Monthly Archives: June 2021

I’m a geek. I’ve gotten used to this fact, as have my children, my wife, and other family and friends. And it’s why, when I fly, sometimes I monitor radiation levels. I’ve learned that this can sometimes make the person sitting next to me a little nervous – I’ve learned not to talk about it much. Luckily my radiation detector has a cell phone app so I can leave it in my carry-on bag and monitor it via Bluetooth from my seat…I just need to remember to turn off the alarm so my bag doesn’t beep annoyingly in the overhead storage bin.

What’s interesting is that, as we ascend after takeoff, radiation levels drop steadily. Actually, this part isn’t very surprising because when we’re on the ground most of the radiation I’m measuring comes from radioactivity in the rocks and soils and, as we gain in altitude we’re getting further and further from the surface, reducing the radiation coming from the rocks and soil. But then when we get to about 10,000 feet the readings stopped dropping – interesting. And by 12,000 feet or so they started rising again – even more interesting. By the time we were at our cruising altitude they were several times higher than they’d been at ground level – not only that, but I was also seeing many more neutrons than I’d seen at the surface. Curious.

What I was seeing was nothing new – except to me the first few times I made these measurements. The first time it was noticed was over a century ago, in 1911 when Austrian physicist Victor Hess was taking a balloon flight, and he had his own radiation instruments with him (albeit without Bluetooth or a phone app) when he noticed the same thing I was to see a century later. Hess was more curious than I was because he was the first person to see or hear of this effect, while I had the benefit of knowing about his work. Hess recognized the physics behind the lowering dose rates that happened at first, but the rising dose rates as he ascended still further had him puzzled. What he finally realized was that he was seeing radiation coming in from outer space – cosmic rays. Then he just needed to figure out what they were.

Hess first thought that the Sun might be the source of this cosmic radiation. But when he arranged to make measurements during a solar eclipse he saw the same effect – while he still didn’t know their source, he could rule out the Sun – cosmic rays apparently originated from somewhere outside of our Solar System. Hess went on to be awarded the 1936 Nobel Prize in Physics (https://www.nobelprize.org/prizes/physics/1936/summary/) for this discovery.

(https://phys.org/newman/gfx/news/hires/2012/1_1911_1912_hess_ballon.jpg)

It turns out that I know more about cosmic rays than Hess did. That’s not because I’m smarter than he was (and goodness knows, I’m not!). Rather, it’s because I’ve been able to read over a century of research on the subject, much of it using instruments that Hess never even dreamed of. Not only that, but cosmic rays have been studied by physicists and astronomers – even by geologists – who have teased out details far beyond the science of Hess’ day.

One of the things we’ve discovered, for example, is that most of the cosmic radiation we’re exposed to originates elsewhere in our galaxy in the form of high-energy particles blasted into space by exploding stars. These are the nuclei of atoms that have had all of their electrons stripped away and that have sailed through space until they encounter our planet – they have so much energy that they punch through the Earth’s magnetic field and stream into our atmosphere. There, they’re likely to smash into an atom in the atmosphere, initiating a chain reaction of events giving rise to a cosmic ray air shower (https://scied.ucar.edu/image/cosmic-ray-air-shower) – a cascade of gamma rays and particles, some of which can create radioactive tritium (H-3) and carbon-14. Other particles can smash into airplane fuselages (where, among other things, they show up on my radiation detector) and still more filter down to sea level to give us radiation dose of between 20-30 mrem annually.

Something else we’ve learned over the decades is that cosmic radiation accounts for about 10% of our annual exposure to natural background radiation – and that this remains relatively constant throughout the solar cycle (the decadal waxing and waning of solar activity). The reason for this is that, when Solar activity is low, the relatively anemic charged particles from the Sun can’t penetrate to sea level, but the much higher-energy galactic cosmic rays can. On the other hand, when the Sun’s activity is high the solar cosmic rays are more energetic and more of them can penetrate more deeply into the atmosphere, exposing us to more radiation from the Sun; at the same time the stronger solar wind helps to sweep the galactic cosmic rays from the inner solar system, reducing our exposure to them. These two factors tend to cancel each other out with the net result that our exposure to cosmic radiation remains relatively constant throughout the Solar cycle.

