News has come from Japan that the police have delivered water cannon to the reactor site.
Now I am sure that no rioting will be occurring at the plant, while some radioisotopes are nasty they are not criminals.
I think that the water cannon are wanted as they increase the distance between the humans and radioactive objects. For a point source of radiation the following rule is true.
Dose rate = k / distance2
k is a constant
So by doubling the distance the dose rate goes down by a factor of four.
The following table can be calculated for a radioactive source which gives a dose rate of 1 Gy per hour at one meter. It is clear that by increasing the distance between the source and the person that the radiation level will go down.
|Distance (meters)||Dose rate (Gy hr-1)|
I expect that the water cannon will be used if the workers at the site need to spray water at a radioactive object or a building which is emitting lots of radiation.