Monthly Archives: July 2015

Plutonium – The Power Behind New Horizon’s Trip to Pluto

Dear Dr. Z – I heard something about plutonium powering the New Horizons spaceship that just visited Pluto. And come to think of it, I’ve heard that other spaceships are powered by plutonium. Do we have nuclear reactors up there? How do they use plutonium to power spaceships? And is there any risk to us if one of these blows up (or reenters) during launch?

Color image of Pluto, photographed by the New Horizons spacecraft on 13 July 2015

Color image of Pluto, photographed by the New Horizons spacecraft on 13 July 2015

I’ve been following the New Horizons mission as well – I remember reading about the discovery of Pluto when I was just a kid (I even sent a letter to Clyde Tombaugh at one point, trying to hook him up with my grandmother). I’ve also followed many of the other space missions, including reading up on their power supplies. First, I can tell you that there are no nuclear reactors on any US spaceships today – we designed a few nuclear-powered spacecraft in the early years of the space program, and NASA was looking into them more recently for prolonged missions to Jupiter and Saturn. But at the moment the US has no nuclear reactors in space. But let’s back up a little bit and talk about where our spaceships get their power from.

One of the most notable aspects of many of our spacecraft is their solar panels – for anything operating on this side of the asteroid belt there’s enough solar energy to power a spacecraft and it will be there as long as the Sun is still shining. But when we go past the asteroid belt into the outer solar system sunlight falls off quite a bit – solar intensity drops by a factor of 4 if you double your distance to the Sun – so solar power just won’t hack it at Jupiter, Saturn, and beyond. We need something different. That “something different” comes from radioactivity.

When any radioactive atom decays away it carries with it some energy – in the case of plutonium (the isotope Pu-238 to be precise) each decay contains about 5.5 million electron volts (MeV) of energy. This is not a huge amount of energy in and of itself, but if you get enough plutonium atoms decaying at the same time it can really add up. If you have one curie of Pu-238 you have 37 billion atoms decaying every second – that comes out to about 200 billion MeV per second, or about 0.00003 BTUs per second. This isn’t a whole lot of energy, but Pu-238 packs a lot of activity into a gram – one gram of Pu-238 puts out about a half watt of power, one kilogram will produce 500 watts – enough to do something with.

A glowing cylinder of Pu-238

A glowing cylinder of Pu-238

Pu-238, in fact, puts out so much energy from its radioactive decay that a large enough chunk (about a kilogram) will actually glow red from the internal heat. Spacecraft use thermocouples to turn this heat into electrical power in what’s called an RTG – a radioisotopic thermal generator.

Diagram of an RTG

Diagram of an RTG

Plutonium-238 is not the only isotope that’s been used to make RTGs – the Soviet Union used to use huge quantities of Sr-90 (strontium-90) for this purpose. Polonium has been used as well, although with a short half-life Po-210 RTGs don’t last very long. Plutonium and strontium are ideal because they have high-energy radioactive decays coupled with relatively long half-lives – at 84 years, Pu-238 can power a spacecraft for decades. In fact, the doughty little Pioneer craft beamed back data for 30 years before running out of energy and the Voyager probes are still operational after nearly 40 years.

New Horizon's RTG

This is the plutonium-powered radioisotope thermoelectric generator (RTG) for the New Horizons spacecraft, which is also seen in the background. Source: NASA

As far as blowing up goes, that can’t happen with Pu-238. There are a number of isotopes of plutonium and only one is used in nuclear weapons – Pu-239. The isotope used in RTGs simply can’t explode (it really doesn’t fission well at all) so it cannot explode as a nuclear weapon.

Lastly, you asked about the risk from launching all of this plutonium into orbit – or, rather, the risk should there be a problem with the launch. Sadly, rockets have blown up on the Launchpad and in the air; others have reentered the atmosphere and burned up unexpectedly. Plutonium is a toxic heavy metal (although, contrary to popular opinion, it is NOT the most toxic substance known to science) – do we have to worry about an accident poisoning people on Earth?