We’ve also found out that cosmic rays can induce radioactivity in rocks that are exposed at the surface – cosmic ray air showers produce neutrons and when neutrons are captured by stable atoms they can become radioactive. My MS advisor made use of this fact – it turns out that you can tell how long ago a rock first became exposed to cosmic rays by studying the induced radioactivity and this can be used to determine the rate at which glaciers are retreating. By analyzing rocks from Antarctica, for example, my advisor was able to tell when rocks were last covered by glaciers; using similar analyses a friend of mine was able to tell when the glaciers melted back from his front yard in Kansas.

Another Kansan, astronomer Adrian Melott has other speculations about cosmic rays – he thinks they might be linked to the manner in which life has evolved on Earth. He notes that our Solar System bobs up and down through the galactic disk every 40-50 million years or so and postulates that when we’re on “top” of the disk (the side facing the direction of our travel through intergalactic space) we might be exposed to more cosmic radiation than when we’re on the “bottom” of the disc, and that this extra smidgeon of radiation might be enough to trigger faster rates of evolution. And there does seem to be some correlation between the fossil record and the timing of our excursions above the plane of our galaxy…so he might have a point.

But then there’s the stuff we’re still trying to figure out….

On October 15, 1991 a cosmic ray observatory (the Fly’s Eye camera – https://en.wikipedia.org/wiki/High_Resolution_Fly%27s_Eye_Cosmic_Ray_Detector) in the Utah desert recorded the debris of a cosmic ray with a staggering amount of energy – in physics terms it was about 300 billion billion (3×1020) electron volts; in more prosaic terms this was a single atom that packed the same wallop as a Little League pitcher’s fastball. Not only was this unprecedented, but it was also beyond anything science could explain.

The first thing that science was unable to explain was how an atom could have been endowed with so much energy. First, even exploding stars – supernovae – lack the power to impart so much energy to a single atom, and scientists were having problems thinking of any other phenomena that could do the trick. But then there was another problem – even if they could find a mechanism to accelerate an atom to so high an energy, it should have been whittled away while the atom was in transit across the galaxy through collisions with the rare atoms of hydrogen or even by running into photons – for the most energetic particles in the universe, anything they encounter will have a lower energy that will slow them down ever so slightly. As far as scientists could understand there was no way particles with so high an energy could exist…and yet they did.

As things stand now there are a few candidates that might be able to boost particles to so high an energy – the super-massive black holes in the cores of quasars are a candidate, as are the huge lobes of energetic gas near some galaxies that emit powerful beams of radio-frequency radiation. Another hypothesis dates back to the earliest days of the universe – speculating that cosmic-scale shock waves from the epoch of galaxy formation might have accelerated particles to these velocities. This latter is the most ancient, but there are some that are even more exotic, including the decay products from supermassive particles formed by what are called topological defects in the fabric of space itself (and no, I don’t really know exactly what they mean by that either, but it sounds pretty cool). But we still have to figure out how these particles can hold on to their energies for so long – especially when we consider that they seem to originate from outside of our own galaxy.

(a radio galaxy – the jets that form the large lobes might be one source of ultra-high energy cosmic rays https://www.nasa.gov/images/content/709514main_hs-2012-47-a-print.jpg)

Interestingly, these highest-energy particles provide a degree of comfort to the designers of our highest-energy particle accelerators. Every now and again when a new accelerator pushes to higher energy levels there’s a concern that it might produce new types of particles (https://phys.org/news/2014-02-chances-particle-collider-strangelets-earth.html) or miniature black holes or that it might even damage the structure of space – any of which could be dangerous. So physicists examine the matter – to date they’ve always decided that the new accelerator was safe to operate. Of course, they might have made a mistake, or there might be gaps in our understanding of the physics. But then they remember the ultra-high energy cosmic rays – boosted by mechanisms we still don’t understand to energies a million times greater than any accelerator we can yet make. The fact that the Earth still exists tells us that even these highest-energy particles can’t do extensive harm, which tells us that our accelerators are most likely safe to operate.

