Monthly Archives: April 2021

Nuclear Medicine

This month marks the 80th anniversary of doctors injecting radioactivity into their patients – not as part of evil experiments, but to help to diagnose and treat disease. And in spite of being only 80 years old, the field of nuclear medicine has become impressively versatile and valuable over the decades, giving physicians the ability to, in effect, “see” within their patients’ bodies to learn what was ailing them. Here’s how it works.

Tracking tagged molecules

Every organ, every tissue in our bodies has a different biochemistry and uses different molecules and elements. The thyroid, for example, uses iodine to function and to produce the hormones that help to regulate our metabolism; the brain thrives on an exclusive diet of glucose; our bones incorporate calcium, phosphorus, and a handful of other elements. So if we want to learn about how the thyroid is functioning – or to learn where in the body metastatic thyroid tissue has migrated – injecting radioactive iodine will cause the thyroid tissue to become radioactive, making it easy to find. Similarly, adding radioactive atoms to glucose molecules can help us to see what’s going on in a patient’s brain while adding radioactivity to molecules that are used by growing bone can tell a physician where there is a small fracture that has begun to knit together.

Physicians have know for decades that certain organs and different tissues have different biochemical characteristics. But until the middle of the last century this knowledge, while interesting and important, was of no help whatsoever in diagnosing illness – but with the advent of nuclear technology (specifically, nuclear reactors and accelerators) this began to change. Radioactive atoms emit radiation (of course!) and that includes gamma rays that can penetrate through the overlying tissues to be detected from outside the body.

So say a patient has metastatic thyroid cancer – when I-131 is injected into the patient the iodine will seek out thyroid tissue, including any metastases where cancerous thyroid cells have spread through the body; when the patient is scanned with a gamma camera (more on this shortly) these clumps of now-radioactive thyroid tissue will show up as bright spots on the scan. Give the patient a higher dose and, instead of revealing their locations, the radioactive iodine will deliver a high enough dose of radiation to destroy the cancerous tissue.

In 1934, Frederick and Irene Joliet Curie formed radioactive P-30 by bombarding stable aluminum with alpha particles. Not long after, Enrico Fermi, speculating that neutrons’ lack of an electrical charge might make them even more effective, began slamming neutrons into as many elements as he could get his hands on to see what would happen. Four years later Fermi was awarded the Nobel Prize for this work.

The Second World War slowed non-war-related research; at the same time, it provided wonderful new tools for producing artificial radionuclides – nuclear reactors and particle accelerators – as well as new instruments with which to measure and characterize the radiation they gave off. After the war, the stage was set for putting these to use in medicine.

While there had been some early work with nuclear medicine in the 1930s, the first bona fide treatment came about in 1946 with the use of I-131 to stop the growth of a thyroid tumor and the subsequent realization that lower doses could be used for imaging rather than destroying the diseased tissue. Over the ensuing years and decades, nuclear medicine physicians learned to insert radioactive atoms into molecules that would be taken up by other organs, by cancerous tumors, by healing bone, and more – the current list of nuclear medicine procedures published by the Nuclear Medicine Technologist Certification Board lists 78 procedures using radiopharmaceuticals not even dreamed of in 1946 ( And the most recent additions – positron-emitting radionuclides (often used in conjunction with CT scanners) – make use of a particle of antimatter, most commonly encountered in science fiction.

Collecting images

It’s possible to find out where the radioactivity is collecting without creating an image at all. If I give someone I-131, for example, I can start scanning their body with my Geiger counter – as I survey I’ll notice I get a low count rate at their feet, higher when surveying their pelvis and abdomen, and highest when my detector is over their throat. This would tell me that there’s not much in the feet or legs that absorbs iodine, that it collects in the bladder and colon while waiting for the person to urinate or defecate, and that the thyroid collects iodine more effectively than any other part of the body. On the other hand, if I got a high count rate from, say, the underarm, groin, or liver then I might suspect a metastatic tumor that had spread to lymph nodes or to the liver.

