Civil Engineering Reference
In-Depth Information
2
10
4
for PWRs
change in k
eff
per degree of temperature increase of the water
1
:
3
10
3
ð
Þ
for BWRs
change in k
eff
per
%
increase of steam volume void coefficient
The negative moderator temperature- or void-coefficient determine the safety
behavior of LWRs during coolant loss accidents [
11
,
36
]. Contrary to the fuel-
Doppler-temperature coefficient which acts in case of power increases instanta-
neously, the coolant/moderator-temperature coefficient can react only with a
certain time delay during power transients. This is due to the fact that in the case
of a fuel temperature increase the moderator temperature or vapor production
increase only after a certain time delay (thermal conductivity in the fuel and
cladding). However, in case of moderator/coolant depressurization (pipe break or
faulty opening of a valve in the primary coolant system) the void coefficient also
reacts instantaneously.
Figure
2.10
shows the criticality or effective multiplication factor for low
enriched uranium fuel and for low enriched plutonium/uranium fuel. The curves
for these two fuel types are shifted against each other. Usually, for uranium fuel a
design value of V
H2O
/V
UO2
¼
2.1 is selected for PWRs to obtain a sufficiently
negative moderator/coolant-temperature coefficient. For plutonium/uranium fuel a
ratio V
H2O
/V
UO2
¼
3.0 is chosen.
Figure
2.10
shows another important result for the case of a molten core as a
result of severe core melt accidents. In case of a molten core, the water in
LWRs is evaporated, the lattice structure (Fig.
2.5
) is destroyed and the fuel is
arranged in form of a core melt. In this case V
H2O
/V
fuel
!
0 and k
eff
<
0.9,
i.e. the molten rearranged core material is subcritical (see Chap.
9
).
2.11 Behavior of the Reactor in Non-steady State
Conditions
As has been explained above, k
eff
¼
1 corresponds to the steady state condition of
the reactor core, in which case the production of fission neutrons is in a state of
equilibrium with the number of neutron absorbed and the number of neutrons
escaping from the reactor core.
For k
eff
6¼
1, either the production or the loss term become dominant, i.e., the
number of neutrons n(t) and k
eff
(t) vary as a function of time.
Axial movements of the absorber rods in the core change the loss term of
neutrons and influence k
eff
(t). The relative change as a function of time k
eff
(t) is
called reactivity
ρ
(t).