Sometimes when I’m flying and looking at my radiation instruments I think about a star that exploded somewhere in space – maybe in our galaxy, maybe in a galaxy halfway across the universe – and that spat out a flurry of atoms at incredible velocities. I think about these atoms speeding through space for eons – maybe since before our planet was even born – deflected time after time by the magnetic fields of stars, of clouds of interstellar gas, and that are woven into the fabric of our galaxy and in the spaces between the galaxies. And after so long a journey, to come to a rest in my radiation detector…just seems too mundane an end for so long and exotic a trip.

A bit over six billion years ago a star exploded somewhere in our galaxy – we can’t be sure where the star was so long ago, but we know that it was within shouting distance (on a galactic scale) of a cloud of gas and dust. A shock wave from the supernova slammed into the cloud, compressing it and setting in motion a series of events that would, millions of years later, lead to the birth of a star and the formation of a number of planets – we’re currently all sitting on the surface of one of those planets.

When stars are active they produce energy by fusion – first of hydrogen, then of helium, and working up to iron. But fusing iron doesn’t produce energy, it sucks it up – when a star starts burning iron it collapses, forming a neutron star or a black hole and the outer layers rebound out into space, blowing the rest of the star apart into space. During this explosion, every element heavier than iron is formed, including gold, lead, and uranium. This debris emerges as a shock wave that can trigger the collapse of existing clouds of gas and dust; whatever is left will eventually become part of such a cloud itself. Thus, the cloud from which the Solar System formed contained uranium, and the shock wave that caused its collapse contained still more.

Over time, as the Earth cooled it began to solidify, the first rocks forming a thin skin that floated like pond scum atop the underlying magma as the magma itself circulated, driven by convection. As the world churned the various elements began sorting themselves out with the large atoms (including uranium) partitioned into the solid rocks of the nascent crust, iron and nickel sinking to form the core, with the mantle lying in between.

Over the eons, the rocks that contained the uranium began to weather and the grains bounced their way down the stream beds, collecting in areas where the speed of the water slowed, just as grains of gold do. But the early atmosphere lacked oxygen, as did the water and since uranium is insoluble in anoxic water the grains just sat there. And then, two billion years ago, that changed – photosynthesis produced oxygen that flooded the Earth’s atmosphere and saturated the water and the uranium began to dissolve, precipitating out again in places where chemical reactions had again stripped oxygen from the water.

One such location was in a part of the planet that would one day become part of the nation of Gabon. Here, the water was percolating through sandstone and depositing nodules of uranium in the vicinity of some hydrocarbon deposits, bathed with groundwater. And this is where things start to get interesting!

Nuclear reactors consist of clumps of uranium surrounded by water – when a uranium atom fissions it emits (among other things) neutrons. The neutrons bounce off the hydrogen atoms in the water molecules and slow down, just as a cue ball slows down as it bounces off the other balls of the same size – this is important because slow neutrons are more likely to be absorbed by a uranium atom and to cause it to fission. If one of those neutrons is absorbed by another uranium atom and causes a second fission the reactor is said to be critical. In our sandstone, the uranium nodules were surrounded by water…just as in a nuclear reactor. And, just as in a nuclear reactor, the water slowed down the fission neutrons to the point where they could be absorbed by another uranium atom.

Uranium is comprised of two isotopes – one has a mass of 235 atomic mass units (AMU) and the other is slightly heavier with a mass of 238 AMU. The lighter atoms of U-235 fission fairly easily, but they account for less than 1% of uranium atoms found in nature – too few to sustain a criticality. That’s why we have to enrich uranium – to increase the amount of U-235 to somewhere around 3-6% of the uranium atoms. And this is the last piece of the puzzle.

U-235 and U-238 have astonishingly long half-lives – about 4.5 billion years for U-238 and a mere 700 million years for U-235. This means that the fissile U-235 decays more rapidly than does U-238; calculating the amount of each that was present 2 billion years ago reveals that natural uranium at that time was around 3.5%…at the lower end of the band of concentrations we have in reactor fuel today. Thus, in one place there were lumps of uranium containing enough U-235 to sustain a chain reaction that was sitting in a water-saturated sandstone formation. And about 1.8 billion years or so ago, enough uranium had precipitated that the occasional spontaneous fission sparked a chain reaction – the uranium ore deposit had become a reactor.

