Gamma and X Radiation
These are both electromagnetic waves similar to light but having higher energy. Gamma and X radiations are. essentially the same, only the origins of the radiations are different. They have great penetrating power but may be absorbed by a sufficient thickness of heavy materials.
Fission products created in a nuclear reactor tend to emit both beta and gamma radiations. Some materials produced in a reactor emit alpha particles, for example plutonium and americium. Generally speaking these alpha emitters are not released in any great quantity following a major reactor accident.
Exposure to alpha, beta, gamma or X radiation cannot make other objects radioactive. Clearly however objects that have become contaminated with radioactive materials must be treated with appropriate care.
3.3 Half-Life
An important feature of all radioactive materials is that their activity decays with time. Each material is characterised by a 'half-life', the time taken for half the activity to decay. After two half-lives the activity will have decayed to a quarter of the original value, after three to one eighth, and so on, see Figure 3.1. Half-lives from a fraction of a second to millions of
vary years. Table 3.1 gives the half-lives of the radionuclides which are likely to be of importance in a reactor accident situation.
3.4 Units of Radioactivity and
Radiation Dose
The radioactivity of a material is measured in terms of the number of atoms which decay every second. The unit is called the 'becquerel' (Bq) which is a very tiny amount of radioactivity and therefore multiples such as mega (one million) or giga (one thousand million) are more commonly used. A source of one megabecquerel therefore has one million atoms decaying every second. Until recently a unit called the 'curie' was used which was based upon the radioactivity of one gram of radium and is equivalent to 37 000 000 000 becquerels (37 GBq).
Radiation dose is measured in terms of the amount of radiation energy deposited in an amount of material, with a correction factor applied for the different biological effects of different types of radiation, (this is more correctly referred to as 'dose equivalent'). The unit used is called the 'sievert' (Sv). This is a large unit and submultiples, particularly the millisievert
TABLE 3.1 RADIONUCLIDES WHICH MAY BE OF POTENTIAL SIGNIFICANCE IN A NUCLEAR REACTOR ACCIDENT SITUATION
Radionuclide
Half-Life
Krypton-85
10.7 y
Krypton-85m
4.48 h
Krypton-87
1.27 h
Krypton-88
2.84 h
Strontium-89
50.5 d
Strontium-90
29.1 y
Zirconium-95
64 d
Ruthenium-103
39.3 d
Ruthenium-106
368 d
Tellurium-1 3 2
3.26 d
Iodine-131
8.04 d
Iodine-1 3 2
2.3 h
Iodine-133
20.8 h
6.61 h
Xenon-133
5.25 d
Xenon-135
9.09 h
Caesium-134
2.06 y
Caesium-137 Barium-140
30.0 y
12.7 d
Lanthanum-140
1.68 d
Cerium-144
285 d
Neptunium-239
2.36 d
Iodine-135
h hours
d days
y - years
14