The thing is, while many scientists and engineers are comfortable with numbers, they’re a little unusual in that; most people can get more information from a picture than from a table of numbers. Finding a way to produce images of where the radioactivity was collecting was a way to make this technique accessible to a much wider range of physicians

The Moritz Orthodiagraph – devised by Friedrich Moritz, of Munich (1861-1938)

The problem is that gamma rays are given off in all directions – so if I’m holding my radiation detector over, say, an I-131 patient’s nose, it’s going to show a high count rate. This isn’t because some iodine ended up in the person’s nose – it’s because the iodine in the patient’s thyroid will emit gamma radiation in all directions, including into my radiation detector. Unless I can find a way to screen out all of the gammas except for those that are directly beneath my detector.

One way to do this is to take a piece of lead and drill an array of holes through it, putting an array of radiation detectors on the other side of the lead. If a particular detector registers a “hit” then I can be sure that the gamma was emitted directly beneath that hole in the lead. Doing this with the entire array of holes and detectors gives us an image of the part of the body beneath the array; scanning the array over a part (or all) of the body can tell us where the radionuclide has collected. The former (parking the array over, say, the neck) can help us to locate small nodules within the thyroid while the latter (assembling an image of the entire body) can show us where metastatic thyroid tissue has landed.

Newer methods

One of the problems with nuclear medicine is that it shows where the radionuclide ends up, but not necessarily what structures it ends up in. For example, a “hot spot” in the abdomen might be the result of radioactivity in the liver, the gall bladder, one of the ducts between the liver, gall bladder, and digestive tract, the connective tissue in the area, or maybe even in the muscles that line the abdominal cavity. Taking “pictures” from different angles can help to narrow down the possible location – a hot spot halfway between the front and back walls of the abdomen, for example, is more likely to be in the liver than in the abdominal muscles. But even the best gamma camera still has limits to their ability to precisely resolve the location – precise information requires a higher-resolution image; we can get that sort of precise three-dimensional imaging from a CT scan. Thus, the PET-CT – a device that can be used to show not only metabolic or biochemical activity (where the radiopharmaceutical collects) but also the structures in which the hot spots lie.

Images from a Ga-68 PSMA PET-CT in a man with prostate cancer shows tumors in lymph nodes in the chest and abdomen. Credit: Adapted from Int J Mol Sci. July 2013. doi: 10.3390/ijms140713842. CC BY 3.0.

Radiation safety

Injecting a patient with radioactivity raises some safety concerns – for the patient, for the medical staff, for their family, and for others with whom they might come in contact.

Consider the I-131 patient we discussed earlier. I-131 gives off both beta and gamma radiation; the beta radiation is not much of a concern because it’s trapped within the body, but gamma rays can (and do) expose others. The key is to limit that exposure.

When a patient is in the hospital, workers can use the principles of Time, Distance, and Shielding to reduce their exposure and that of others. Reducing the amount of time anyone is near a patient reduces their exposure, as does increasing the distance to the patient. Nurses, for example, can stand at the foot of the bed instead of at the head to be further from the thyroid; if they must be at the head of the bed, they can stand back a step instead of standing at the bedside. Simply taking one step away from the patient can reduce radiation exposure by a factor of four or greater. In addition to this, many hospitals install lead shielding in the walls surrounding the rooms where their I-131 patients remain during their stay; this reduces radiation exposure to those in adjacent rooms.

Radiation safety for the patient primarily takes the form of carefully calculating the amount of I-131 that’s administered – a high enough dose to obtain a good image or to ablate the thyroid tissue, but not much more than that to avoid excessive radiation exposure to healthy tissues. In addition, nuclear medicine patients are usually given instructions on how to reduce radiation exposure to their family and friends when they return home.

These instructions usually include cautions to use a separate bathroom than other family members (if possible), not to share cups or cutlery with others, not to hold babies and young children on their lap, to sleep in a separate bed from spouse and children, and so forth – the idea is to not only reduce radiation exposure from the gamma radiation emitted from their bodies but also to reduce the chance for family members to come in contact with radioactive contamination. In cities with widely used mass transportation (or patients who rely on mass transit) there is also a common admonition to try not to sit or stand too close to other passengers if possible. These precautions might last for only a few hours (with most PET radionuclides), for a few days (in the case of Tc-99m), or for a few weeks (in the case of I-131).