There was one moment in time when this could have happened. Before about 2.2 billion years ago there wasn’t enough oxygen in the environment to mobilize the uranium and after about 1.5 billion years ago there was too little U-235 to sustain a chain reaction. But for about 700 million years the Earth could make it work.

Fast-forwarding to the 1970s, French geologists located a rich uranium ore deposit in Gabon that they started to extract at the Oklo uranium mine. As the uranium was extracted and enriched the radiochemists carefully tested the uranium enrichment at various stages of the process and were surprised to see that there was less U-235 than they expected. Through some impressive investigatory work they came to realize that what they were mining was the remnants of a natural nuclear reactor – the first (and, to date, only) ever found. And in the 40-odd years since it was discovered further study has helped to tease out some of the details of its operation.

As the reactor operated the fissions heated up the water percolating through the sandstone; as it warmed up it became less dense – it might even have boiled off entirely from time to time. When this happened, the neutron moderation ground to a halt and the reactor shut down. When the rocks and the water cooled the chain reaction restarted and the reactor would fission some more. It seems to have continued in this vein for about 100,000 years. And that leads us to another intriguing detail – with some implications for our storage of nuclear waste.

The natural nuclear fission reactors of Oklo: (1) Nuclear reactor zones (2) Sandstone (3) Uranium ore layer (4) Granite

We have a very good understanding of how fission works and what’s produced when uranium atoms split. And when examined in detail, physicists realized that virtually all of the fission products formed during the millennia of operation are still present in the rock – in spite of being located in porous and fractured rocks that were saturated with water for the better part of 2 billion years. Such a location would never be approved for radioactive waste disposal today yet, in spite of the rocks’ unsuitability, the fission products are still there. With no planning, no engineering, and a lousy (by our standards) location Nature managed to store the waste safely for eons – this bodes well for our ability to store radioactive waste in a well-designed and well-constructed disposal site located in an impermeable rock formation.

And, interestingly, the conditions under which the reactor formed and the rock formation that hosted it are hardly rare…what’s rare is the preservation of such a formation for so long. Earth might have once been littered with natural reactors and we might never know. And think about it – uranium from a dying star flew through space, causing the collapse of the pre-solar nebula (itself the detritus of other dying stars) and formed the Sun and the Earth. There it sat, gradually moving into the continental crust, eroding and collecting in streams…and waiting for oxygen levels to increase to the point where it could form an ore, and waiting until there was enough uranium in a suitable rock formation to achieve criticality. We don’t know how many times it might have happened on Earth. But we do know that it happened once – and that’s pretty cool.

Why we should care about noble gases

So a friend sent me an email on Monday (June 14, 2021) that linked to a news story reporting that China’s Tianshan nuclear reactor seemed to have elevated levels of noble gases in the reactor coolant. This caused some mental alarms to sound – muted at this point, but there nevertheless. But the reason for these alarms isn’t very obvious to the vast majority of the population – and why noble gases should cause any sort of reaction calls for a little explanation. So let’s see if we can figure out where these noble gases come from and what they might portend. But to do that we’ve got to get into things like how fission works, how reactor fuel is made, and how we can use this knowledge. So…let’s get started!

Where noble gases come from

When uranium atoms fission they split into two radioactive atoms that are called fission products (also called fission fragments). These aren’t equal in size – most are clustered around masses of about 135 and 95 atomic mass units (AMU). And as it turns out, there are two noble gases (krypton and xenon) that fall into these peaks. This means that fission causes (among other things) these fission product noble gases to be produced and they accumulate in the fuel as the reactor operates. Most have relatively short half-lives, but some (such as Kr-85) stick around for a while. Luckily they’re normally trapped in the fuel matrix – we’ll talk about that next.

