by Andrew Maynard on March 13, 2011
Twenty five years ago, during the Chernobyl disaster, I was a physics undergraduate in the UK. I clearly remember our lecturers at the time scrambling as they realized that, falling into complacency since earlier nuclear facility disasters such as Winscale in the UK and Three Mile Island in the US, they had neglected to give us a sound grounding in the health implications of exposure to radioisotopes and ionizing radiation.
Twenty five years on, watching the events unfolding around the tragic earthquake in Japan and the failure of the Fukishima nuclear power plant, I’m experiencing a strong sense of déjà vu. Except this time round I’m the lecturer, and it’s my students that are asking the tough questions.
As the situation develops in Japan, I am having to brush off stuff I haven’t thought about in over two decades as I try to make sense of what is happening, and its potential implications to the health of those in the vicinity and further abroad. And it isn’t helping that there seems to be a dearth of concise and accessible information on the web that might help interpret the news reports on radiation leaks and exposures.
Realizing that others are probably struggling with the same issues, here’s a really quick run-down of some of the top questions I’m currently grappling with:
What are current radiation exposure levels around the Fukushima reactor site?
On Saturday March 12, the International Herald Tribune was reporting radiation levels outside the plant as 1.2 micro Sieverts per hour(µSv/h). According to a March 13 news release from the Japanese Nuclear and Industrial Safety Agency boundary measurements on March 13 recorded 40 µSv/h at the boundary of reactor unit 1, and between 0.07 µSv/h and 4.5 µSv/h elsewhere around the plant’s boundary. At 15:29 (local time) on March 12 a measurement exceeding 500 µSv/h was monitored at the site boundary. And Geoff Brumfiel at Nature News reported this morning measurements of 21 µSv/h at the plant
And what is a micro Sievert anyway?
Rather confusingly, radiation measurements are a mess of non-standard units, measures of radiation emission, measures of how much your body will absorb, and measures of how much potential damage exposure might be caused. So here’s a quick run-down of what’s what:
Radioactive material activity. The activity of radioactive materials is characterized by the number of times per second atoms within them undergo radioactive decay, releasing alpha, beta and/or gamma radiation. The old unit here – which is still widely used – is the Curie (Ci). One Ci is equivalent to 37 billion decays per second.
In SI units, activity is measured in Becquerels (Bq) – one Bq is equivalent to one disintegration per second, so one Ci is equivalent to 37 giga Becquerels – or GBq.
Radiation Absorbed Dose. Radioactivity itself doesn’t indicate how harmful something is – this depends as much on the energy and the type of radiation. To get a better idea of how damaging a particular source of ionizing radiation might be, Radiation Absorbed Dose is used. The non-SI unit for radiation absorbed dose is the rad, and is equivalent to the absorption of 100 Ergs of energy per gram of absorbing material. An Erg by the way is an archaic (in my books) measure of energy!
In more “sensible” units, radiation absorbed dose is measured in Grays (Gy). One Gy is equivalent to the absorption of 1 Joule of ionizing radiation by kilogram of absorbing material. Named the Gray after the British physicist Louis Harold Gray.
Biological effect of ionizing radiation. Unfortunately, even radiation absorbed dose doesn’t give a clear indication of how damaging exposure to ionizing radiation is likely to be. For this, an equivalent dose is needed that takes into account the different levels of harm that can be caused by different forms of ionizing radiation. The non-SI measure of equivalent dose is the Röntgen Equivalent in Man, or rem. Rems are the product of radiation absorbed dose in rads, and a weighting factor, which depends on the type of radiation and tissue exposed.
In SI units, equivalent dose is measured in Sieverts (Sv). Sieverts are the product of radiation absorbed dose in Grays, and a weighting factor which depends on radiation type and type of tissue that is exposed. Fortunately, the conversion between Sv and rems is relatively simple – 1 Sv is the equivalent of 100 rem
For both rem and Sv, the weighting factor depends on the type of radiation, and the type of biological tissue that is exposed. Wikipedia has a useful list of weighting factor components.
Because the effects of radiation are cumulative, biologically-relevant exposure is usually measured with respect to time – usually Sv per hour (Sv/h) or Sv per year (Sv/year). And because a whole Sv is a massive dose – certainly above the level of concern – exposures are frequently measured in micro-Sieverts, or one millionth of a Sievert.
So a micro Sievert per hour – or µSv/h – is a measure of how much biologically relevant radiation exposure someone is receiving each hour.
