[OPINION] Uranium mine waste and the weird idea of half-life

The following is the 23rd in a series of excerpts from Kelvin Rodolfo’s ongoing book project “Tilting at the Monster of Morong: Forays Against the Bataan Nuclear Power Plant and Global Nuclear Energy.“

Let’s now look at how radioactive substances decay, in a useful way, by examining the environmental threats of the waste generated by uranium mining. Huge amounts accumulate in piles like these rock “pyramids” left behind when a German uranium mine closed. Elsewhere, milling sludge is stored in ponds, kept wet to avoid radioactive dust from being blown around by wind. These pictured here on the right are at the Olympic Dam mine in Australia.

The waste, having been crushed and milled, is easily broken down by the weather and dispersed into soils and waters. Like mining for non-radioactive metals, uranium mining also releases mill chemicals and heavy metals such as arsenic into the environment.

But uranium mining waste poses an even worse concern: radioactivity, such as from radon gas. Only tiny amounts of uranium are extracted, leaving just about as much rock waste as was mined. About 85% of the ore’s initial radioactivity remains in the waste, including 5-10% of the uranium, almost all of it U238. Thorium 230, an especially abundant product of U238, also remains.

Earth is 4.54 billion years old; purely by coincidence, almost the same time that it takes for any amount of Uranium 238 to decay until only half of it remains. That’s called its “half-life.” In other words, when the Earth first formed, it had twice the U238 as it contains now.

Shortly, we will look more closely at what the crucially important idea of “half-life” means.

When the politician introduced the legislation to activate BNPP in 2008, he did not understand. Few politicians do; my hope, possibly forlorn, is that this will teach some of them.

First, let’s look at how a U238 atom decays. The drawings look complicated, but the arithmetic involved is actually very simple.

From U238 to Lead 206

Our last foray introduced radioactivity by looking at how U238 decays by emitting an alpha particle to become Thorium 234, which in turn emits a beta particle to become Protactinium 234.

Recall that an alpha particle is composed of two protons and two neutrons, so alpha decay decreases the atomic weight of the daughter atom by four and its number of protons or atomic number by two.

Also remember that beta-decay changes a neutron into a proton, so the atomic number of the daughter atom increases by 1; in our example, Thorium, Element #90, becomes Protactinium, element #91. Its atomic weight remains the same, because a beta particle or high-speed electron has no mass.

But U238 to Thorium 234 to Protactinium 91 are only the first two of 15 steps. As the upper left corner of the next picture shows, the decay series continues with Protactinium emitting a beta to become Uranium 234, which then emits an alpha to become Thorium 230, and so on.

The entire decay series emits eight alpha particles and six beta particles before finally ending up with Lead 206, that metal’s stable and most abundant isotope. A single U238 atom undergoing the entire decay series will exist as 14 different radioactive isotopes, including two of Uranium, two of Thorium, three of Polonium, two of Bismuth, and two of Lead.

Most of the decays also emit gamma rays, which, although dangerous to human health, change neither atomic weight nor number.

The sequence from Uranium 238 to Radium 226 involves all the half-lives longer than a thousand years. The last eight are much shorter, the longest only 22.3 years, the shortest only 16.3 thousandths of a second.

“Half-life” will be important for us more than once. All people involved in deciding whether or not to activate the BNPP owe it to the taxpayers to be familiar with this concept; with luck, some of them will read this and learn.

The first half-life is like human life expectancy

Every atom of a particular radioactive isotope has a “half-life,” a definite length of time during which it has a 50-50 chance of either decaying or surviving.

For our example, let’s use Thorium 230, the fifth form in the decay series. Th230 is prominent in uranium-mining waste.

The half-life of Th230 is 75,400 years: any Th230 atom can decay at any time, but starting with a large number of them at a given time, half will have decayed 75,400 years later.

Isolate a kilo of Th230: after 75,400 years a half kilo will have decayed, and the other half will remain.

Half-life is well-established because atoms, being not only extremely tiny but also enormously numerous, lend themselves very well to precise statistical analyses of how they behave.

The first half-life is somewhat like human life expectancy, a statistic based on large numbers of people. There are so many millions of Filipinos that the population data can be used to estimate how long they live on average.

The National Statistics Office says that Filipino boys born in 2018 can expect to live 66.2 years. This is simply an estimate: by 2084 half of all Pinoys born in 2018 will have died, and half will still be alive. Let’s call 66.2 years their “half-life”.

Filipinas born in 2018 have life expectancy of 72.6 years; half will have survived by 2090, many of them widows. Perhaps Pinays owe their longer “half-life” in part to their safer life styles: no testosterone poisoning, drinking much less San Miguel beer, no drunken brawls, less reckless driving, smoking much less.

Life expectancy does not apply to individual girls or boys, only to entire groups. Some born in 2018 died in infancy; others will live to ripe old ages.

But the similarity of human life expectancy and atomic half-life must end there! Humans born in a certain year have only one “half-life.”

Unlike us, atoms don’t become frailer and closer to death over time. A radioactive atom stays exactly as fresh as it was when it came into being. And so for however long any Th230 atom survives, it has a 50-50 chance of surviving another 75,400 years – until the instant it actually decays.

So, let’s say a kilogram of Th230 containing 262 trillion atoms exists at a given “time zero.” After one half-life (75,380 years), half of the atoms, 131 trillion atoms together weighing ½ kilo will remain…

After two half-lives or 15,760 years, 65.5 trillion atoms weighing ¼ kilo …

After three half-lives, 226,140 years, almost 33 trillion atoms weighing 1/8 kilo…

After four half-lives or 321,520 years, more than 16 trillion atoms weighing 1/16 kilo would still exist.

So: how many more half-lives would it take for only a trillion Thorium atoms to remain? How many years?

Fuel for an activated BNPP would every year add 100,000 tons to radioactive mine-waste somewhere else in the world. It’s a good thing that the Philippines has no mineable Uranium, yes?

But the idea of half-life will become important if BNPP is activated. First, it will expose people living nearby to the health risks of tritium and other short-lived isotopes. Second, it will generate much radioactive waste that would last virtually forever, with no safe, permanent repository.

Finally! We are now ready to examine how BNPP would work in our next foray. – Rappler.com

Born in Manila and educated at UP Diliman and the University of Southern California, Dr. Kelvin Rodolfo taught geology and environmental science at the University of Illinois at Chicago since 1966. He specialized in Philippine natural hazards since the 1980s.

Keep posted on Rappler for the next installment of Rodolfo’s series.

[OPINION] Uranium mine waste and the weird idea of half-life

From U238 to Lead 206

The first half-life is like human life expectancy

Previous pieces from Tilting at the Monster of Morong:

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