How reactor fuel is made

Reactor fuel is made up of uranium oxide that’s compressed into a pellet that’s an inch or so in diameter and about the same length:

These fuel pellets are loaded into fuel rods that are a few tens of feet in length, clad with an alloy of zirconium, a tough and corrosion-resistant metal. These fuel rods are then assembled into fuel bundles that are arranged in the reactor core in a pattern that will sustain a critical reaction while permitting the fuel to be cooled and that leaves room for neutron-absorbing control rods to be inserted to help control the chain reaction. As long as the cladding remains intact the fission products are contained safely in the fuel pellets. But if the cladding cracks or otherwise becomes compromised then these fission products can be released into the reactor coolant. And the cladding isn’t made of pure zirconium. The process of making it and loading it with fuel results in traces of uranium (called “tramp uranium”) to be present in the cladding.

What it all means

Radioactive krypton and xenon are not found in nature, and they’re certainly not found in the ultra-purified water circulating through the reactor plant. So if we find evidence of these in the reactor fuel it tells us that there might be a crack of some sort that’s letting the fission products into the reactor coolant.

But here’s the thing – the tramp uranium is also fissioning and since it’s in the cladding some of the fission products can shoot out of the cladding and into the coolant. This means that there are always traces of fission product noble gas in the reactor coolant. So simply finding them in the coolant doesn’t necessarily mean that there’s a problem with the fuel – before we can determine that we need to know how much noble gas is in the coolant and how the levels we’re measuring differs from what we normally see.

Something else we have to be aware of is that the amount of this noble gas in the coolant depends on the reactor power – if reactor power increases then we see more in the coolant than we do at low powers. But even more than that – since these radionuclides don’t decay immediately, they can stick around for hours after reactor power drops. So we need to calculate a “power-corrected” fission product activity to see how what’s measured compares to what we expect to see. The power-correction calculations aren’t necessarily highly complex; at the same time, it’s easy to make a mistake, especially if power is changing frequently. On the nuclear submarine I was on power changed frequently – every time we changed speed – and commercial reactor plants change power as well, responding to changing electrical demand during the day.

What’s the situation at Tianshan?

And that brings us to Tianshan. The Tianshan reactors were built by a consortium that includes the French utility EDF (Électricité de France), Framatome (a French design and construction firm), and the Chinese government. Framatome got word that the concentrations of fission product noble gases were higher than expected and they asked the US government for help in interpreting the laboratory results.

The problem is that there’s very little information that’s been released to date. The Chinese government notes that fission product noble gases are normally found in reactor coolant, which is true, and that the levels that have been found are more or less normal. The problem is that we don’t know what the normal levels are on the Tianshan reactor, we don’t know the levels that have been found, and we don’t know the power history so we can’t do the power corrections. So the information that’s available isn’t sufficient to know if the noble gasses are really elevated or if they just seem that way. And without knowing that we don’t know how significant these lab results might be.

What comes next?

This is another question that we can’t really answer at this time, but there are a few possibilities.

If there really is a defective or compromised fuel rod then we’ll likely continue to see noble gas concentrations rise for a while, after which they should stabilize. We might also start to see other radionuclides show up, especially if the crack or defect gets worse over time. And the radionuclides we see ought to follow a predictable pattern – first we can expect to see volatile elements such as iodine and cesium, which are in the fission product peak close to 135 AMU. This is what I saw when I was in the Fukushima area after the accident there – I-131, Cs-134, and Cs-137 were all present, and they were exactly what I expected to see (the device I was using wasn’t the correct instrument to measure noble gases).

At the moment we can be reasonably confident that the reactor isn’t melting down – if that were the case then we’d see a wider variety of radionuclides, including some that have higher melting and vaporization temperatures such as strontium, molybdenum, and so forth. The fact that these have not been reported suggests that the reactor is not suffering a meltdown.

