Radioactivity
Amount, dose and
units
There are many ways of expressing the scale of the radioactive waste problem.
Sometimes volumes are used - there are large volumes of low level waste but tiny volumes of high level waste.
Of importance is the amount of radioactivity in the volumes.
This is measured in units of becquerel (Bq)
1 Bq = 1 decay per second
For large quantities, typical prefixes are used:
k = kilo = 1000 1 kBq = 1000 Bq,M = mega = 1 million,
G = giga = 1 billion,
T = tera = 1 trillion
Sometimes older units are used.
The curie is the old form - 1 Ci = 37 GBq.
Even this does not necessarily tell us how dangerous the amount is.
1 GBq of thorium-230 is far less hazardous to health, because kg for kg, it decays more slowly than the same amount of caesium-137.
This is a big simplification, there are many other factors to be taken into account, decay energy, type of radiation, etc.
The damage that can be done to the body is measured by the amount of energy absorbed.
The unit is the gray (Gy).
Alpha, beta and gamma radiation have different effects and the unit used to express the health effects on an equal footing is the sievert (Sv).
Doses above around 1 Sv can be very serious and are certain to lead to harm. Such situations arise very rarely, usually as a result of serious accidents. High doses (more than around 10 Sv) are almost certain to lead to death.
See http://www.iaea.org/worldatom/inforesource/bulletin/bull384/camp.html for more information
Up to around 0.1 Sv means that there is a non-predictable chance of developing cancer or other effects.
It is assumed that there is no safe dose of radiation below which no health effects will be felt.
Typical average annual exposure to all forms of environmental radiation is between 2 and 5 mSv per year (0.002 - 0.005 Sv/y).
Source: Galson Sciences Limited