Civil Engineering Reference
In-Depth Information
10
5
(K
1
)
- Doppler coefficient in the range of
2.5
10
4
(K
1
)
- coolant temperature coefficient in the range of
2
For a 1.3 GWe BWR, they are a
10
5
(K
1
)
- Doppler coefficient in the range of
2
10
3
(per % steam volume increase)
- coolant void coefficient in the range of
1.3
10
5
(s) for all LWR
cores. The negative safety coefficients, together with the delayed neutrons (see
Sect.
2.10
), guarantee a safety-oriented feedback and control behavior of LWRs.
This will be demonstrated for two examples below.
The effective prompt neutron life time l
eff
is about 2.5
5.6.2.1 Stable Time Behavior of Power When Absorber (Control) Rods
Are Withdrawn in a PWR
Starting from a constant reactor power level, P
0
, of a PWR, which is lower than the
nominal power, limited withdrawal of the absorber (control) rods by a few cm
(Fig.
5.4
)—as an example—shall produce a positive ramp type increase in k
eff
resulting in a k
eff
¼
1.002 within an interval of 20 s (Fig.
5.5
). Initially, this raises
the relative power, P(t)/P
0
, and the fuel temperature in the reactor core. After a
delay of several seconds, radial heat conduction in the fuel rods of the reactor core
also increases the cladding tube temperature and, as a consequence, the coolant
temperature T
C
as well. An increase in fuel temperature by
T
F
(t), through the
negative fuel Doppler coefficient, practically instantaneously causes a negative
Doppler reactivity,
Δ
10
5
Δρ
D
¼ Δ
T
F
2
:
5
and, after a short delay, through radial heat conduction in the fuel rods, the coolant
temperature increase
Δ
T
C
causes a negative coolant temperature reactivity,
:
10
4
Δρ
C
¼ Δ
T
C
2
Both negative feedback reactivity contributions counteract the positive initial
reactivity produced by withdrawal of the absorber (control) rods until the total
reactivity becomes zero. This stabilizes the reactor power at a slightly higher level
of roughly P(t)/P
0
¼
1.17 when