I should also note that fuel element defects are not unheard-of – they don’t happen frequently, but they do happen and they’re not necessarily a disaster. In fact, they happen often enough that the International Atomic Energy Agency has even written a document titled Review of Fuel Element Failures in Water-Cooled Reactors (https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1445_web.pdf). On the other hand, if there’s a problem with a fuel element then the utility will have to do its best to limit future damage to the fuel element and might need to try to locate the exact fuel rod that’s damaged to that it can be removed and replaced. This can be a long and expensive process – if the defect isn’t too bad then the best course of action might simply be to operate the reactor carefully until the next refueling when the damaged fuel can be removed and replaced with a new one.

The bottom line

At the moment we just don’t know very much, which means that there’s far more speculation than fact. It is entirely possible that the levels of fission product noble gases that so concerned Framatome are actually no more than what’s to be expected for the Tianshan reactor. It’s also possible that there is a fuel element defect similar to what a number of other reactors have experienced. Or this could be the prelude to something more serious – there’s just no way to know. But it seems fairly reasonable to assume that, as of now, the reactor is not melting down or we’d be seeing a lot more than noble gas in the coolant.

So at the moment, I’m interested – but I’m not worried. At the same time, I’m going to keep looking for more information to see what else I can learn.

So…I’m a radiation safety professional, which means that I have a bunch of radiation detectors at my home. And every now again I turn on my meters to see what they read – sometimes I’m teaching a class via Zoom, sometimes I’m checking to make sure the instruments are working properly, I might be checking my own radioactive materials that I use for teaching, or sometimes I’m just curious. The other day I was making some measurements and I noticed they were a little higher than I’m used to seeing and it made me think about all the ways that I encounter radiation on a regular basis. And, being a writer, it occurred to me that it might be worthwhile to share with you the sorts of things I run across.

We can start with natural background radiation – every minute of every day we are all exposed to radiation from nature. Potassium, for example, is vital to the proper operation of our bodies (including our hearts and other muscles)…and about one potassium atom in 10,000 is radioactive, exposing us to radiation from within our own bodies. Not only that, but we also have small amounts of radioactive carbon and hydrogen in our bodies – these are formed by cosmic ray interactions in the upper atmosphere and they filter on down to sea level where we breathe them, drink them, and eat them. Incidentally, a colleague of mine once calculated the radiation he received from potassium in his wife’s body due to, as he put it, “spending about 25% of his time at a distance of less than 1 meter from her” (when I asked him about installing lead shielding, he pointed out that the toxicity of lead would be more dangerous than that extra radiation exposure).

The potassium in bananas, salt substitutes, and other high-potassium foods gives us a little radiation as well – as do the traces of radium found in Brazil nuts. Add to that scant amounts of uranium, thorium, radium, and a few other natural heavy elements that lodge in our bones (mostly from breathing and ingesting dust) and we get about 40 mrem every year from radioactivity that’s a part of our bodies.

There’s also cosmic radiation – some from our Sun, but most that originate in exploding stars elsewhere in our galaxy. In fact, every time I fly I can see cosmic radiation exposure increase as we climb to cruising altitude – in 2019 I flew from NYC to Seoul South Korea on a flight that took us over the North Pole and I saw cosmic radiation levels climb even higher as we flew increasingly northward.

Then there’s still more radiation from the rocks and soils as well as from things made from rocks and soil (granite countertops, bricks, concrete, and so forth) – these each account for just under 30 mrem annually. And the radon emanating from the ground exposes us to another 200 mrem a year, although this is variable, depending on the amount of uranium in the soil and the underlying bedrock. All told, we get about 300 mrem (more or less) from natural sources and from things that are built or made of natural materials. And that’s just the start!

In my apartment I’ve got a lot of radiation sources – some of these are pretty common, some are not, but I don’t need to have a radioactive materials license for any of them. There’s the granite countertop that my landlord installed, for example (granite contains potassium as well as uranium and thorium), as well as the brick my building is made of (brick is made of clays that often contain potassium). I’ve got my collection of radioactive rocks and minerals as well – I picked up most of these at rock and mineral shows or shopping online – and I also have a bunch of consumer products, most of which I bought online. Thoriated welding electrodes, “Vaseline glass” and Fiestaware plates colored with uranium, a stainless-steel soap dispenser contaminated with radioactive cobalt, and a few other things.

