This was written for the Potential Energy blogging project I did for the Institute of Physics in 2006. The blog is no longer working. You can view the original blog post and comments at Archive.org here. I’ve taken the links directly from the original piece. I can’t guarantee that they are still active links.

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…