Although metal alloys is not among the most exciting topics for most people, the moment you add the word ‘radioactive’, it does tend to get their attention. So too with the once fairly common Mag-Thor alloys that combine magnesium with thorium, along with other elements, including zinc and aluminium. Its primary use is in aerospace engineering, as these alloys provide useful properties such as heat resistance, high strength and creep resistance that are very welcome in e.g. jet engines.
Most commonly found in the thorium-232 isotope form, there are no stable forms of this element. That said, Th-232 has a half-life of about 14 billion years, making it only very weakly radioactive. Like uranium-238 and uranium-235 it has the unique property of not having stable isotopes and yet still being abundantly around since the formation of the Earth. Thorium is about three times as abundant as uranium and thus rather hard to avoid contact with.
This raises the question of whether thorium alloys are such a big deal, and whether they justify removing something like historical artefacts from museums due to radiation risks, as has happened on a few occasions.
Elemental Facts
Since the (probably machine-generated) article that inspired these questions didn’t bother to include any useful details or references, it’s time to do a bit of a dive ourselves. This starts with the element thorium and its isotopes.
Obviously the problem with thorium here is not so much with the metal itself or its elementary properties, but rather the fact that a small fraction will decay into radium-228 via alpha decay. This has a half-life measured in years before rapidly passing through actinium-228 to become thorium-228, with a half-life of 1.9 years.
The subsequent decay chain is pretty rapid, taking it through very short-lived isotopes of radon-220, polonium-216 and so on until it becomes stable lead-208. Virtually all of this occurs via alpha decay. Of note is again that the initial isotope here – Th-232 – has a half-life of 14 billion years, or roughly the estimated age of the Universe. This makes it by far the most stable unstable isotope, with U-238 having a half-life of only about 4.463 billion years. Effectively, for most intents and purposes it might as well be a stable isotope.
Thorium is found in most rocks and soil, at around 6 ppm, with several minerals like thorite and monazite containing significantly higher levels.
This raises the question of how dangerous Th-232 truly is, such as when you start concentrating it in some fashion. How much radiation exposure do you experience once you take e.g. thorium ore and wear it around, or concentrate it into pure Th-232 and combine it with magnesium into a metal alloy that people regularly spend time around?
Negative Vibes
One persistent fad in the ‘alternative health’ community is that of negative ions and kin, with many shops selling items like bracelets and similar body-worn items that are supposed to generate these chi-balancing vibes via special ions. Interestingly, some of these are sold with thorium or uranium isotopes embedded in them.
Since these items are worn directly against the skin for extended periods of time, they form an excellent test case of the potential harm of such direct exposure to a significant amount of these isotopes.
According to the fact sheet on on the NRC website, as performed by Oak Ridge National Laboratory (ORNL), these items contain sometimes quite significant quantities of the radioactive material that range from Th-232 to U-238 and even Ra-226, some at more than 0.05% by weight to the point where they would have required a radioactive material license. The estimated local skin equivalent radiation dose was said to be more than the IAEA limit of 50 mSv annually. Despite this, these items require no special disposal methods and you are free to keep using them, albeit with some precautions.
Another study showed an annual exposure of 1.22 mSv, which with the assumed validity of the linear no-threshold (LNT) model would lead one to expect to see some kind of negative health effects. So far these have remained absent despite the popularity of these bracelets and the close contact.
TIG Welding
Outside of accidental exposure in the case of weird bracelets, there is a common use case for thorium, with thoriated tungsten welding electrodes. These are used with DC TIG welding, and contain around 1% (yellow band) to around 2% (red band) of thorium oxide (ThO2). Although an alternative exists with cerium oxide (CeO2) in ceriated tungsten electrodes, thoriated tungsten remains popular due to the long lifespan and good performance with common applications.
Although it’s noted that thoriated tungsten electrodes are radioactive due to the small percentage of thorium within the ThO2, it is such a small amount that no special precautions seem to be warranted. Much like with the thorium oxide found in the aforementioned bracelets and kin, you’ll probably be fine if you don’t try eating it.
Since thorium is also not a heavy metal, unlike uranium, it is in that regard significantly safer, as is its oxide form which does not have the pyrophoric proclivity of the metal form.
Alloys
This brings us back to the thorium-metal alloys which started the whole journey. A number of missiles and jet engines have used or currently use Mag-Thor alloys, which has led to for example the Dutch and German defense ministries investigating the radiation exposure from the J-79 jet engines, as found in F-104G Starfighter and F-4 Phantom aircraft.
The reason for this investigation was, as stated, the expected radiation dose when these engines and their respective aircraft are being worked on, handled for disposal, or displayed in a museum or collection. Here we also see the amount of thorium added to the used alloy, at up to 4% by weight, with an average of 1.7%. This means that the overwhelming majority of metal in these alloys is magnesium.
Part of the study was the measured dose at various distances from the components examined, along with a potential cumulative dose. Even in the most conservative scenario the dose came to about 1.2 µSv/hour, or less than 1 mSv/year, since it was probably assumed that people generally do not live 24/7 around these objects.
Realistically, a much bigger potential health risk involving thorium would be something along the lines of incandescent gas lantern mantles, which leads to significant higher exposure to the general public. Not to mention the hazards of the radioactive potassium-40 in something like bananas.