Environmental Engineering Reference
In-Depth Information
In addition to the above three types of radiation, there is the emission of neutrons. Usually,
neutrons are emitted as a consequence of a nuclear fission. We already mentioned the fission of
235
U
with the emission of three neutrons [equation (6.1)]. These neutrons can enter other nuclei of
235
U,
which leads to a chain reaction. Also, neutron irradiation of other nuclei can lead to their destabi-
lization, with the successive emission of
radiation.
9
Radioactivity accompanies the whole
nuclear power plant fuel cycle, from mining of uranium ore, through uranium extraction, isotope
enrichment, fuel preparation, fuel loading, reactor operations, accidents, plant decommissioning,
and, last but not least, spent fuel disposal.
β
or
γ
6.3.1
Decay Rates and Half-Lives
The decay rate of an ensemble of radioactive nuclei, to which the diminishing intensity of radioac-
tivity is proportional, is governed by the law of exponential decline:
−
dN
/
dt
=
kN
(6.8)
where
N
is the number of decaying nuclei, or their mass, present at time
t
, and
k
is the decay rate
in units of t
−
1
. Integration yields
N
=
N
0
exp
(
−
kt
)
(6.9)
where
N
0
is the number of nuclei, or their mass, at the start of counting time. The time after which
the number of decaying nuclei is halved is called the half-life
t
1
/
2
:
ln 2
k
t
1
/
2
≡
(6.10)
Some radioactive nuclei decay very fast, and their half-lives are measured in seconds; others decay
slowly, and their half-lives can be days, years, or even centuries.
The decay rate has great importance in regard to radioactive waste disposal. For example, spent
fuel of a nuclear power plant contains many radioactive isotopes, such as strontium-90 (half-life
28.1 y), cesium-137 (half-life 30 y), and iodine-129 (half-life 15.7 million years). Strontium-90
and cesium-137 will decay to small amounts in hundreds of years, but iodine-129 will stay around
practically forever.
10
Table 6.1 lists some radioactive isotopes that play a role in the fuel cycle of
nuclear power plants, along with their radiation and half-lives.
6.3.2
Units and Dosage
The level of radioactivity of a sample of substance is measured by the number of disintegrations
per second. The SI unit of radioactivity is the becquerel (Bq), which is one disintegration per
second (see Table A.1). A more practical unit of measurement of radioactivity is the curie (Ci),
9
The irradiation by neutrons is called neutron activation. It is used, for example, for identification and quanti-
tation of elements in environmental samples. Research reactors at universities and other institutions are used
to irradiate samples inserted into their reactors for neutron activation analysis.
10
With a half-life of 28.1 years,
90
Sr will decay to 1 % of its initial value in 187 years.
137
Cs with a half-life
of 30.2 years will decay to 1 % in 201 years.