We actually have a few test cases for this and the answer is a resounding no. First, the RTGs are swaddled in protective layers designed to keep them safe from just such an explosion – these are tested extensively before a device is space-rated, and nothing can be loaded onto a rocket unless it has been so tested. As far as reentry goes, there have actually been a few Pu-powered RTGs that have experienced this tribulation with interesting results. One (from a failed 1964 American satellite launch) burned up in the atmosphere, distributing its plutonium into the stratosphere – while this is not ideal, the plutonium was eventually spread globally and no single location was contaminated to the point of causing a health risk. Other RTGs have survived reentry – an RTG in the Apollo 13 lunar module and one on an aborted Russian mission to Mars – without apparently losing any of their plutonium. The bottom line is that RTGs are designed to be safe, even in the event of a launch failure – and this design has proven to be safe in a handful of instances.

One other comment about RTGs – in this case, terrestrial ones. During the Soviet Union’s existence a large number of very high-activity RTGs were built using Sr-90; these were used to power meteorological stations as well as lighthouses along the Arctic coast. While all of these seem to have been accounted for now, this was not always the case. In 2001 some woodcutters in the nation of Georgia found an abandoned RTG in the Georgian mountains. Noticing that snow near the RTG was melting the men carried it into their camp for heating. During the course of the night they received serious radiation exposure leading to radiation sickness and skin burns. All survived, but they were seriously ill for quite some time.

Oklo Natural Nuclear Reactor

Dear Dr. Zoomie – I saw something the other day that scientists found a natural nuclear reactor that’s over a billion years old. Is this for real? Or was this left behind by aliens?

Never fear – there was an actual natural nuclear reactor found on Earth and aliens had nothing to do with it! It was in a place called Oklo, in what is now the nation of Gabon in western Africa. And what happened is pretty cool.

There’s not enough room here for all the details; if you’re interested in some of the nitty-gritty you can check out an on-line article written by a scientist who is conversant with both geology and nuclear reactors. But here’s the big picture – let’s start with the geochemistry of uranium.

Uranium dissolves easily into oxygen-rich water but not at all into water that lacks oxygen. Until about two billion years ago the Earth’s atmosphere was oxygen-deprived which meant that surface water couldn’t dissolve uranium. Ancient algae was producing oxygen, but all the oxygen that was produced was immediately sucked up by iron and other metals in the Earth’s crust – in effect, the Earth was rusting. About two billion years ago the iron was all oxidized and oxygen began to accumulate in the atmosphere – as soon as it did so it also started to accumulate in rainwater (and streams, lakes, and rivers) and when that rain fell on granite rocks (granite tends to have elevated levels of uranium compared to other rocks) it began to dissolve the uranium out of the water.

As the uranium-rich water flowed along it sometimes came to areas where, due to decaying organic matter (more of that algae) the water was oxygen-deprived; when that happened the uranium came out of solution. And in one place in particular, apparently a fluke, enough uranium collected in one place in a configuration that resembled that of a nuclear reactor – a number of lumps of uranium dispersed in a sandstone formation. And when that area became flooded with water (water slows down neutrons, which makes them more efficient at causing fissions) these lumps of uranium began to fission. As they did so they produced heat, neutrons, and fission products (when a uranium atom fissions it produces two radioactive atoms). Of course, there’s more to making a reactor than mobilizing uranium and precipitating it out of solution in lumps. The uranium also has to be enriched so that it will sustain a nuclear chain reaction – that can’t happen today because there simply isn’t enough of the fissionable U-235 in uranium to sustain a chain reaction. At least, not today. But in the past things were different.

Oklo Reactor Zones

Oklo Reactor Zones

Uranium today has a very specific composition – if you take a uranium sample from anywhere on Earth and count the atoms you’ll find that 99.2% of the uranium atoms have a weight of 238 atomic mass units (it’s abbreviated U-238), and U-238 doesn’t fission very well. Virtually all of the rest (0.72%) of the uranium atoms are a little lighter with a mass of 235 – U-235 fissions quite nicely, but uranium that only has seven tenths of one percent of U-235 will not sustain the chain reaction necessary to keep a reactor operating. This is why we need to enrich uranium; so that there’s enough U-235 to achieve and maintain criticality. And, incidentally, in a nuclear reactor “critical” simply means that the reactor is operating at a constant power – all reactors are critical when they are operating. Anyhow – natural uranium today can’t sustain a chain reaction unless we use something like heavy water or graphite to help coax things along. But two billion years ago, things were different.

