutes into 56Mn which has a half-life of about 2.5 hrs and is an intense gamma emitter on the order of 850 keV. [Note: all numbers from memory so they might be slightly off.]. There are a number of other elements in alloy steels, paint pigments, lubricants, et cetera that can also become activated, and even in small quantities can be hazardous for long term exposures but won’t produce radiation like an active neutron source creating new isotopes or high energy cathode ray tube.

The other way that materials ‘become’ radioactive is just to be contaminated the nuclear material. The Chernobyl #4 reactor exploded and caught fire, spewing activated carbon from the graphite moderator as well as a variety of materials from the fuel rods, reactor systems, et cetera, coating everything liberally in radioactive residues. This is why although the Sarcophagus and the New Safe Containment structure enclose the reactor itself, the Chernobyl Exclusion Zone covers many thousands of square kilometers in area thoroughly contaminated with residues, many of which have been uptaken by trees and other plant life and cannot just be removed. This is also why vehicles racing offroad through the zone and the concern about wildfires of the very unhealthy forest cover in the Zone are of significant concern.

With regard to people becoming radioactive, despite the meme that exposure to “radiation” (usually meant as gamma radiation harmful to living beings) will cause people and objects to “glow in the dark”, in fact gamma radiation is just high frequency electromagnetic radiation capable of ionizing substances (i.e. causing electrons to be ejected, giving the substance in question net positive charge). This can certainly be harmful, burning and destroying tissue and damaging the DNA within the exposed cells so that the cell can no longer produce necessary proteins or regulate metabolic functions but it doesn’t cause the tissue or substances to themselves become radioactive, and the damage essentially stops after the source of gamma radiation is removed (although the biological degradation can continue until either the damaged tissue is sluffed off or in extreme cases death through catastrophic organ and tissue failure and collapse of the immune system.).

Although humans and other living beings do have many elements that can be transmuted into radioactive isotopes, the amount of neutron exposure required to do this would result in death within days (see the “Demon Core” criticality accident at Las Alamos that killed Louis Slotin and Harry Daghlian, and the 1999 Japan Nuclear Fuel Conversion Company ultimately resulting in the horrifying deaths of Hisashi Ouchi and Masato Shinohara through eventual systematic organ failure and infections from immune system collapse). When people “become radioactive” (versus just ‘irradiated’, which means exposed to radiation) it almost always means that they are contaminated with radioactive materials like dust or liquids. As long as these are outside the body they can be washed off, usually with little effects beyond ‘beta burns’ and a slight increase in skin cancers, but if they are ingested or inhaled they can cause severe and chronic illness, even alpha emitters that are normally harmless outside the body, as the elements are metabolized into cells where there is no protection provided of the outer epidermis.

Stranger

7

Thank you for the detailed answers. (IIRC, I asked this here once before, some time ago, and didn’t get much of a detailed answer then.)

I get that anything can become contaminated and be radioactive simply by being coated with hot dust. But that could be washed off, if the item is washable, like that Claw maybe. Good luck doing that with a hundred square miles of dirt and trees, or peoples’ lungs. So I was really asking about every-day substances actually getting transmuted into hot stuff.

Can just about any element absorb stray neutrons and become a new isotope? And can just about any element (even some of the lighter ones?) become unstable when they absorb neutrons?

8

It depends on the element. Some elements have a lot of unstable isotopes but either small neutron capture cross-sections or unlikely chains, and some elements only have a few radioactive nucleotides. I think helium is the only element that actually has no possible radionucleotides (technically it does if you consider two protons to be “helium” but the binding is so unstable that it cannot exist except under extreme conditions, and the higher mass isotopes are only formed in particle accelerators or at a transitory product of helium burning fusion) but many elements have radioactive isotopes with such short half-lives that they are never observed in nature. Some elements, however, have large neutron capture cross-sections and half-lives that are long enough to persist in the environment but short enough that they produce intense radiation flux; many of these can also be absorbed by the body (either as isotopes of elements normally utilized in biological processes like calcium, or close enough to such elements to ‘replace’ them metabolically, like strontium).

This is an expansive topic and there are people who make entire careers in nuclear and medical physics just advancing the understanding of isotope production, decay processes, and biological uptake and use. I have a cursory knowledge (and a CRC handbook on the topic) but I’m far from a subject matter expert. I once worked on a project with some people at a government lab who did their Ph.D. theses on identifying isotope ratios from of different nuclear reactors as a means of nuclear arms control and tracing atmospheric nuclear testing.

Stranger

9

Not sure what you mean by “it wouldn’t last that long.” a 5-year half-life means that in five years, half of the cobalt-60 in a sample will have decayed; another five years, another half of what’s left will have decayed (leaving 1/4 of what you had at year zero). 35 years? 1/(2^(35/5)) = 1/128th of the original quantity is left. Whether that amount is a problem or not depends entirely on how irradiated the specimen was at year zero. If it was really bad at year zero, it could still be very dangerous at year 35.

10

FYI, 10 half-lives is generally considered de minimis.

11

I feel like we should point out that the neutron capture cross-section is a measurement of how easily any particular element can capture a neutron, and so potentially become a radioactive isotope. Each nucleus has its own unique neutron capture cross-section, and so while technically, any element could, in theory, be transmuted, some are much easier to transmute than others.

Here’s a pdf that lists a lot of known cross-sections, and you can see there’s quite a wide range. Just in the first few entries, it goes from 10^-3 to 10^3. So quite a large difference.

12

So, based on what I read here, those now radioactive Russian soldiers have to be isolated because they could irradiate other people, places, and things they might come in contact with? If so, for how long before they can be considered “safe”, and will they even be alive that long?

13

No, once any reside is washed off they can’t make other things radioactive, and the amount of radiation they put out is small, possibly of concern for small children or immunocompromised but tolerable to caregivers just wearing normal medical PPE. Their wastes may be mildly radioactive but quickly dilucted in the sewage stream. Any isolation would be to protect the patients from infection since that much exposure would weaken or completely suppress their immune systems. Victims of radiation poisoning who don’t die from catastrophic tissue damage usually succumb to infections, often by normally mild pathogens that a healthy person fights off every day.

Stranger

14

I would expect “de minimis” to depend on what one plans to do with the material. 10 half-lives gets you to 1/1024th of the original quantity. If I start with a large enough quantity, then surely I could end up with a remainder that couldn’t be considered “de minimis” for certain uses/dispositions.

15

And similarly, if you start off with something that’s at twice the “considered safe” level, then a single half-life would be enough to make it safe.

As a nitpick, any kind of radiation at all can transmute materials, but most forms other than neutrons either do so at a very low rate, or only do so at energies much higher than you’d usually encounter. And in some cases, you might be able to render a substance radioactive without, properly speaking, transmuting it, if you put a nucleus into an excited state (from which it could release gamma rays).