Preparing for Nuclear War - Answers to the Quiz Questions

Here are the answers to the questions I posed last time.

I should point out that this information doesn't really come under the heading of "Preparing for Nuclear War", as it's not especially relevant to the sort of things you'll have to deal with after the bomb goes off.

The intent of bringing this up was to illustrate that the sort of nuclear physics you get taught at high-school can potentially lead you astray, and, more importantly, it shows why seemingly knowledgeable people pontificating on the subject on the net are more often than not talking complete bollocks. This stuff is harder than it seems and it's easy to think that you know more than you do.


The first question was:

Strontium-90 is essentially a pure beta emitter. Beta particles are stopped by only a few millimetres of metal shielding. Would a strontium-90 source (of non-trivial size) encapsulated by a few mm of metal be safe to come close to?

The answer is, as I mentioned, "no" and the reason is "bremsstrahlung". You would be forgiven for replying something like "Never heard of him, wasn't he a composer?". Bremsstrahlung - which is a German term and not someone's name - is a type of secondary radiation. I won't even try to give a technically complete and correct description of this effect as this would be too complicated for the purposes of this post and I'd probably get it wrong anyway, but it can be summed up by saying that when beta radiation is stopped by shielding, it creates gamma radiation which is known as bremsstrahlung.

Therefore to safely shield a beta source it is necessary for the shielding to be sufficient to not only stop the beta radiation but also stop the much more penetrating bremsstrahlung.

BTW, Wikipedia will often be quite helpful regarding topics like this - at least when they have no political dimension to them - but the article on bremsstrahlung is an impenetrable mass of equations and looks like someone copied out their second year university physics notes on the subject.

With an intense enough beta source, the bremsstrahlung can be a very serious hazard. The IAEA (International Atomic Energy Agency) have a comprehensive account of an incident in Georgia (the central European country, not the US state) where three guys came across a pair of abandoned strontium-90 sources in a forest and used them as personal warming devices. The report is at http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1660web-81061875.pdf and although it goes into a lot of technical detail, it's very readable - but be warned that it contains some almost unbelievably gross pictures of radiation burns.

Second question:

All radioactive elements decay over time at a rate defined by their half-life. If you took a pure piece of the radioactive element uranium-238 and placed it in a sealed container, would the level of radiation within that container gradually reduce over time at a rate determined by the half-life of this element?

The answer is, again, "no". In fact the radiation level inside the container will actually gradually rise over time. In fact, if you come back after 100,000 years, the radiation level will still be rising.

The reason for this is when uranium-238 decays, it decays into thorium-234 which is also radioactive. The thorium-234 in turn decays into other elements which are also radioactive. In fact, U-238 decays through a series of thirteen radioactive elements (known as decay products, daughter products or sometimes just daughters) before eventually ending up as stable lead. So as the uranium decays, these daughter products build up and contribute to the total radiation being emitted.

It sounds a bit scary, but each daughter product only builds up to a certain level (known as an equilibrium level), so the total radiation level stops rising once all of the daughter products reach their equilibrium levels. After that, the overall level will decrease over time.

Fortunately, this is not an effect you really have to worry about when it comes to nuclear fallout. Most of the bad stuff from a reactor accident, and to an even greater extent a nuclear bomb, has a comparatively short half-life and a short decay chain and radiation levels continually fall over time.