About that Plutonium

By Dr. Y

Every now and again plutonium makes the news. Traces of it were found in Japan after the Fukushima reactor accident in early 2011, traces were found in some uranium enrichment facilities in 1999, plutonium-contaminated lands were engulfed in wildfires near national laboratory facilities several times in the 1990s and 2000s, and a laboratory worker at the National Institute of Standards and Technology was exposed to small levels of plutonium in 2008 – not to mention an entire book (The Plutonium Files) that was written about human plutonium experiments carried out in the first decades of the atomic age. It seems that every time the issue of plutonium arises there are similar concerns – that plutonium is uniquely deadly (usually using the phrase “the most deadly substance known to humanity”), that plutonium is not found in nature, and that the plutonium found near the Fukushima reactors could only have come from ruptured reactor fuel disseminated into the environment. Each of these points is flawed – here’s why.

The most toxic substance known?

Contrary to popular opinion, plutonium is far from being the most toxic substance known. True – it is a radioactive and toxic heavy metal, as are lead, uranium, mercury, and cadmium (to name only a few). It is also true that plutonium is highly toxic – a fraction of a microgram can kill, but the deadliness comes from the radiotoxicity more than from the chemical properties of the element. But polonium – such as what was used to kill Alexander Litvenenko in London in 2006 – is far more radiotoxic, and any number of organic poisons, including ricin, botulism toxin, some spider venoms, amatoxin (found in poisonous mushrooms) and some shellfish toxins are also far deadlier than plutonium.

Ironically, the book The Plutonium Files helps to make this point that plutonium is not uniquely toxic – the author mentions a number of experiments in which subjects who were thought to have only days or weeks to live were injected with plutonium for purposes of scientific research and that many of these people were still alive decades later, having been exposed to the injected plutonium far longer than expected. What the author did not comment on was that this very longevity suggests that trace amounts of plutonium are not deadly – were that the case these subjects would have succumbed from plutonium toxicity. Putting all of this together – yes, plutonium is highly toxic but no, it is far less toxic than many natural venoms and toxins.

Plutonium artificiality

The first plutonium that was identified was produced in nuclear reactors and, in fact, the Manhattan Project put a huge effort into producing even the first few micrograms of plutonium and an even greater effort into making the kilogram quantities needed to produce the weapon that was dropped on Nagasaki Japan in August, 1945. In nuclear reactors plutonium is produced within a few days after the nucleus in the uranium-238 atom captures a neutron, transforming it into U-239. This then undergoes radioactive decay and becomes Np-239, which soon undergoes another radioactive decay to become, Pu-239, a fissionable nuclide that can be used to make nuclear weapons.

Surprisingly, this happens in nature as well, most often in uranium ore deposits. The surface of the Earth is subject to a continual neutron flux caused by cosmic ray interactions in the atmosphere – these neutrons can be captured by U-238 (which makes up over 99% of natural uranium atoms), producing U-239 and, thence, Pu-239. In addition, U-238 is a large atom that sometimes fissions spontaneously, emitting neutrons that can be captured by neighboring atoms with the same result. The first natural plutonium was identified in uranium ore by Charles Levine and Glenn Seaborg in 1951. Further, in 1967 geochemist P.K. Kuroda also identified the remnants of Pu-244 in meteorites, apparently produced by supernovae in distant parts of the galaxy (http://www.terrapub.co.jp/journals/GJ/pdf/2601/26010001.PDF). It is safe to say that large quantities of plutonium on Earth are bound to be man-made, but trace amounts of plutonium are produced in nature as well.

Plutonium from reactor fuel

As mentioned above plutonium is produced in nuclear reactor fuel – in fact, a substantial fraction of the energy produced in a nuclear reactor comes from the fission of plutonium produced during normal reactor operations. But the fact that plutonium was found near the Fukushima reactors does not mean that it came from them. In actuality there is plutonium found in all of the soils of the Earth. Some of this plutonium comes from nuclear weapons testing – only a fraction of the plutonium in the warheads fissions and the remainder is thrown into the atmosphere, settling back to Earth gradually over the following years. Then there was a plutonium-powered satellite that was incinerated in the atmosphere in 1967 – plutonium from that spread across the planet as well.

Plutonium is a refractory element that is locked up within the reactor fuel – even during a fuel meltdown it is not easy to spread the plutonium far afield. This is not to say that there have not been reports of plutonium from the damaged reactors – an article in the October 2, 2011 Financial Times (http://www.ft.com/cms/s/0/7e3af460-ece6-11e0-be97-00144feab49a.html#axzz1eI6gJlxa) reports that traces of plutonium from the reactor were found up to 40 km distant. What is difficult is trying to untangle the plutonium that we expect to see from the sources mentioned in the first paragraph from those that came from the reactors – any short-lived isotopes of plutonium (especially Pu-241) is more likely to have originated in the reactors while the longer-lived nuclides (Pu-238, Pu-239 primarily) may have its origins in the era of atmospheric nuclear weapons testing.

The bottom line is that plutonium can be nasty stuff – especially when used as a nuclear explosive. But we should give it the respect it is due – neither short-changing its dangers nor exaggerating its threat. Only by so doing can we take reasonable steps to assure our safety.

 

Dr Y is a certified health physicist, trained in nuclear power plant design and operations, with experience in nuclear power, environmental science, and planning for radiological and nuclear emergencies. He has 30 years of experience in the areas of nuclear and radiation safety.

 


 

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