U-238 has an incredibly long half-life – it takes almost 4.5 billion years for half of it to decay away, which means that over the entire life of our planet only half of the U-238 it formed with is left. On the other hand, U-235 has a half-life of “only” about 700 million years – there was around 75 times as much U-235 on Earth when it first formed compared to today. And two billion years ago – at the time that the Oklo reactor formed – U-235 comprised about 5% or so of natural uranium; this is about the same amount that’s found in the fuel for commercial nuclear reactors today.

Oklo could not have happened at many times in history. Earlier in Earth’s history there was enough U-235 but not enough oxygen to mobilize the uranium; later in Earth’s history there was plenty of oxygen but not enough U-235 to sustain a chain reaction. But for one brief moment – actually for about a half-billion years – conditions were about perfect for uranium to dissolve, move, and concentrate in a manner that would sustain a criticality. During that time, a body of water-saturated sandstone with a number of uranium deposits achieved criticality.

The reactor operated only sporadically – it depended on water to sustain the chain reaction and, as the reaction progressed, the water would boil away, shutting down the reactor. So the reactor would operate and then shut down; while shut down it would cool off until water could re-saturate the sandstone and the reaction would start up again. Each time this happened a little more U-235 would fission and the concentration dropped slightly – eventually there was too little left to sustain a chain reaction and the reactor shut down for good. All in all scientists estimate that the Oklo reactor operated for about 100,000 years.

Now let’s fast-forward a few billion years to the mid-1970s. French geologists located a rich body of uranium ore and they started evaluating it as a source of reactor fuel. But when they started feeding the uranium into their uranium enrichment system they found they weren’t getting the amount of enrichment they expected – some good scientific detective work showed them that the ore itself was deficient in U-235, something that had never been seen before. Eventually they accepted that this uranium ore body was different than any other on Earth. Then they saw evidence of fission products in the ore. It seemed reasonable to assume that the presence of fission products was linked to the lack of U-235 – when they looked at the geology they had to accept the conclusion that they had found a fossil nuclear reactor – nature (not aliens!) had preempted Enrico Fermi (the Italian Nobel laureate who built the first artificial nuclear reactor) by a couple of billion years.

Why Are There So Many Radioactive Sources Being Stolen in Mexico? What is the Risk?

Dear Dr. Zoomie – what is it about Mexico and all the stolen radioactive sources? Why is it happening there? And what sort of risk do these thefts pose?

Good questions! It sounds like you’ve heard about news reports concerning another stolen radioactive source in Mexico.

Let’s take them one at a time, along with a little background information.

We’ve talked about the security of radioactive materials in the past in this blog – there’s a certain level of security that the International Atomic Energy Agency (IAEA) recommends and that the Nuclear Regulatory Commission (NRC) requires. For the most dangerous sources, these security requirements are most stringent – if you have enough radioactivity in one place you’ll have to perform background checks (including fingerprinting) of anyone who will have unescorted access to these sources.

There are also transportation requirements – not only security, but also to move the materials safely. For example, high-activity sources are required to be secured in very strong containers (called Type B containers) that are marked with the radiation symbol and that are secured in the beds of trucks that must carry the radiation symbol. There are additional requirements, but these are the most relevant to your questions. So now let’s see what it is about Mexico that makes it the recent poster child for radioactive materials insecurity.

The biggest thing is that effective radiation safety regulation requires a strong and effective central government and a generally law-abiding society – Mexico has neither of these things at present. Organized crime – particularly the drug cartels – consumes so many of the government’s resources that there is little left to enforce compliance with radioactive materials regulations. Because of this, there is incentive for licensees to follow the rules – it’s easy to cut corners, reducing security for example, or neglecting to put the radiation symbol on vehicles or containers. This, in turn, means that thieves are unlikely to understand that the vehicle they are stealing is carrying radioactive materials. In addition, the general destabilization of the government and the general level of violence in society makes crimes (not just murder, but theft and hijacking as well) more common. So this answers the “why Mexico” part of your question.

