With the ongoing disaster at Japan’s Fukushima nuclear plant, questions are inevitably being raised more generally about the safety of nuclear power. Meanwhile, on the news, we are frequently hearing reports of how the levels of radiation are measured in terms of milli-Sieverts, and reassured that certain levels of milli-Sieverts are “safe”, but what does this all actually mean?
The Sievert is the SI derived unit of “dose equivalent radiation”. This means that it is not simply a measure of how much radiation has been absorbed, but an attempt to qualitatively evaluate the resulting biological damage to anyone who is unlucky enough to have been exposed. The idea is simple enough. Each tissue type in the human body absorbs different amounts of ionising radiation. Think about X-rays (which are a kind of lower energy version of gamma rays): the bones absorb most of the X-rays and so show up on the photograph as shadows, whereas softer tissues absorb far less and are nearly transparent. Knowing the precise level of absorption for each tissue type means we can therefore calculate the amount of radiation absorbed throughout the whole body. So far so easy. There is, however, another difficulty, which arises because ionising radiation exists in a variety of different forms. Anyone who has studied physics, even at the most foundational level, will probably be aware that radioactive materials emit three different kinds of radiation. The reality is a little more complicated (as reality generally is), though for purposes of explanation let’s stick with these three most familiar types.
Each of the three different kinds of radiation causes damage in different ways. At one extreme there are gamma rays, a type of very high-energy “light”. It happens that gamma rays are the least absorbed and therefore the least damaging (although dangerous enough). At the opposite extreme, there are alpha particles, heavy and relatively slow-moving, easily stopped and thus easily absorbed. Alpha particles are the most highly damaging of the three, but as fortune would have it, they are generally stopped before they ever reach us — giving up the ghost after travelling just a few centimetres through air and stopped almost entirely by something like the thickness and density of a cigarette paper. Between these extremes there are the beta particles. These are very high energy electrons that penetrate further than alpha particles but not as far as gamma rays. Their middling penetration means they are also middling in terms of the damage they cause, which is not coincidence, but a direct consequence of damage being dependant (to great extent) simply upon levels of energy absorbed — any particles or rays that passed through you without interaction, and thus losing no energy, couldn’t do any harm.
In measuring the biological effects of radiation, all of this has to be taken into account, and so it is, with Sieverts calculated on the basis of different weighting factors (based on measurements) applied to different kinds of radiation. There is however one huge problem with this whole analysis, which is that it works on the presumption that all of the sources of radiation are on the outside coming in. So what about the damage being caused by radioactive sources that have entered the body? The dust in the air that gets inside our lungs. The isotopes we have unavoidably swallowed and pass through our gut, or worse, are absorbed into parts of our own tissue. The recognised measure of “dose equivalence”, the Sievert, takes no account of these secondary effects; effects that may, especially in the case of alpha emitting sources like Uranium and Plutonium, actually lead to more significant and lasting damage.
Some experts, such as Christopher Busby1, are saying it’s time we changed the way we measure the risks associated with radioactivity. They argue that the current methodology underplays the dangers, especially in the case of alpha-emitting isotopes when absorbed internally. This has important consequences not only for assessing the dangers of radioactive waste and leaks from the nuclear industry, but also in underestimating the harm caused by the use of so-called depleted uranium (DU) in the weapons deployed both in Afghanistan and Iraq (click here for link to Uranium Weapons: why all the fuss?).
With regards to the current crisis in Japan, Busby and his colleagues at the Low Level Radiation Campaign, have also been highly critical of “official attempts to play down the radiological impact of this disaster”, saying on their website, “There appears to be no monitoring of alpha emitting radionuclides.” This is in part because ordinary Geiger Counters do not in fact measure levels from pure alpha sources at all (the alpha particles being unable to penetrate the window of the counters). Click here for further analysis and information.
Professor Christopher Busby, Scientific Secretary of the European Committee on Radiation Risk, also spoke to BBC News about serious potential dangers following the explosions at the Fukushima nuclear power plant.