Civil Engineering Reference
In-Depth Information
becomes slightly
1. This is balanced by absorber materials (moveable absorber
rods, burnable neutron poisons, e.g. gadolinium or boric acid) which keep the
reactor core always at k
eff
>
1.
After shutdown of the reactor the gradually decaying fission products and the
radioactive decay of higher actinides creates afterheat in the reactor core. This
afterheat (decay heat) must be transferred by the coolant water to outside coolant
towers or to river or sea water.
Prior to the description of Light Water Reactor designs some basic characteristics of
reactor physics and reactor safety will be presented. For a deeper understanding of
these characteristics the literature given in the reference is recommended [
1
-
8
].
2.1 Radioactive Decay, Decay Constant and Half-Life
Radioactive decay changes the number of unstable (radioactive) isotopes, N(t),
existing per cm
3
as a function of time, t. This change can be described by the
exponential law of
ðÞ¼
N
0
ð
λ
Þ
Nt
exp
t
is the decay constant and N
0
the number of atomic nuclei per cm
3
at
the time t
where
λ
¼
0. Instead of the decay constant,
λ
, one can also use the half-life,
T
1/2
¼
(ln2)/
λ
, which is the time by which half of the nuclei existing at t
¼
0 have
decayed. The decay rate,
N(t), is called the activity of a specimen of radioactive
material. This activity is measured in units of Curie or Becquerel [
1
,
2
].
One Becquerel, denoted Bq, is defined as one disintegration per second. One
Curie, denoted Ci, is defined as 3.7
λ
∙
10
10
disintegrations per second, which is
approximately the activity of 1 g of radium. Low activities are also measured in
mCi
10
3
Ci or
10
6
Ci [
1
,
2
].
¼
μ
Ci
¼
2.2 Fission Process
If a neutron of a certain velocity (kinetic energy) is absorbed by a fissile heavy
nucleus, e.g. U-233, U-235 or Pu-239, the resulting compound nucleus can become
unstable and split (fission) into two or even three fragments (Fig.
2.1
). The fission
fragments are created essentially according to a double humped yield distribution
function with mass numbers between about 70 and 165. The mass yield distribution
functions are similar for heavy nuclei fissioned by neutrons with kinetic energies of
0.0253 eV (thermal spectrum reactors) up to about 0.2 MeV for Fast Breeder