Now we get to the risks, and let me look at both the risks to individual people as well as the risk to our society from these losses.

The risks to individuals from these sources can be substantial. The sources that were stolen in late 2013 contained over 2500 curies of cobalt-60 – this amount of radiation can give a person a fatal dose of radiation in just a few minutes at arms’ length from the sources, far lower-activity sources have caused deaths when the sources were found by unsuspecting members of the public. In fact, even sources with as little as 5 curies have given a fatal dose of radiation to people – a 5-Ci source of Co-60 can give a person a fatal dose of radiation in about two weeks or less, depending on the amount of exposure each day and the location of the source relative to the people exposed. Even the source stolen last week (some reports say it contains 120 Ci of activity) is a potentially dangerous source. Any individual would finds any such source needs to back away to a distance of at least 100 feet, contact the authorities (police, fire, or radiation regulators), and keep an eye on the source until help arrives. As long as you keep your distance – and NEVER try to recover or to shield a source yourself – you will be safe.

A somewhat larger question is the risk to our society, and the answer here is that we just don’t know. The three thefts in the last 15 months are troubling, but they seem to be accidental; the thieves seem to have stolen vehicles that just happened to hold radioactive materials – as opposed to stealing them because of the radioactive materials. This tells us that the thieves were most likely not terrorists attempting to construct dirty bombs, which is good. On the other hand, these thefts have given ample evidence that radioactive materials are poorly secured in Mexico – this might encourage the deliberate theft of radioactive materials from Mexico by groups who wish to cause us harm. So here we can only say that, to date, these thefts have been accidental and don’t seem to pose a risk to the US, but this might not always be the case.

Helen Caldicott – Anti-Nuclear Alarmist – Keeps Spreading False Information

Dear Dr. Zoomie – I was browsing the web the other day and came across an editorial by Dr. Helen Caldicott where she said that radiation from Fukushima is a huge risk. Is she right? Do I need to be worried?

Helen Caldicott is a pediatrician and anti-nuclear activist who used the nuclear reactor accident in Fukushima as an opportunity to express her concerns about nuclear energy – a calling she has followed since the Three Mile Island reactor accident. Unfortunately, Dr. Caldicott included a number of errors in her editorial that are sufficiently serious as to invalidate her conclusions. I’d like to take an opportunity to take a look at these mistakes and to explain the science behind them.

In the first paragraph of her article, Caldicott states that “the mass of scientific and medical literature…amply demonstrates that ionizing radiation is a potent carcinogen and that no dose is low enough not to induce cancer.”

To the contrary, even the most conservative hypothesis (linear no-threshold) holds that low doses of radiation pose very little threat of cancer. Using a slope factor of 5% added risk of cancer fatality per 1 Sv (100 rem) of exposure, the risk of developing cancer from 1 rem of radiation is about 0.05% (5 chances in 10,000). This risk is far lower than the risk of developing cancer as a habitual smoker, from working with a number of solvents (e.g. benzene), working with a number of laboratory chemicals, and so forth. Epidemiologists have noted no increase in cancer rates among people living in areas with high levels of natural background radiation, as well as among the lowest-dose groups of atomic bomb survivors (in fact, people living in the states with the highest levels of natural radiation have lower cancer rates than do those who live in the lowest-dose rate states). Not only that, but age-adjusted cancer rates have dropped steadily (with the exception of smoking-related cancers) over the last century, in spite of dramatic increases in medical radiation exposure. In the words of respected radiation biologist Antone Brooks, these observations show us that “if (low levels of) radiation cause cancer it’s not a heavy hitter.” The bottom line is that, if even the lowest doses of radiation can cause cancer (which has not yet been shown to be either correct or incorrect), radiation is a weak carcinogen – not the “potent carcinogen” that Caldicott would have us believe.

In the second paragraph of her article, Caldicott states that “Large areas of the world are becoming contaminated by long-lived nuclear elements secondary to catastrophic meltdowns: 40% of Europe from Chernobyl, and much of Japan.”

