Civil Engineering Reference
In-Depth Information
Fig. 5.4
Fuel element of a
pressurized water reactor
with a rod cluster control
element [
2
,
3
]
Control absorber
rods of a PWR
Control absorber
rods of a PWR
Fuel element
of a PWR
Fuel element
of a PWR
Δρ
CR
þ Δρ
D
þ Δρ
C
¼
0
is reached.
Figure
5.5
shows the different time-dependent reactivity contributions,
ρ
CR
,
ρ
D
,
and
ρ
C
, and the associated time-dependent relative reactor power, P(t)/P
0
.This
indicates the importance of the negative safety coefficients to power stabilization at
higher relative reactor power levels. If the coefficients of reactivity (Doppler fuel
temperature coefficient and coolant temperature coefficient) were not negative, the
relative reactor power will rise uncontrolled (dotted line).
Conversely, inserting the absorber (control) rods would give rise to a negative
reactivity ramp. The relative reactor power P(t)/P
0
will drop and, as a consequence,
also the fuel and coolant temperatures would decrease. These negative changes of
temperatures now produce a positive reactivity feedback.
This type of automatic feedback control of a PWR core can be supplemented by
minor movements of the control rods. Such support by the control rods allows the
change in power to occur faster [
2
,
3
].
