Half-Term Half-Life
OK. OK. I was supposed to get this article up yesterday. My excuse: it’s half-term. For those non-Brits reading this, half-term is when all of the schools in the country have one week’s break. The kids are at home… with their parents…
I start every half-term full of energy, excited- ‘We’ll bake cookies!’ ‘We’ll paint pictures!’ ‘We’ll make things out of clay!’- by the end of the first 24 hours, we haven’t done a thing and half my energy is gone… and I flop into bed with a grunt a couple hours earlier than normal.
I start Day Two a little bit later than usual, still with big plans though, but before I know it the day is finished, no cookies have been baked and another half of my half of my energy is gone so I’m left with a quarter of my original energy.
By the end of the next day, half of my half of my half of my energy is gone…and so on… day after long, 24-hour day…
So, yes, I was supposed to have this article up yesterday. It’s now Friday- counting the weekend, it’s the seventh day of half-term, and my half-term ‘half-life’ is 24 hours (the amount of time it takes for half of my energy to disappear)… so, I’m now operating at one one-hundred and twenty-eighth of my original self… forgive me.
But my exhaustion has provided a convenient way of explaining another ’scary’ nuclear-related term ‘Half Life’. My last post explained what ‘Radiation’ was and I finished it by saying that I wanted to talk about radiation’s effect on the human body. As I was looking into it, I learned about half-life and realised that I never really knew what half-life was and suspected a lot of other people don’t either so I wanted to explain that first.
As you learned in my last post, radiation is excess energy released as electromagnetic waves or particles. When an atom emits alpha or beta particles, it becomes a new element. Radiation is basically the way in which an unstable atom becomes- or decays into- a stable atom. Like a working mum during half-term, all it’s trying to do is rest… that’s all it’s trying to do.
Now, atoms decay in a completely random way. If you were looking at one Uranium-238 atom (which makes up the majority of depleted uranium), you would have no way of knowing exactly when it would decay. If you were looking at 5 billion Uranium-238 atoms, however, you would know that in 1.41 × 1017 seconds (or 4.47 billion years), half of them on average would have decayed and you’d be left with 2.5 billion Uranium-238 atoms. After another 4.47 billion years, half of those atoms (again on average) would have decayed and you’d be left with 1.25 billion Uranium-238 atoms… and so on. This is ‘Half-Life’.
Uranium-238 is only mildly radioactive - approximately one atom in every 5 billion will decay in one year. It’s an alpha emitter which means it releases an alpha particle when it decays. You will remember from my last post that an alpha particle is essentially the same as a helium atom, two protons and two neutrons, except that it doesn’t have any electrons. An alpha particle can be stopped by a piece of tissue paper.
So unlike what you hear most of the time, depleted uranium isn’t extremely radioactive… at all. Having an incredibly long half-life does NOT mean that it is incredibly radioactive. The problem with depleted uranium isn’t the mild radioactivity, it’s that, like most heavy metals, it is chemically toxic… like mercury, arsenic or lead.
But our poor decayed atom isn’t able to rest quite yet because it hasn’t become something stable- it’s become a Thorium-234 atom (notice the number 234 is 4 less than the uranium’s 238. The alpha particle it emitted has how many protons and neutrons?…). Thorium is a beta emitter (it releases an electron which can be stopped by a thin sheet of aluminium) and its half-life is 24.5 days… which means that in just under a month half of the Thorium-234 atoms on average will have decayed.
Yet, still it hasn’t decayed into a stable atom… So I don’t have to go through the whole decay chain, here it is. It shows the type of decay and the half-life of each element.
Along with the alpha and beta radiation which can be stopped by a piece of paper and some aluminium respectively, radioactive decay also produces gamma rays which are high frequency electromagnetic waves. They are pretty penetrating, but can be stopped by concrete.
Long term storage of depleted uranium is done by vitrifying it into in glass, then encasing it in steel and placing it in a concrete store underground. People are often frightened that the radiation can somehow leak out and as these things are going to be there long after the human race has ceased to exist.
Now I’ve pointed out several times the things that stop various types of radiation, but let me show you.
I have to say that after learning about alpha, beta and gamma radiation, looking at the decay chain, seeing the illustration of what stops radiation and reading about nuclear waste storage, this is the first time in my life that I am not afraid of stored nuclear waste. I may even go so far as James Lovelock and say that I’d have no issue with it being stored in my back garden.
So I hope you understand half-life a little bit more… Interestingly, my half-term half-life is 52.731 times longer than that of Lead-214’s which is 26.8 minutes. Although, if I had Lead-210’s half-life of 22.3 years, I’d definitely have fewer wrinkles…
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June 2nd, 2006 @ 9:45 am
The other thing I learn every half-term is that teachers definitely should be paid more.
June 2nd, 2006 @ 2:09 pm
Thank you Gia.
We’re not all science types and some simple clarification of the relevant terms and concepts is exactly what this site needs. More please.