This is a difficult statement to parse because it is such a nebulous statement. If, by “contaminated,” Caldicott means that radionuclides are present that would not otherwise be there, she is wrong – in fact, you can find traces of artificial radionuclides across virtually every square mile of Europe, Asia, and North America. But all that this means is that we can detect trace levels of these nuclides in the soil – doing the same we can also find traces from the atmospheric nuclear weapons testing in the 1940s through the 1960s. And for that matter, we can find lead contamination over virtually the entire world as well from the days of leaded gasoline. But lead contamination goes much deeper as well – scientists found traces of lead in Greenland glaciers that date back to the Roman Empire. But nobody is getting lead poisoning from the Ancient Romans’ pollution, just as nobody is getting radiation sickness (or cancer) from the traces of Cs-137 and Sr-90 that can be found across the Northern Hemisphere. But Caldicott can’t really comment on the fact that artificial nuclides have contaminated the world for nearly 70 years because this would shatter her claim that radioactive contamination is causing death and destruction in Europe and Japan.

In the third paragraph, Caldicott states that “A New York Academy of Science report from 2009 titled ‘Chernobyl’ estimates that nearly a million have already died from this catastrophe. In Japan, 10 million people reside in highly contaminated locations.”

Caldicott is incorrect…again.

The New York Academy of Science “report” wasn’t actually a report, but a translation of Russian papers published on their website. After Caldicott’s letter was published the New York Academy of Science later updated the webpage referencing the Russian papers with the following text:

“In no sense did Annals of the New York Academy of Sciences or the New York Academy of Sciences commission this work; nor by its publication does the Academy validate the claims made in the original Slavic language publications cited in the translated papers. Importantly, the translated volume has not been formally peer‐reviewed by the New York Academy of Sciences or by anyone else….”

Furthermore, the World Health Organization has concluded that in the first 20 years, fewer than 100 people could be shown to have died from radiation sickness and radiation-induced cancers and they further concluded that, even using the worst-case LNT model, fewer than 10,000 would eventually succumb from radiation-induced cancer as a result of this accident. This is not a trivial number – but it is less than 1% of the one million deaths the NYAS claims. And in fact the actual number is likely to be far lower, as physician Michael Repacholi noted in an interview with the BBC. In fact, even the WHO’s International Agency for Research on Cancer acknowledges that “Tobacco smoking will cause several thousand times more cancer in the same population.” Even if contamination from Chernobyl and Fukushima are sufficient to cause eventual health problems, we can do far more good to the public by devoting attention to smoking cessation (or, for that matter, to childhood vaccinations) than by spending hundreds of billions of dollars cleaning up contamination that doesn’t seem to be causing any harm.

In the fourth paragraph of her piece, Caldicott notes that “Children are 10 to 20 times more radiosensitive than adults, and fetuses thousands of times more so; women are more sensitive than men.”

To the contrary – the National Academies of Science published a sweeping 2006 report that summarizes the state of the world’s knowledge on the “Health Risks from Exposure to Low Levels of Ionizing Radiation” in which they conclude that children are between 2-3 times as sensitive to radiation as are adults – more sensitive as adults, but a far cry from Caldicott’s claim.

The reproductive effects of radiation are also well-known – fetal radiation exposures of less than 5 rem are incapable of causing birth defects according to our best science, and the Centers for Disease Control flatly states that exposure to even higher radiation doses is not a cause for alarm under most circumstances. This conclusion, by the way, is based on studies of hundreds of thousands of women who were exposed to radiation from medical procedures as well as during the atomic bombings in Japan – it is based on a tremendous amount of hard evidence.

This claim of Caldicott’s, by the way, is particularly egregious and has the potential to do vast harm if it’s taken seriously. Consider – in the aftermath of the Chernobyl accident it is estimated that over 100,000 women had abortions unnecessarily because they received poor medical advice from physicians who, like Caldicott, simply didn’t understand the science behind fetal radiation exposure. There are estimates that as many as a quarter million such abortions took place in the Soviet Union, although these numbers can’t be confirmed.