One other consideration in recent years is that many cities have established networks of radiation detectors as part of their counterterrorism efforts, and nuclear medicine patients cause the majority of radiation alarms that come in. In the decade or so that I’ve worked with law enforcement on radiological interdiction, I’ve seen far more alarms from nuclear medicine patients than from all other sources combined. For this reason, many nuclear medicine physicians will give notes or cards to their patients so that, if they do trigger a radiation alarm, they can show the cards to the police.

After 80 years nuclear medicine is a mature methodology with a solid technological and scientific foundation. Over the years it’s been developed to a high level, and in recent years there have been about 15 million procedures annually – both diagnostic and therapeutic. For what it’s worth, I account for some of those procedures – I’ve had a number of scans over the last decade or so, so I can personally confirm that it doesn’t hurt, it doesn’t make one dangerously radioactive, and it doesn’t put others at risk. And the information that it provides can either be used to help guide treatment – or to put one’s mind at ease. Either way, it’s useful to both patients and physicians – probably one of the reasons it’s still in use.

The Demon Core

On May 21, 1946 physicist Louis Slotin was conducting a dangerous experiment with a plutonium sphere – designed and constructed to the exacting standards required to achieve criticality and to explode with the force of tens of thousands of tons of TNT. The experiment Slotin was conducting involved bringing the two halves of the sphere close together – close enough for neutrons in one half to start to cause plutonium atoms in the other to start fissioning, but barely far enough apart to keep those fissions from increasing uncontrollably. To do this, Slotin had placed one hemisphere on top of the other, using a screwdriver to hold the two pieces a safe distance apart. To cause the fissions to start he’d move the screwdriver slightly to lower the top half ever so slightly, noting the increase in neutron levels; moving the screwdriver in the other direction increased the separation and shut the reaction down.

And then his hand slipped.

The top hemisphere moved too close to the bottom and everyone in the room saw a blue flash and felt a pulse of heat. Realizing what had happened, Slotin quickly used the screwdriver to flip the top hemisphere away from the bottom, stopping the reaction.

Slotin died nine days later, his estimated radiation exposure was well over the 1000 rem (10 Sv) that is invariably fatal. Of the other seven people in the room at the time of the accident, the highest radiation exposure was 182 rem and nobody else exceeded a dose of 62 rem. Two later died of cancer – both of different forms of leukemia – but their doses were low enough and the cancer appeared late enough (19 and 29 years after the accident) that it’s unlikely that radiation from this accident played a role in their illness.

The image above is a re-creation of the 1946 experiment. The half-sphere is seen, but the core inside is not. The beryllium hemisphere is held up with a screwdriver. Reference: Los Alamos National Laboratory – Taken from “A Review of Criticality Accidents”, LA-13638, Figure 42, page 75, Los Alamos National Laboratory

The thing is, this same plutonium core had killed before – in August, 1945. That time its victim had been a physicist name Haroutune (Harry) Daghlian.

While Daghlian was performing a different experiment than Slotin he, too, was working by hand, manually moving pieces of beryllium around the core to change the number of neutrons that were reflected off the beryllium atoms back into the plutonium. Like Slotin, Daghlian’s hand slipped, dropping a beryllium block onto the experiment and causing the core to become critical. Also like Slotin, Daghlian was able to disassemble the critical configuration, shutting down the reaction, and receiving a fatal dose of radiation in the process. Receiving a dose of about 310 rem (3.1 Sv), Daghlian died 25 days later.

I had known about the accidents and the deaths; I learned about them both in the 1990s during a class I took on nuclear criticality safety. But it turns out that the history of this particular core – re-nicknamed the “Demon Core” in the aftermath of Slotin’s death (its original nickname had been “Rufus”) – is far more interesting than this, and extends over a longer period of time.

