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The water cannon and the inverse square law

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)
0.5 4
1 1
2 0.25
3 0.11
4 0.0625
5 0.0400
10 0.0100
20 0.0025
40 0.000625

 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.


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