But even in this country we see this level of misinformation causing problems today – during my stint as a radiation safety officer I was asked to calculate nearly 100 fetal radiation dose estimates – primarily in pregnant women who received x-rays following serious traffic accidents – and many of the women were seriously considering therapeutic abortions on the advice of their physicians. When I performed the dose calculations there was not a single woman whose baby received enough radiation to cause problems. And it doesn’t stop there – we also had parents who refused CT scans for their children, preferring exploratory surgery and its attendant risks to the perceived risks from x-ray procedures. The bottom line is that this sort of thinking – that children and developing babies are exquisitely sensitive to radiation – can cause needless abortions and places children at risk; by espousing these views, Caldicott is transgressing the Hippocratic oath she took to “first do no harm” and she should be taken to task for doing so.

Finally, in the last paragraph of her tirade, Caldicott claims that “Radiation of the reproductive organs induces genetic mutations in the sperm and eggs, increasing the incidence of genetic diseases like diabetes, cystic fibrosis, hemochromatosis, and thousands of others over future generations. Recessive mutations take up to 20 generations to be expressed.”

All that I can say to this is that Caldicott decided to go out with a bang. The fact is that there is not a single case in the medical or scientific literature in which birth defects or genetic disease is linked to pre-conception radiation exposure. This is not my conclusion – it’s the conclusion of Dr. Robert Brent, who knows more about this topic than anyone else in the world. Eggs and sperm might be damaged, but Dr. Brent notes that there is a “biological filter” that prevents cells that are damaged from going on to form a baby. Another line of reasoning supports Brent’s claim – areas with high levels of natural radiation also have no increase in birth defects compared to areas with lower levels of natural radiation. Caldicott’s claim that low levels of radiation exposure cause long-term genetic damage are simply not supported by the scientific or medical literature or by any observations that have been made.

Caldicott’s claim that radiation is also responsible for a host of genetic diseases is similarly dubious. The world’s premier radiation science organizations (the International Council on Radiation Protection, the United Nations Committee on the Effects of Atomic Radiation, and the National Council on Radiation Protection and Measurements) all agree that, if radiation contributes to multi-factorial disease then the effect is very weak indeed – possibly too weak to be distinguished from natural sources of these diseases. Specifically, UNSCEAR calculated that – if pre-conception radiation exposure can cause these problems – exposing the population of each generation to 1 rem of radiation each might lead to an additional 100 cases of dominant genetic disease per million births per generation and 15 cases of recessive genetic disease (ICRP calculated similar, but lower rates). This is far lower than the background incidence of genetic disease in the population as a whole. Oh – UNSCEAR also determined that “multifactorial diseases are predicted to be far less responsive to induced mutations than Mendelian disease, so the expected increase in disease frequencies are very small” – a statement with which the ICRP is in agreement. In other words, Caldicott’s claim runs contrary to the best work of the most-respected scientific organizations that specialize in radiation health effects.

With respect to the length of time required for genetic effects – if any – to manifest themselves, I honestly don’t know where Caldicott pulled the number of 20 generations. This is a number I haven’t seen anywhere in the scientific literature, nowhere in any of the genetics classes I took in grad school, and nothing I ever calculated or saw calculated. As near as I can tell, she is either repeating something she heard somewhere or she made the number up to impress the reader.


The bottom line is that there is not a single statement in Caldicott’s editorial that seems to be based in scientific or medical fact. The Fukushima accident was bad, but it pales in comparison to the natural disaster that set it off. The aftereffects of the accident are bad enough – thousands of families displaced, hundreds of thousands of Japanese who were evacuated from their homes, along with the stress, anxiety, and depression they have been suffering. TEPCO and the Japanese government will have to spend billions of dollars tearing down the plant and billions more cleaning up the contaminated area – in many cases, cleaning up places not because they pose a genuine risk to life and health but because contamination levels exceed an arbitrary level. Things are bad enough, and Caldicott is making claims that have no connection to scientific or medical reality, simply in order to score a few cheap points to advance her anti-nuclear agenda. Her article does nothing to advance the debate – it only serves to use the tragedy in Japan to inflame the public’s fears.