Fukushima radiation levels have hit a new high this week and boy is it a new high. The problem with radiation level measurements are there are so many different units. Some measure the physical dose of radiation an others the effective biological dose. To make it more confusing units have changed over the years to SI (systeme internationale) units. As someone who has done radiation training even I find it confusing.
The usual accepted dose unit is the sievert. This is a measure of the effective biological dose. Usually its expressed in millisieverts (mSv) since the sievert is regarded as too large. 1 mSv is an energy dose delivered to 1 gramme of living tissue. The annual average dose is 2.4mSv from natural sources but over 6 from one computed tomography (CT) scan. In the US workers are allowed a dose of 50mSv a year, conveniently measured using badges with film inside (I’ve worn one of these). At Fukushima the limit was raised to 250mSv for workers. To put this in perspective 1sievert (1000mSv) is enough to cause sickness 5 and 10sieverts are enough to kill you although over different time periods (months and weeks respectively).
The measured dose at the plant this week was 530sieverts. I’ll leave it to readers imagination about the speed that these radiation levels will kill you but suffice to say it does a robot in within 2 hours. This week there was also some “good news” from the plant in that melted fuel had been located. This is good news since they know where it is. The problem is with such high levels of radiation cleanup is going to be much much harder. Costs of cleanup are running at £150 billion pounds but looks like a considerable underestimate.