There used to be even more than this – I recently got a “Revigator” that’s almost 75 years old. The Revigator is a ceramic crock that’s lined with what looks like concrete…except that the concrete is impregnated with radium-bearing rocks. The premise was that people would fill this with water and, overnight, the radium would “invigorate” the water with energy that, when drunk the next day, would improve one’s health. I only made a few measurements on my new acquisition, but it looks like the most radioactive thing in my collection. Having said that, it still gives off too little radiation to pose a risk to me – especially since it sits about six feet from my desk. Another “back in the day” source of radiation were the cathode ray-type television sets and computer monitors – they never gave off enough radiation to cause problems, but they did give off radiation.

Interestingly, a few months ago I turned on one of my radiation detectors and noticed that dose rates were higher than I’m used to seeing. At the time I was borrowing a gamma spectroscopy device from a colleague – I identified the nuclide as I-131, which is commonly used for treating thyroid cancer and other thyroid diseases. My guess is that one of my neighbors was having thyroid problems – I probably could have figured out which one by checking the walls, floor, and ceiling…but decided to leave my neighbors with a bit of privacy, especially since the dose rate wasn’t at all high enough to be a concern (for me or for them). Of course, nuclear medicine is hardly rare – when I was working for the police (as a civilian scientist) I made hundreds of radiation surveys, both on the ground and from the helicopter, and we picked up nuclear medicine patients on our instruments all the time.

This is a photo of my radiation detector display when we were flying circles over the Brooklyn Bridge and the East River. The high readings showed up when we were over the bridge.

Getting back to building materials, granite’s a big one – due to the geochemistry of uranium, thorium, and potassium (and due to the way that minerals crystallize in magma chambers – https://en.wikipedia.org/wiki/Bowen%27s_reaction_series) many light-colored igneous rocks, including the gray, pink, and red granites, have more radioactivity than many other types of rock. Enough, in fact, to sometimes set off radiation alarms for ground-based surveys and to show elevated dose rates from the air. Flying over the granite Brooklyn Bridge, we always saw higher readings than when we flew over the East River; flying over cemeteries gave us higher readings due to all of the granite headstones.

I’ve also been called on to respond to other sources of radiation – loads of ceramic tiles for example that were coated with glaze that included uranium for the bright colors (mostly yellow and orange) it could produce. And then there was a time we detected radiation from industrial radiography – using radioactivity to take images of pipes, welds, structural steel, and the like – anyplace with a lot of construction and a lot of welding is likely to have radiography taking place on a regular basis. Here, too, the radiation levels aren’t nearly high enough to cause problems, providing the radiographer is doing their job properly – in the US and Europe that’s likely to be the case, but there have been radiography accidents in a number of nations over the years.

I teach a lot of classes on radiation safety – many of my students work in industries that use radioactive sources to gauge the levels of tanks or to control various manufacturing processes. If you put anything between a radiation source and a detector, the radiation levels drop – the more material, the more the levels go down. So radiation levels from a source at the top of one side of, say, a tank filled with caustic chemicals will suddenly drop when the tank fills too high, letting operators (or automatic systems) know it’s time to stop filling the tank; a source at the bottom of the tank will keep the tank from emptying out and possibly ruining a pump. Similarly, I’ve seen radioactive sources used to check the levels of beer bottles on an assembly line, to control the thickness of paper or steel, to check the density of soil, even to detect clogged conveyor belts at a gold mine in Nevada. While none of these expose me on a daily basis, they’re examples of how radiation and radioactivity are used on a daily basis in the nooks and crannies of industry.

In fact, radiation is present in all sorts of society’s nooks and crannies, whether it’s the natural radiation we’re exposed to the mountains, in our basements, at high altitudes, or in the bunch of bananas we stash in our kitchens; in our workplaces and in the workplaces of others; or in the hospitals and clinics and contained within the patients who have visited them.

What’s interesting is that people are exposed to more or less radiation depending on where they live, where they work, what they buy, what they do for a living, and so forth…but these don’t seem to affect the rates of cancer. This suggests that the radiation we run into on a daily basis isn’t likely to hurt us.