Weighing a mere 14 pounds and measuring 3 ½ inches in diameter, this particular bit of plutonium had actually been manufactured in 1945 and it was originally intended to be placed in the third weapon to be dropped on Japan in the closing days of the Second World War – tentatively scheduled to be used on August 19, 1945…or on the first day after August 18 with good weather over the target area. As it was being prepared for shipment, Japan surrendered and it remained in the US, at the Los Alamos Laboratory site in New Mexico – the same site at which both Daghlian and Slotin worked.

In the aftermath of Slotin’s accident, the core received its nickname. But, more importantly, the lab also implemented a number of new safety measures that were intended to reduce the risk of such an accident occurring as well as moving the scientists far enough away to keep them safe even if a criticality did occur. The criticality safety training I attended in the mid-1990s was a continuation of these changes, as was a series of criticality safety inspections and audits I conducted at a uranium enrichment facility several years later.

But the history of the Demon Core didn’t end with Slotin’s death. As the US moved into the post-war years it realized that it needed to better understand the effects of these powerful new weapons; when the Soviet Union set off its first nuclear weapon in 1949 these tests gained a new urgency. One series of tests, dubbed Operation Crossroads, was planned for 1946 and was intended to learn about the impact of nuclear weapons on naval ships – the Demon Core was to be shipped to a balmy tropical island where it would be placed into a nuclear weapon and detonated.

The second accident put an end to that plan. The reason for this is that, when plutonium and uranium fission they split roughly in half, and the two fission fragments that are formed are radioactive. Not only that, but some of these fission fragments absorb neutrons quite efficiently – in these early days of nuclear weapons design and fission research, nobody was quite sure if these fission products and “poisons” would affect the test explosions. Why, the thinking went, use an uncertain weapon core when they could use a new one with no such issues instead? So instead of a working vacation on a tropical island, the Demon Core remained locked up in New Mexico.

Unable to be used in Operation Crossroads, the Demon Core (or, if you prefer, Rufus) was eventually melted down and mixed in with other plutonium, eventually ending up in a number of nuclear weapons cores in the nation’s nascent, but growing, nuclear stockpile.


Nuclear Death in the Desert: the SL-1 Accident

On my very last day at Naval Nuclear Power School, in addition to doing a lot of paperwork and receiving our orders and still more paperwork to bring to our next duty stations, we were sent to the only room at Nuke School that had room for our entire class. There, we heard about the loss of the USS Thresher (SSN 593) and what happened to cause it to sink. And then we were shown a movie about a nuclear reactor accident in the desert of southern Idaho. The discussion of the Thresher got my attention – I had already volunteered for submarine duty (and, as it turned out, I ended up on one of Thresher’s sister ships – but that’s a story for another time) – but there was something about the reactor accident that riveted my attention, including the fact that, since all three operators died, nobody was quite able to figure out exactly why the accident happened. Even today, in spite of the engineering investigations, scientific publications, and popular accounts, we’re still not quite sure about what, exactly, caused the accident – the reason is that it was due to the actions of a single person, and that person is dead and can’t tell us why he took the actions he took. It was this human factor that caught my attention.

But I’m getting ahead of myself – let me go back to the start.

In the 1950s it was clear that the Navy’s nuclear power program was going to be a success and other branches of the military became interested in what nuclear energy might do for them. The Air Force began experimenting with, among other things, nuclear-powered aircraft and the Army looked into developing small land-based nuclear power plants that could be easily transported by truck or airplane and used to power remote bases – the SL-1. As an aside, there were some non-military programs aimed at developing spaceships powered by nuclear reactors or driven by nuclear explosions – which sounds like a good topic for a future blog posting!

The SL-1 reactor was the Army’s first design of a “stationary, low-power” nuclear reactor. The prototype of this reactor was built at a government facility in the desert of southern Idaho, not far from the Navy’s prototype submarine and carrier reactors, the Air Force’s nuclear airplane project, and a few other military or military-adjacent nuclear projects.

