122 Nuclear Forensic Analysis
the coolant, caused by an unplanned rise in the nuclear reaction rate, reduces the
d
ensity
o
f
t
he
s
urrounding
w
ater
a
nd
m
akes
i
t
a
p
oorer
m
oderator
o
f
n
eutrons,
ca
us-
ing
a
d
ecrease
i
n
t
he
r
eactivity
o
f
t
he
c
ore.
A
s
a
n
e
xample
o
f
a
n
e
ngineered
s
afety
f
eature,
m
ost
r
eactors
h
ave
b
oth
c
ontrol
a
nd
s
hutdown
r
ods
f
or
c
ontrolling
t
he
r
eac-
tivity
o
f
t
he
c
ore.
S
hutdown
r
ods
a
re
s
pecically
d
esigned
f
or
r
apid
a
nd
f
ail-safe
t
er-
mination
of the chain reaction. Although a normal operational change in power level
t
hrough
t
he
a
gency
o
f
t
he
c
ontrol
r
ods
m
ay
t
ake
p
lace
o
ver
a
n
h
our,
a
n
e
mergency
s
hutdown (or “scram”) may take only seconds.
Even
w
hen
t
he
c
hain
r
eaction
i
s
s
topped,
h
owever,
t
he
c
oolant
s
ystem
m
ust
c
on-
tinue
t
o
o