June 3rd, 2006 @ 3:45 am
The problem, though, isn’t the depleted uranium- it’s the witches’ brew of other nasty stuff that spent fuel contains. You see, because you’re not just letting uranium decay, you’re actually splitting it (by pelting it with neutrons). The broken pieces are much smaller atoms than the very havy transuranic elements (the series starting with uranium, being the heaviest element to be present in significant quantities in our solar system). These smaller atoms tend to have much more internal stress because of their proportionally more unbalanced proton to neutron ratio. This means that they, like the half-term mum, need to have a lot more relaxation, and so they fall apart with great vigour. They tend to emit high-energy beta particles, lots of ‘hard’ (high energy and very penetrating) gamma rays and even the occasional neutron.
They’re really horrible stuff. Spent fuel is really, really nasty.
June 3rd, 2006 @ 3:01 pm
You and your “cohorts in crime” are doing a wonderful job removing the veil of mystery from the science of nuclear energy. It’s a goal of mine as well, but you’re pulling it off in a much more effective fashion. The world will be a better place as a result. Thanks!
John
“This Week in Nuclear” Podcast
http://thisweekinnuclear.com
June 5th, 2006 @ 10:35 am
The ‘broken pieces’ or fission products of which you write, Jas, are indeed highly unstable, which is why they have such a short half-life, decaying to approximately 1000th of their initial radioactivity in less than 40 years and to below the level of the mined ore in only a few hundred years. Modern industrial society deals with - and makes secure - many waste streams which could be described by such emotive terms as ‘witches’ brew’ and will continue to do so regardless of whether we commission a new nuclear build.
June 6th, 2006 @ 5:01 pm
Just because we already do something bad does not mean that it is okay to continue to do something bad, or, as you seem to be implying here, MCrab, that because we are doing something bad it’s okay to do something else really nasty.
FYI, fission products may not be a long-term problem in storage, but they remain a major obstacle for process stream waste in repro plants. They increase the activity of what would otherwise be low-level waste streams, and have to be dealt with.
There are some singularly unpleasant chemicals in the fission waste stream- iodine, caesium and strontium are all taken up and concentrated by the body, which makes them particularly dangerous. Even though radioiodine has a really short halflife, it is still dangerous; Cs-134 and Cs-137 from Chernobyl will be problematic for at least a hundred years from now, in many parts of northern Europe. Cs-134 has a halflife of about 2 years, which may seem short compared to the thousand-year halflives of transuranic elements, but its short halflife makes handling the stuff much more dangerous in the near term. Indeed, your statement isn’t even accurate- Cs-135 has a halflife of 2.3 million years, and emits relatively penetrating beta particles (whereas most transuranics are alpha emitters), and worse still, the lovely bone-seeking Sr-90 has a half life of about thirty years. while emitting nice beta particles.
Seriously, nuclear waste is a mixture of different substances, and all of them will kill you if you get them in you.
June 7th, 2006 @ 2:38 pm
Substances which could be labelled ‘bad’ and ‘nasty’ are created in many industrial processes and it is how they are contained, treated and disposed of that concerns me, not that they are produced in the first place. Indeed, it is naïve to assume that any advanced society could survive without ever producing anything toxic or harmful. It is in that context that nuclear waste should be judged and it is worth noting that unlike certain toxins, nuclear waste decays with time and becomes less dangerous for future generations.
You’re point about the shorter lived fission products being particularly dangerous because of their short half-lives is certainly valid, Jas. But it is only relevant in considering the accidental release of radioactive material in a Chernobyl style event. Thus you have to factor in the probability of such a release from a modern Western reactor which, even in the unlikely event of a meltdown, is still miniscule.
As you wrote in your previous post, Jas, the fission products do not present a long term storage problem which is one of the reasons why talk of nuclear waste being a problem for thousands or millions of years is overblown. The activity associated with the longer lasting fission products is shown in this graph. You can see that Cs-135 has an activity roughly a thousand times lower than the original uranium ore. Its slow decay rate and the low energy of its emitted beta ray make it a particularly low hazard - something you failed to note in your post.
Your last sentence is simply wrong. Dose, exposure and type of radioactive substance all determine the biological hazard of nuclear waste. It is as inaccurate as saying that CO2 from a coal powered station is lethal (which it can be in high enough concentrations). Such misleading and simplistic scaremongering surely has no place in a detailed debate on the real merits and drawbacks of nuclear power in the 21st century.
July 21st, 2006 @ 8:01 pm
Good article, Gia–isn’t it amazing what a little research will do to your perception of a problem.
For you and our friend Jas on here, you might like to take a look at a program I wrote recently. It allows you to see which isotopes are created in nuclear fission and how long it takes them to decay to stability. You’ll see that most decay chains proceed to stability very quickly.
And Jas, please don’t eat the nuclear waste. We wouldn’t want to remove your genes from the gene pool.
Visualization Tool for Decay Chains