US Atomic Energy Commission image of SL-1 reactor core being removed from National Reactor Testing Station facility in Idaho

Along with its other specifications, the Army wanted a reactor that could be operated by a small number of people – the design that was constructed in Idaho was fueled with uranium enriched to the same level as the uranium used in nuclear weapons, arranged in an array that included nine control rods. In a nuclear reactor, neutrons caused by one fission fly out of the fuel rod and into the water in which the core is immersed; while in the water the neutrons lose energy and slow down, becoming more effective at causing fission. Neutron-absorbing control rods can be inserted into the reactor core to soak up these neutrons, shutting down the reactor; pulling the rods from the core will allow the reactor to start up. Normally a reactor’s control rods are moved by a motor, hydraulics, or some other sort of mechanism – in the case of the SL-1 the reactor sat in a pool on the main floor of the containment structure, beneath some radiation shielding. The operators could actually walk out on top of the reactor when the reactor was shut down and radiation levels were low.

So…with that as a prelude, here’s what we know about the events of January 3, 1961.

Following a holiday shut-down and maintenance period the three operators, Army Specialists John A. Byrnes (age 22) and Richard Leroy McKinley (age 27), and Navy Seabee Construction Electrician First Class (CE1) Richard C. Legg (age 26) were starting the reactor. At 9:01 PM Byrnes abruptly pulled one of the control rods, causing a sudden and dramatic increase in power. In the next few seconds, much of the fuel either melted or was vaporized, the sudden input of thermal energy caused some of the water in the core to flash to steam and the steam explosion shot jets of water out of the reactor core and caused the entire reactor to jump upwards by about nine or ten feet and spraying the inside of the containment structure with radioactive water. The pressure also shot the #7 shield plug out of the top of the reactor vessel; standing above it was Richard Legg, who was impaled by the plug and pinned to the ceiling of the containment structure.

The other two operators were apparently standing nearby at the time of the accident and were also killed by the accident – it’s possible that they were killed by the shock when the reactor jumped upwards so violently and the impact of the operators with the reactor. Byrne and Legg appear to have died instantly, McKinley seems to have survived for an hour or two before succumbing to his injuries. Although all three men died of physical trauma from the explosion, they received a dose of radiation that would have been fatal even in the absence of their injuries.

The aftermath of the accident unfolded over the next several years and included an extensive investigation as well as decontamination and demolition of the reactor plant along with radiation surveys and decontamination of areas surrounding the reactor site. But – and this is the part that fascinated me – there is no definitive way to know why Byrnes was standing over the reactor, manually pulling the control rod.

The story I heard was anecdotal; one of my classmates said he’d heard that Byrnes had learned that his wife was having an affair with one of the other two operators – that he had deliberately caused the explosion to either commit suicide, to try to kill his wife’s lover, or both. That was the conclusion of one popular account of the accident as well. The problem is that there’s no way to prove – or to disprove – this hypothesis. Not only that, but there are plausible explanations other than murder and/or suicide.

As one example, the control rods sometimes stuck in their channels; it’s possible that Byrnes was trying to free a stuck rod and, when it finally came free, he was pulling so strongly that the rod traveled far past the point at which the reactor became critical, causing it into a dangerous condition known as “prompt criticality” in which power increases uncontrollably.

The rod might have been pulled inadvertently, perhaps by an operator expecting some resistance and finding none.

Or the operators, noticing the rod having a tendency to stick when moved, might have been trying to move it up and down by hand until it was able to move smoothly, accidentally jerking it upwards.

Unfortunately, the operators failed to make any notes in their operating logs that they were having problems with any of the control rods or that they would be working on them.

Thus, my fascination with SL-1. First, at the very start of my stint in the Navy’s nuclear power program, it was sobering to hear of an accident that had cost the lives of three military operators; just as hearing the technical details was fascinating. Hearing how the accident was re-created and what the investigators were able to learn was fascinating, sparking that part of my mind that had so enjoyed reading Sherlock Holmes stories when I was younger. And adding in the fact that we might never know exactly why it happened – plus a little bit of spice with the suggestion that it might have been the result of a love triangle…that made it irresistible.